THE CONCEPT OF PEST

A pest can be defined as any organism which injures man, his property, or his environment, or which just causes him annoyance. Such organisms include principally certain insects, nematodes, fungi, weeds, birds and rodents, or any other terrestrial or aquatic plant or animal life, or virus, bacteria, etc. In agriculture, concern is normally expressed when the damage done to a crop by a specific crop pest or a group of pests causes a loss in yield or quality because this would mean a reduction in profit. When a loss in yield reaches certain proportions, the pest can be designated an economic pest. According to Edward and Heath (1964), the pest status is reached when there is a 5 percent loss in yield in a particular crop. In pest management, the economic appraisal of the pest status and justification of the need to embark on control measures is defined in relation to the following concepts: economic damage, economic injury level and economic threshold. Economic damage can be defined as the amount of injury done to a crop that will justify the cost of artificial control measures. Economic injury level is the lowest pest population density that can cause economic damage, which will vary from crop to crop, season to season, and area to area. For practical purposes, there is an economic threshold defined by Stern et al. (1959) as the pest population density at which control measures should be initiated or started to prevent an ever increasing pest population from reaching the economic injury level. The economic threshold is conceptualized by the farmer as the level above which it will pay him to control his pests and below which control is deemed uneconomical. In this regard, of fundamental importance is the cost/benefit ratio of such an operation. The mathematical formula for calculating the economic threshold or action threshold involves 4 determinants, and is presented as follows: Where C= cost of implementing control measure, P= price of crop per tonne, D= loss in yield (tonne ha-1) associated with unit number of pests, and K= reduction in pest attack caused by control measure. The concept of economic threshold is predicated on the fact that organisms in a relatively undisturbed environment reach a state of equilibrium with their environment over a period of time. This is a dynamic state of equilibrium, which implies that although population densities vary from season to season, year to year, or place to place, for a particular place there is an average population density which is stable over a period of time. The economic threshold and economic injury level are usually above this average population. Based on the concept of economic threshold, pests are grouped into the following three major categories: (1) the regular pests, (2) the occasional pests, and (3) the potential pests. The regular pests are those that perennially inflict damage on crops, and whose population levels hardly fall below the economic threshold. Common examples are Maruca testulalis on cowpea, Dysdercus spp. and red boll-worm on cotton, sorghum midge, and Quelea birds in most parts of the grain-producing Guinea and Sudan Savanna of tropical Africa. The occasional pests are those that normally have their population levels below the economic threshold, although their levels may rise above it occasionally. Insects featuring such examples are those that periodically damage cereal crops and grasses in sub-Saharan Africa. They include the stem borers and armyworms of cereals in western and eastern Africa, the variegated grasshopper, Zonocerus variegatus, in West Africa, and a host of other lepidopterous larvae attacking deciduous forest trees, arable, as well as plantation crops. The potential pests are those whose population levels are usually considered to be far below the economic threshold but which can become highly injurious under changed cultural conditions or as an introduced pest. Common examples include many species of grasshoppers and caterpillars in western and central Africa.

DEVELOPMENT OF PEST STATUS

The development of pest status by insects could result from any of the following activities:
Increase in numbers- This is the most common way by which an insect species can attain pest status. The natural control of insect population by parasites and predators is upset by the practice of agriculture, which provides an unlimited food supply for a potential pest. Under these conditions, the population of harmful insect species may still be kept in check by parasites and predators, but the natural factors do not act quickly enough to check the pest from increasing in numbers. In this situation control measures must be initiated should the farmers want to avoid crop losses. Increase in numbers of harmful insect species may also occur through population resurgence. Resurgence is the term used to express a sudden rise in population density. This may occur when the target species, hitherto suppressed by insecticidal treatment, undergoes rapid recovery following the decline of the treatment effect. It may also occur as a result of development of new mutant species of the pest or if the insecticide kills a disproportionate number of the natural enemies of the pest. Increase in numbers coupled with shortage of food supply may induce pest migration. In migration pest species move from one area into another where they cause serious damage. Locusts and armyworms are migratory tropical insects of considerable economic importance. Seasonal increases in number of pests are usually controlled by climatic and biological pressures. The climatic factors are temperature, humidity and sunlight, while the biological pressures include competition, both intra and interspecific, predation and parasitism.
Ecological change- An ecological change, such as the growing of a susceptible crop on a large scale, can convert a harmless insect into a pest. The major ecological reasons for an insect developing pest status are as follows: Character of food supply- Man usually selects his crops for desirable agronomic qualities, such as high yield, succulence, and high nutritional value. Crops having these qualities are more attractive to pests than their wild relatives. For example, maize and sorghum are more attractive stem borers than wild grasses. Monoculture- This is the cultivation of a single crop species on an extensive scale. Under this system insect pests are apt to multiply rapidly owing to an abundant food supply. Essentially, monoculture is similar to the climax conditions of natural vegetation, where large areas are dominated by a very few plant species. The devastating attack of trees by defoliating caterpillars in North America and Europe is the equivalent of a field crop heavily infested by insect pests. Minimum cultivation technique- Minimum cultivation is an agricultural technique employed in seed-bed preparation. It consists essentially of a chemical destruction of old crop remains and weeds, followed by a subsequent planting of the new crop into the undisturbed soil. Ploughing and harrowing before planting is replaced by spraying with paraquat (Gramoxone®) 3-5 days before planting. This technique allows for pest population build-up, since the larvae of beetles, moths, and flies that feed on the aerial parts of the plants normally pupate in the soil. The conventional method of ploughing and harrowing brings the larvae of the foregoing pests to the soil surface, thereby exposing them to predators and sunlight. Multiplication of suitable habitats- Insect pest species are apt to multiply rapidly where there is concentrated food supply. This phenomenon is usually observed in granaries where there is an unlimited food supply. Loss of competing species- At times, some specific pest control measures may remove a pest, but another insect released from competitive pressure may increase in numbers and become a new pest. As soon as the pest is established mutations make the relationships between it and the crop to be closer, because as numbers increase more mutants appear and can be selected to consolidate their niche as pests. Change of host/parasite relationships- Most insects are kept in check by their natural enemies, although a time-lag exists between the population build-up of pests and that of their predators/parasites. The greater the time-lag between pest population increase and that of the predator/parasite, then the more likely is the species to be a serious pest. Agricultural operations involving large-scale insecticide applications may affect the predators/parasites more than the pest. A case in point is the Red Spider Mite (Metatetranychus ulmi) which ultimately became a pest on fruit trees after widespread use of DDT (Dichloro diphenyl trichloroethane) in orchards. Another example is the Giant looper caterpillar (Ascotis selenaria) on coffee, a minor pest, which became a serious pest where parathion had been used regularly over a long period of time. Spread of insects and crops by man- The development of transport and intercontinental trade has brought about the introduction and establishment of new pests from other areas. Most contemporary pest outbreaks occur when pest species are accidentally or otherwise introduced into new habitats or countries where their natural enemies are absent. For example, the cassava mealybug, Phenacoccus manihotis, is believed to have originated in South America, but now a major pest in Nigeria. Similarly, when crops are introduced into a new environment, local pests may find them to be more suitable food plants and become serious pests on them. For example, cocoa is attacked by fewer pests in its native land in South America than its new land in West Africa.
Economic change- An insect reaches pest status in relation to the value/magnitude of damage done by it as assessed by man so that changes in the value of a crop will affect the importance of pest status. Economic factors that affect pest status cause price changes and include changes in demand and supply as well as changes in production costs. Damage which is not important when prices are low can become very serious when prices are high. At times the converse situation is true if an important food crop is in short supply then some damage may be tolerated. Similarly, a pest may become economically important when agricultural practices change. If a new high yielding variety is developed, minor pests that attack it become of economic importance.

INSECT PEST DAMAGE IN CROP PRODUCTION

Insect damage to crops may be direct or indirect. In direct damage, the part of the plant to be harvested is the part attacked. For example, leaves of tobacco, fruits of citrus, the tubers of yam and potato. Indirect damage occurs when the part of the plant attacked is not the part to be harvested, e.g., leaves of tomato. Farmers, as a rule, tolerate indirect damage, but are affected by the slightest direct damage. Based on the method of feeding, insects may be classified as mandibulate, i.e., those having biting and chewing mouthparts, or haustellate, those with piercing and sucking mouthparts. Damage done to crops by these two groups may differ.
Insects with biting and chewing mouthparts may damage plants as follows:
(a) Eat up crop foliage and stems, thereby reducing the amount of leaf-assimilative tissue and hinder plant growth. The leaf margin may be irregularly eaten or the lamina wholly eaten, leaving only the main veins. Adults and nymphs of the variegated grasshopper, Zonocerus variegatus, may eat up leaf margin or reduce lamina of leaves of several crops. Larvae of several families, e.g., Epiplemidae and Bombycidae and adults and larvae of certain beetles, e.g., Epilachna spp. are also defoliators. Damage to leaves may also be as a result of mining activities of larvae of several beetles, e.g., Buprestidae, Chrysomelidae, and Curculionidae. Also in the Order Diptera, Agromizidae, Ephydridae, Chironomidae are also leaf miners. In the mining activities only the chlorophyllous tissue are eaten up.
(b) Burrow into the plant vascular system and interrupt sap flow, often destroying the apical part of the plant; these are stem borers and shoot flies, such as the Yellow Headed Stem Borer, Dirphya nigrocornis in coffee, Earias biplaga in cotton, and Busseola fusca in maize.
(c) Ring-bark stems, for example, Authores leuconotus, the White Coffee Borer, in coffee.
(d) Destroy buds or growing points and cause distortion or proliferation, as with Earias biplaga in cotton shoots.
(e) Cause premature fruit fall (abortion of fruits, as with Mango Fruit Fly (Ceratitis cosyra (Wk).
(f) Attack flowers and reduce seed production as with Maize Tassel Beetle, Megalognatha rufiventris Baly.
(g) Injure or destroy seeds completely, or reduce germination due to loss of food reserves; examples are Sorghum midge, Maize weevil, Coffee Berry Borer, and Pea Pod Borer.
(h) Attack roots and cause loss of water and nutrient absorbing tissue; as in Black Maize Beetle, and various Chafer larvae (Scarabaeidae).
(i) Remove stored food from tubers and corm, and affect next season’s growth; examples are Cylas weevils (both adults and larvae) in sweet Potato Tuber Moth larvae and Yam Beetle.
Insects with piercing and sucking mouthparts may damage plants as follows:
(a) Cause loss of plant vigour due to removal of excessive quantities of sap and in extreme cases wilting may result as in the case of stunting of cotton by the whitefly, Bemisia tabaci and Aphids on different crops.
(b) Damage floral organs and reduce seed production, e.g., the pod-sucking Coreid Bug Complex in cowpea, which include Anoplocnemis curvipes, Riptortus dentipes, Acanthomya spp., and Mirperus jaculus.
(c) Cause premature fruit-fall; example is Coconut bug (Pseudotherapterus wayi).
(d) Cause premature leaf-fall, as do many diaspidid scales (armored scales-Homoptera) in cassava.
(e) Inject toxins into the plant body, causing distortion, proliferation (galls), or necrosis; examples are Lygus Bug damage on cotton leaves, and the stem necrosis on cashew by Helopeltis anacardii, and cotton boll abortion by Calidea bugs.
(f) Provide entry points for pathogenic fungi and bacteria as in the case with Cocoa Capsids or mirids, Sahlbergella singularis and Distantiella theobroma which provide entry point for the parasitic fungus Canonetria rigidiscula, causing canker-formation and die-back.

Indirect effects of insects on crops

Irrespective of whether they have mandibulate or haustellate mouthparts, insects feeding on crops may have the following indirect effects: (a) Insects render the crop more difficult to cultivate or harvest; induce abnormal growth in plants as do the larvae of Earias biplaga in cotton, where they cause the plant to develop a spreading habit that makes weeding and spraying difficult. Also, the feeding activities of insects may delay crop maturity as is the case with the bollworm (Pectinophora gossypiella) on cotton. (b) Insect infestation results in contamination and loss of quality in the crop. The quality loss may be due to a reduction in nutritional value or in marketability. For example, stored grain infested by the tropical warehouse moth, Ephestia cautella and the red flour beetle, Tribolium castaleum usually suffer nutritional quality loss. Quality loss due to changes in appearance of the crop, e.g., skeletonized or discoloured cabbages have a lower market value than the intact ones. Similarly, citrus fruits with blemished skins and hard scales have poor quality. Contamination of crop produced by insect faecal matter also reduces the marketability of the produce. (c) Many insects are involved in the transmission of pathogens by serving as vectors for such disease agents. The two methods of disease transmission by insects commonly recognized are mechanical and biological. Mechanical transmission also termed passive transmission takes place through feeding lesions or wounds created by the insects. At times the pathogen (usually fungi or bacteria) is carried on the mouthparts of the insects, or sometimes on the body of the tunneling insects. The transmission of fungal diseases in cotton by Dysdercus superstitiotus and Nozara vividula, the Green Stink bug (or Green Vegetable bug) during which spores are carried in the saliva of the bugs is also mechanical. In biological transmission an insect vector is actively involved in the dissemination of plant pathogens, which are mostly viruses. The vector is usually also an intermediate host as in the case with most aphid and whitefly hosts. Diseases transmitted in this way include Groundnut rosette, Tobacco mosaic, Cassava mosaic, and leaf curl of cotton.

FORECASTING PEST ATTACK/OUTBREAKS

A major strategy in contemporary pest management is the accurate forecasting of pest outbreaks before they actually take place so that control measures can be efficiently planned. The following studies relating to the biology and ecology of pests are necessary for successful predictions of pest outbreaks:
1. Quantitative seasonal studies conducted over several years to determine population and geographical distribution of the pest. The success of these studies will depend on appropriate sampling procedures, and the seasonal estimate should be related to weather and topographical data.
2. Life history studies on the field and in the laboratory to find out the developmental period, fecundity, food consumption, longevity, free oviposition period of the pest. The limits of survival of the insect with respect to temperature and relative humidity should also be determined. The life history of the pest in relation to the host plant phenology should be studied.
3. Field studies on the effects of weather on the pests and their natural enemies- The essential aspect of forecasting is to predict the timing of critical pest population or population reaching the economic injury level. In practice, the forecasting of pest attack is a difficult operation. Insect populations as well as climatic conditions are subject to unpredictable fluctuations, and this can upset well established prediction techniques. With the exception of armyworm and locust forecasting there are not many pest forecasting schemes in Africa.

FORECASTING METHODS

By sampling: The study of the development of pest population is commonly referred to as pest monitoring. By sampling immature stages of insect pests it is possible to arrive at an approximate estimation of numbers expected in later stages. Pest sampling can be achieved using a variety of methods. One reliable method is to inspect soil samples for the presence of insect eggs. In the United Kingdom, taking soil cores for the eggs of carrot fly (Psila rosae) and the cabbage rootfly (Delia brassicae) is quite helpful in determining whether or not to apply insecticides. With many lepidopterous pests it may be possible to determine the best spraying date by detecting the eggs or the instar larvae on the crops. This is the method currently employed with Pea moth on pea crops, and bollworms (Diparopsis and Heliothis) on cotton. Pests having alternative hosts may be sampled while on the other host, so that an estimate of their probable pest density on the crop can be made. For instance, the population density of Bean Aphis is predicted based on the number of eggs on the alternative winter hosts.
By prediction: This depends on critical observations of weather conditions. Environmental factors, such as temperature and rainfall have been used to prognosticate the incidence of pest attack. Other climatic variables commonly monitored for prognosis include wind velocity, relative humidity, soil temperature, solar radiation, etc. An empirical method using mean temperature of 2 months has been developed to predict the date of emergence of the adult of rice stem borer, Chilo simplex in Japan. It has also been used in the USA for predicting the outbreaks of the European corn borer, Ostrinia nubilalis. Rainfall has also been to forecast the likelihood of pest attack. In Tanzania, rainfall index of previous years has been used to predict the occurrence of the red locust, Nomadacris septemfasciata. Also, in the Sudan the amount of pre-planting rains has allowed damage by Empoasca spp. (Cotton jassids) to cotton to be predicted. The likelihood of pest outbreaks can be inferred from a simple diagram of the climate or weather characteristics of an area. If the monthly mean temperatures are plotted against relative humidity, a polygonal diagram called a climatograph is obtained. If temperature and rainfall parameters are used the diagram is a hythergraph. Climatographs can be used to identify the area of prevalence of a particular pest and to determine its status. Prediction can also be based on observations of climatic areas since the geographical distribution of many pests is dictated by some limiting climatic factor. The major factors controlling a pest population build-up may be biotic, climatic as well as topographical, though a combination of temperature and humidity (or rainfall) is probably the most important. The distribution of a pest can be divided into three zones: 1. Zone of natural abundance-where the insect is always present in detectable numbers, and is a regular pest. 2. Zone of occasional abundance- where the population is kept low by climatic conditions and only sometimes can it rise to pest proportions. 3. Zone of possible abundance- where the climate at times permits an outbreak to occur, but not often. The insects often migrate from zones 1 and 2 into this zone, where it may be a pest for a while before being finally controlled by climate.
The knowledge of both temperature and relative humidity requirements for the different instars of an insect species enables the likelihood of an outbreak of that pest to be predicted when the climatic conditions of that area are known.

FACTORS AFFECTING THE CHOICE OF PEST CONTROL MEASURES
The different methods of insect pest control available to the crop protectionist are manifold. However, the choice of which control measure should be applied is based on several considerations. 1. Degree of risk- Some crops in certain areas are grown at a high risk because of the presence in large numbers of pests affecting such crops. Under such situations, prophylactic/preventive measures also called insurance measures may be justified. 2. Nature of the pest complex- Usually many different insects will be interacting in the form of a complex. The control strategy should be aimed at the major or key pests. When these are controlled other minor pests are also put under control. 3. Nature of the crop and system of agricultural practice- The crop type, spacing, height and whether it is grown in pure or mixed stands will determine the feasibility of any specific control measure. 4. Economic factors- Cost of chemicals and specialized equipment and the value of the crop being cultivated are all important factors to be considered before embarking on control measures. Control measures initiated should be justified in terms of monetary considerations and must not be deleterious to the environment. 5. Ecological factors- The extent and type of natural control and water availability all have a role to play in deciding on control measures in any locality. 6. Consumer pressure- Consumer demands also exert an influence on insect control. In the agriculturally advanced countries and in some developing countries too certain fruits may be rejected on account of their poor quality. This frequently compels the farmers to use chemical control whether this represents the most ecologically sound method or not.
There are some other factors which are prerequisite to the conduct of control programme, and these are: 1. Insect classification and life history- It is important that the pest be correctly identified and their general biology be known. In biological control, misidentification of a pest may result in the search for parasites and predators in countries other than the native home of the pest, thus leading to a wastage of funds. It also provides a basis for selecting the appropriate insecticide should immediate control be imperative. Life history data is important in timing control, and one of the most familiar principles of insect pest control is that the weakest link or the most vulnerable part of the life cycle must be identified with a view to gaining effective control. 2. Establishment of Economic Injury level (EIL) and Economic threshold (ET) for the various pests. 3. Pest Sampling, Survey and Detection- Competent personnel must detect low level infestation of pest species before they become damaging. Sound control programme should be based on accurate knowledge of the distribution and abundance of the insect pest. This should be gained through survey and appropriate sampling technique. 4. An appreciation of the Natural Control- This should precede the conduct of artificial control. The presence of parasites and predators often permits modification of standard chemical control programmes and reduction of undesirable side effects resulting from the use of insecticides.
It should be emphasized that community projects are required in coping with some pest control problems. For example, insect vectors of virus diseases and those that readily migrate from one area to another can be controlled through communal efforts. In order to minimize crop yield losses, pest control methods have been devised and constantly improved to reduce the numbers of pests and decrease the losses they cause. These methods constitute pest control and are broadly based on the principles of (i) prevention, (ii) control, and (iii) eradication. Preventive measures are undertaken where pest problems can be anticipated and steps taken to prevent damage or loss. Control measures are applied once the effects of the pest incidence become noticeable, and if timely, can prevent the occurrence of economic losses. The application of the principle of eradication can only be relevant and satisfactory where pests are new introductions and their distribution is restricted. Otherwise the cost of eliminating an established pest from an invaded area can be prohibitive and complete eradication can hardly be guaranteed. The application of the basic principles of control can be translated into specific control methods, of which the following are the most important: 1. LEGISLATIVE AND REGULATORY CONTROL METHODS 2. PHYSICAL AND MECHANICAL CONTROL METHODS 3. CULTURAL CONTROL 4. VARIETAL CONTROL 5. BIOLOGICAL CONTROL 6. CHEMICAL CONTROL 7. INTEGRATED PEST MANAGEMENT

LEGISLATIVE AND REGULATORY CONTROL METHODS
Most countries have strict quarantine laws (or legislations) which are aimed at preventing the introduction of exotic (foreign) pests. In most tropical African countries, this function is performed by the national plant quarantine services empowered by law to control the movements of agricultural produce, and to certify, quarantine or destroy such produce in case of danger of the introduction of pests. The Plants (Control of Importation) Regulations, 1964, based on the Agriculture (Control of Importation) Act (1959) in Nigeria is an example of such control by legislation. The activities of such services are coordinated on a continental scale by the Inter-African Phytosanitary Commission with its Headquarters in the Cameroons. This commission has drawn up a series of important recommendations on checking the spread of new pests and diseases in Africa. It will however have little impact on the pest situation in the continent unless it has imaginative programmes based on scientific research and the support of member African nations. Pest-free plants can usually be imported with the proviso that they are accompanied by the necessary documents from the country of export. However, certain plants are completely prohibited because of the extreme likelihood of their carrying noxious pests. For example, the importation of pome fruit into many African countries from Asia and America is rigorously controlled because of the danger of importation of San Jose scale (Quadraspidiotus perniciosus), a potentially dangerous orchard pest known to attack all types of fruit crops including ornamentals.

PHYSICAL AND MECHANICAL CONTROL METHODS
These refer to methods of mechanical or physical removal of insect pests or their destruction through manipulation of physical factors. Physical and mechanical methods of pest control differ from cultural techniques in that they are applied directly to the pest. For example, tomato hornworms may be picked directly from tomatoes and flies can be swatted. Physical or mechanical control is based on the ecology of the insect pest and a realization of the fact that in the biology of species there are threshold levels of tolerance, such as extremes of temperature, humidity, sound, and responses to various portions of the electromagnetic spectrum. Physical control may be divided into three categories: 1. Mechanical removal 2. Use of electromagnetic energy 3. Use of physical factors
Mechanical removal
Handpicking- In this case adults and larvae of insect pests are removed by hand. The method is simple and cheap but can hardly be practiced where crops are grown on a large scale. Handpicking of pests is still a profitable method for the removal of Papilio caterpillars from young citrus trees.
Use of mechanical drags- This involves the use of chains, brushes or tarred paper which are dragged over the crops so that pests are crushed. This has been used against armyworms (Spodoptera spp.), but this practice is now generally obsolete.
Use of barriers and adhesives- These devices prevent the migration of pests within the plant and have been particularly used for insect control on citrus trees. The trees are banded with a suitable barrier such as creosote, corrugated paper bands, lime, etc. and adhesives, such as hydrogenated castor oil, natural gums, resins, vegetable wax and these prevent insects from migrating from the ground into the tree to damage foliage or fruit. 15 cm-high aluminum strips or trenches are sufficient to stop bugs or crickets from migrating from field to field. Filling the trench with kerosene often kills the bugs.
Use of traps- Insect trapping devices have the advantage of being clean, easy to use and effective under a variety of conditions. They may be baited with a variety of attractants including food and oviposition lures, virgin females and pheromonal attractants. Nocturnal flying insects are attracted to the ultra-violet (UV) portion of the electromagnetic spectrum and collect around UV or ‘black light’ made from fluorescent lamps of mercury vapour discharge. The insects attracted include Lepidoptera, Hemiptera, Hymenoptera, Orthoptera, Neuroptera, Trichoptera, Diptera, Ephemeroptera, and Coleoptera. UV traps are often used in conjuction with electrocuting wires or fans, the blades of which pulverize the insects. Other successful lures include traps baited with pheromones. Daytime fliers are often attracted by food baits, often used in conjunction with a poison. Trimedlure (Pherocon MFF®) is a synthetic food bait for the Mediterranean fruit fly, Ceratitis capitata, and sprays of yeast, protein and malathion have also been used successfully against this fly. Ammonia yielding chemicals mimic decaying organic matter, thereby attracting ovipositing flies. Corn earworm adults, Heliothis armigera, will oviposit on twine impregnated with corn silk juice. Despite the variety of attractants, insect traps alone are rarely sufficient to achieve adequate control. As a rule, they are used with other methods of management.
Use of electromagnetic energy Another physical method of control of insect pests relates to the use of electromagnetic energy. The absorption of radio-frequency energy by biological material results in the heating of the tissues. Control of insect pests by such heating is only practicable in enclosed spaces of small or moderate size (food stores, warehouses, timber stores). The nature of absorption of radio-frequency energy by materials in a high frequency electrical field is such that for certain combinations of hosts and insect pests, their dielectric properties are favourable for differential absorption of energy, hence insects can be killed without damaging the host material. Timber beetles in wood blocks have been controlled in this way. Ionizing radiations, such as γ and X-rays are sterilizing at lower dosages, but lethal at higher. The use of these radiations in controlling pests of stored products, particularly in grain, is being quite extensively studied in many countries.
Use of physical factors
These factors include temperature, humidity, sound, asphyxiation, etc.
Temperature- Insects are poikilothermic and therefore depend on environmental temperature for the maintenance of body metabolic activities. Prolonged high and low temperatures have been used against pests of stored products, such as grain. Most pests are readily killed when exposed to 3-4 hours at 52-55ºC in a high-frequency electrostatic field. The sun-drying of grains, which is widely practised to achieve a reduction in grain moisture level, usually results in lower pest infestation, especially in stored products. Stored grains may also be heated artificially to kill pests in an enclosure. In the Sudan, the heating of cotton seeds to kill the larvae of the pink bollworm, Pectinophora gossypiella, is an effective method. Plant bulbs infested with mites and nematodes can be successfully controlled by hot-water treatment (dipping). Most pests are killed when exposed for one or two days at -22ºC. However, care must be exercised where crop preservation is essential since seed viability is likely to be affected by prolonged extremely low temperatures.
Humidity- Terrestrial insects must maintain their body water content within reasonably narrow limits in order to survive. Therefore, humidity may also be manipulated for insect control, especially in stored products. Insect damage caused by stored-grain pests increases to a certain extent with the rise in moisture content of the grain. This phenomenon explains the rationale for maintaining the moisture content of grain below 12% during storage. Insect mortality under a CO2-rich atmosphere is largely due to the excessive water loss from insect bodies caused by prolonged opening of their spiracles. It has been shown that many storage pests cannot reproduce normally at ambient relative humidity of 50% or less. The manipulation of relative humidity in limiting insect population in field crops is almost impracticable and has not been widely practised.
Sound- Most insects produce sound and most likely all are sensitive to sound vibrations. Many species respond to artificial sound and many more have communication system based on sound production. The use of sound for insect control is still in the experimental stage. It is possible that sound may be marketed for dry application in the future. Experiments have been conducted which prove that ultra-sonic waves can, in some cases, bring about the control of insect pests. Three basic methods of sound application for control of insect pests have been suggested. They are: (1) the use of very high intensity sound for physical destruction; (2) the use of loud noises to repel the pests, and (3) the use of recorded sound produced by insects or imitations of this to influence behaviour. Insects that produce or respond to vibrations include the grasshoppers and crickets, cockroaches, flies and mosquitoes, moths, butterflies, termites, ants and beetles.
Asphyxiation- Control of insects by depriving them of O2 is used against storage pests. This is the principle behind the hermetic storage of grains. The small quantity of O2 enclosed within the container is quickly used up and the subsequent accumulation of CO2 results in the death of both arthropods and microbial pests.

CULTURAL CONTROL
Cultural methods include regular farm operations designed to destroy pests or prevent them from causing damage of economic proportions. These operations include proper crop rotation, changes in the time of planting, tillage practices, time of harvesting, destruction of crop residues, amendment of soil fertility, adjustment of depth of planting and spacings, use of irrigation, mixed cropping or intercropping.
Crop rotation- This method can be quite effective against insects having a restricted host range, i.e.,monophagous and oligophagous species and those possessing limited migratory capability. For effective pest control crop rotation must separate crops in time and space. The most common type of rotation involves alternating legume crops with cereals. This has been found to reduce damage by the stem borers and sweet potato weevils.
Changes in the time of planting- By sowing early or sometimes late, it is possible to avoid the egg-laying period of an insect pest so that the vulnerable stage in plant growth may have passed by the time the insect numbers reach damaging proportions. Groundnuts grown in northern Nigeria suffers considerable losses from rosette virus disease transmitted by Aphis craccivora. However, it has been observed that groundnuts sown early in the wet season already mature and become less attractive at the time of aphid invasion, and suffers less damage than those planted late. Early sowing is regularly practised in many parts of Africa against the sorghum midge, Contarinia sorghicola, and has been suggested against the cassava mealybug, Phenacoccus manihotis.
Tillage practices- Ploughing and harrowing some days before planting may bring lepidopterous larvae and pupae including beetle larvae up to the surface to be killed by predators or unfavourable weather conditions. Also, most soil pests are reduced in number by a period of fallow. When a fallow is bare the chances of insect pests using weeds and other volunteer plants as hosts are also reduced.
Time of harvesting- Prompt harvesting of maize and cowpea may prevent these crops from becoming infested by maize weevil (Sitophilus zeamais) and cowpea beetle (Callosobruchus maculatus), respectively.
Destruction of crop residues, alternative hosts, volunteer plants and weeds- The foregoing may carry pests over from one season to another. The recommended method of destruction is by burning and other methods may not be as much effective. For instance, stubble serves as a reservoir for infestation by most cereal stem borers so that its destruction minimizes re-infestation. Similarly, destruction of the weeds, Euphorbia hirta and Boerhavia diffusa, has been recommended for reducing the incidence of groundnut rosette disease. The foregoing weeds harbour the vector of the disease, Aphis craccivora, during the dry season. Experiments conducted at the International Institute of Tropical Agriculture (IITA) have shown that weeded cowpea and soybean plots suffer less damage from Hemipterous bugs and the cowpea moth, Cydia ptychora, than unweeded plots.
Amendment of soil fertility and pH- High soil fertility helps plants to overcome insect damage, but may also make some crops to be more attractive to certain insects. Stalk borer damage in rice has been observed to occur in acidic soil (pH 4.8) than in neutral to slightly alkaline soil of pH 7.8.
Adjustment of planting depth and spacing- Planting depth and spacing may influence the degree of damage done to a crop. For example, sweet potato tubers planted deep suffer less damage from boring beetles than those planted shallow. Close spacing has been demonstrated to reduce the incidence of Aphis craccivora in groundnuts.
Irrigation practices- Management of water can favour or hinder the development of insects. Proper manipulation of water level in irrigated rice has been shown to reduce the level of damage by borers.
Mixed cropping and intercropping- These traditional cropping principles adopted by the African farmers have been shown to minimize damage and stabilize crop yields. For example, interplanting cowpea with maize reduces damage by the pod sucking bug, Anoplocnemis curvipes, and the flower beetle, Mylabris spp., in cowpea.

VARIETAL CONTROL
This involves the use of host-plant resistance in insect pest control. It has been established that some varieties of plants are more resistant to pest attack than others. Crossing resistant with high-yielding varieties can produce a crop that is both pest-resistant and high-yielding. Resistance may be provided by one gene (monogenic or vertical resistance) or by many genes (polygenic or horizontal resistance). Monogenic resistance confers good protection against host-specific pests or strains and is fairly easy to select for in crosses. The mechanism for breeding for resistance through one gene and high yield through another gene is:
High-yielding, susceptible variety × wild strain: resistant, not high-yielding (gene for resistance)

F1- All resistant because wild gene is dominant

(F1 selfed)

F2- Segregation and assortment of gametes according to Mendel’s 1st and 2nd laws ( Resistant form backcrossed with original High yielder. Backcrossing continued for 6-7 generations)

Pure-breeding, high-yielding, and resistant variety

Resistant varieties have been developed in this way for tomatoes, maize and rice. Polygenic resistance generally provides protection against a wider range of pests, but, because of its polygenic nature gives less protection against specific pests. Polygenetically resistant forms are much more difficult to select for because many genes are involved. This type of resistance has less chance of being circumvented by new pest strains.
Resistance may be described as real or apparent. Real resistance varies from complete immunity, to high resistance, to low resistance, to susceptibility, to high susceptibility. Apparent resistance includes early-maturing crop varieties, which evade peak pest population densities, or varieties with low water content.

MECHANISM OF RESISTANCE
Resistance of plants to insect attack is manifested in several different ways. Painter (1951) classified the mechanism of plant resistance into antibiosis, tolerance, and preference or non-preference.
Antibiosis in plants causes adverse effects on the biology of the insects. These adverse effects may include: (i) death of 1st instar larvae, (ii) a reduced reproduction by females reared or feeding on the resistant plant (low fecundity), (iii) smaller size and lower body weights of adults reared on resistant plants, (iv) abnormal life span, such as longer nymphal and adult life compared with insects reared on susceptible hosts. A longer nymphal or larval period exposes the young insects to enemies for a considerable length of time and may lead to fewer generations per year. Short adult life limits the time available for females to mate and lay eggs, (v) smaller food reserves are accumulated which would affect the ability of the insect to withstand unfavourable conditions, and (vi) abnormality in growth and structure. Antibiosis may be due to the presence of toxins, growth and reproduction deterrents, absence of some nutritional requirements, such as specific amino acids, or imbalance of nutrients, e.g. sugar-protein ratio and sugar-fat ratio. Some plants actually produce ecdysones that act as insect anti-hormones, interfering with juvenile hormone production and so preventing successful metamorphosis. For example, the bedding plant, Ageratum haustoneanum, contains ecdysones, called Precocene I and II, which induce precocious metamorphosis, shortened life-cycle, reduced feeding and sterile females in the milkweed bug, Oncopeltus fasciatus, and cotton stainer, Dysdercus suturelles, and induce diapause in the Colorado beetle, Leptinotarsa decemlineata.
Tolerance: Tolerance is the ability of the plant either to repair an injury very well or tolerate the injury. This is characteristic of healthy vigorous plants that can heal quickly and show compensatory growth after insect attack. As a matter of fact, many plants bear more foliage than they actually need and can sustain a certain amount of defoliation without yield reduction. However, the replacement, regrowth and repair of damaged plant tissues depend on the growth stage of the crop during the time of insect pest attack. Plant tolerance is mostly displayed towards insects with sucking mouthparts, such as aphids, leafhoppers and true bugs.
Preference or non-preference- Some plants possess features that discourage pests from feeding, ovipositing or sheltering on them. Sticky glandular hairs on potatoes and tomatoes discourage feeding by aphids, which stick to them. Mustard oils in the Cruciferae family generally discourage insect feeding. However, certain cabbage caterpillars use mustard oils as a feeding signal (cue) and will only feed on leaves containing, or painted with, these oils.

ADVANTAGES OF HOST-PLANT RESISTANCE 1. It is environmentally safe and is often helpful when used in conjunction with other methods of control. 2. Adoption of the system is easy and does not require the user to be educated. 3. Varietal resistance is usually maintained for long periods and the effect is cumulative because fecundity is reduced in every successive pest generation.

DISADVANTAGES OF HOST-PLANT RESISTANCE 1. The time required for development is usually too long (between 10 and 15 years) and damage must have been done. 2. Resistant varieties may have undesirable agronomic characteristics or qualities. 3. Development of biotypes (pest strains) that have the ability to circumvent the resistance.

BIOLOGICAL CONTROL
The use of natural enemies to check insect pest population occurs in two (2) ways: 1. Natural biological control- This is the suppression of insect pests by their natural enemies without the interference of man. 2. Classical biological control- This is the deliberate introduction of predators, parasites and pathogens to reduce pest population to a level at which it is no longer considered a pest. Of recent development is the use of sterilization and sex attractants in the control of insects, both of which can be considered as specialized methods of biological control.
Several types of organisms that have been used in biological control include predators, parasites and pathogens.
Predators- These organisms destroy pests by preying and devouring them. Among the vertebrates, birds are obviously important as insect predators although their effectiveness has not been studied in Africa. However, chickens are employed in certain parts of East Africa to control cotton strainers. Invertebrate predators include various Coccinelidae (Lady bird beetles), both as larvae and adults, and the larvae of many species of Syrphidae. These collectively prey mostly on aphids and coccids. Certain predatory mites are used to control phytophagous mites. For example, some measure of control has been achieved using Phytoselius mites as predators on Tetranychus spp. in some countries.
Parasites- Parasites reproduce at the expense of other insects by planting their eggs in or on any developmental stage of the host, from egg to adult. Some species lay their eggs on leaves, which when consumed by the pest insect, become active inside its body. The most important group of parasites belongs to Hymenoptera (Ichneumonidae, Braconidae and Chalcidoidea), which attack the eggs, larvae, pupae and sometimes adults of many other groups of insects. All members of the family Ichneumonidae are parasites on insects or spiders, their favourite hosts being the larvae of Lepidoptera. For example, Glypta rufiscutellaris, is a parasite of the oriental peach moth. In addition, a number of ichneumon flies parasitize the larvae of Coleoptera, Hymenoptera and Diptera, and a few other insects. Certain Chalcidoidea (especially Trichogrammatidae) are solely egg parasites.
Pathogens- The various microorganisms engaged in biological control include fungi, bacteria, and viruses. Fungi are not so important at present although some 300 antibiotics do show promise as pesticides; these act directly as killing agents or inhibitors of growth or reproduction. Among the fungal formulations, beauverin obtained from Beauveria bassiana is used to control Colorado beetle. Another formulation prepared from the fungus of the genus Ashersonia has been tested against the greenhouse whitefly on vegetable crops and has shown good results. The use of bacteria in biological control is promising, and two bacterial formulations are now available on the market for the control of harmful Lepidoptera. The first, Entobacterin, is used against the cabbage and small white butterflies while the second, Dendrobacillin, is effective against bollworms and armyworms. It has been reported that a bacterial agent, Bacillus thuringensis, was responsible for a high mortality of the Coffee Berry Moth (Prophantis smaragdina=Thliptocera octoguttalis) on coffee in Tanzania. Viruses are the most commonly found pathogens attacking insects and have been most effectively used against Lepidoptera (armyworms in E. Africa), some Hymenoptera (sawflies) and a few beetles (Melolontha and Oryctes). There is some evidence from other countries that viruses may be effective against some red spider mites.
Sterilization. This usually refers to the sterilization of males by X-rays or ɣ-rays and is called the Sterile Male Technique- control of a pest by this technique is termed autocide. Sterilization can also be done by exposure to various chemicals and this practice is called chemosterilization. The rationale behind this method is that male sterilization is effective in species where females only mate once and are unable to distinguish or discriminate sterilized males. A classical example was in 1940 on the Island of Curacao against screw-worm (Callitroga) on goats- the male flies were sterilized by exposure to ɣ-rays, and dropped from planes at a rate of 400/square mile/week. The whole pest population was eradicated in twelve months. This method of control could quite possibly be effective against Oryctes spp. attacking coconuts and work is in progress on the feasibility of autocidal control as a method for controlling tsetse flies.

CHEMICAL CONTROL
Chemical control is the reduction of pest population or prevention of pest injury by the use of toxic materials to poison, attract, or repel them from specified areas. A chemical that exercises lethal action against insects are called Insecticides. Chemical attractants are substances whose odour and taste attract insects and animals. In contrast, repellents are substances whose odour and taste repel insects. Toxic chemicals are still the main defense against pest attack and they are likely to remain so for many years to come. The appeal of chemical pesticides for use in pest control can be attributed to the following reasons: (i) They are easy to apply, (ii) They are highly effective, their action on pests being almost immediate upon application, (iii) Their use is economical provided they are applied timely, and (iv) They can readily be employed as needed if there is no financial constraint. Alternative means of control having all the foregoing attributes can seldom be found. Insecticides remain the single most important control technique for insect vectors of animal and plant diseases, particularly plant viruses. Without insecticides, it is estimated that half as much land again would be needed to produce the same amount of food as we currently harvest. In the US alone losses of over $2 billion would occur as a result of pest damage is spraying ceased. Yield increases due to insecticide use are often dramatic. In Ghana, cocoa production has gone up 300%, and cotton production throughout the world has doubled.

INSECTICIDE STRUCTURE AND CLASSIFICATION
Insecticides can be referred to by (i) their common names, (ii) the registered trade names, which may differ according to different manufacturers, (iii) the scientific names and (iv) the structural formula.

This is called (i) carbofuran, (ii) Furadan, (iii) 2, 3-dihydro-2, 2-dimethyl-7-benzafuranyl methylcarbamate.

Most contemporary insecticides work as contact poisons, whereas older arsenic derivatives, such as Paris Green, lead arsenate and calcium arsenate were stomach poisons. Insecticides are classified principally according to their chemical composition and mode of action. Based on chemical composition, three main groups of insecticides are distinguished: 1. Inorganic compounds (compounds of lead, mercury, fluorine, barium, sulphur, copper, chlorates and borates). 2. Insecticides of plant, bacterial, and fungal origin (pyrethrins, bacterial and fungal preparations, antibiotics). 3. Organic insecticides- This is the most extensive group including insecticides with a high physiological activity: organochlorine or chlorinated hydrocarbons, organophosphorus compounds, derivatives of carbamic, thio- and dithiocarbamic acids, nitroderivatives of phenols, mineral oils, formamidines.
According to their mode of action, insecticides are divided into stomach, contact formulations and fumigants. 1. Stomach insecticides are poisonous materials which must necessarily be taken in through the mouth of the insect before they can act. They are usually applied to the part of the crop that serves as food for the pest and are ingested in the normal process of eating. 2. Contact insecticides refer to toxic materials that are capable of producing symptoms of poisoning in an organism after it has made a physical contact with such materials. These symptoms may be instantaneous, when the materials are called ‘knock-down’ poisons or they may take some hours to manifest themselves, as is the case with some of the newer and effective contact insecticides. 3. Fumigants are volatile chemical substances that give off vapours intended to destroy insects, pathogens and other pests. They also include insectoacaricides having a fumigant action that destroys insects and mites on entering through the respiratory tract.
The above classification is provisional to a certain extent because many insecticides exhibit more than one type of action. Examples are hexachlorocyclohexane (HCH) and heptachlor.

ORGANOCHLORINE COMPOUNDS
The insecticides comprising this group vary in their chemical structure, but due to several common characteristics (high insecticidal activity, chemical and biological persistence) they are classified into one group. All organochlorine insecticides are poorly soluble in water and well soluble in organic solvents, including fats. Many of them are quite volatile, thermally and chemically stable, and can withstand the action of various environmental factors, such as temperature, solar radiation, moisture, etc. This underlies their prolonged protective action against pests, but at the same time creates a hazard of contaminating the environment and agricultural products. Representatives of this group may persist in the soil up to 15 years, remaining for a long time in the top soil and slowly migrating along the soil profile. Organochlorine incorporated into the soil in large doses may inhibit nitrification processes for a period of 1-8 weeks and briefly suppress its general microbiological activity. However, they do not substantially affect soil properties. Organochlorine compounds contain C, H, O, and a Cl group.

The most famous organochlorine is probably DDT (Dichloro-diphenyl-trichloroethane), which was synthesized in 1939 and later banned in the United States in 1973. Organochlorines may be subdivided into several groups- the three most commonly used groups are represented by DDT, BHC, and aldrin (cyclodienes). They all alter Na and K concentrations in nerves (lipoid equilibrium of nerve cell membranes), preventing the transmission of nerve impulses, thereby causing the muscles to twitch spontaneously. Most organochlorine compounds are toxic to the animal kingdom in general, including birds, mammals and fish as well as insects. Important diphenyl aliphatics include DDT, TDE (or DDD), methoxychlor, dicofol, chlorobenzilate and Perthane. The cyclodienes include chlordane, aldrin, dieldrin, heptachlor, endrin, mirex, endosulfan and chlordecone (Kepone ®, 1958), which severely poisoned some industrial workers in 1976. A good number of cyclodienes are soil insecticides, dieldrin and chlordane being effective against termites for more than 25 years.

ORGANOPHOSPHORUS COMPOUNDS.
Organophosphorus compounds are amongst the most important groups of modern pesticides. They were discovered and developed during the Second World War by a German research team responsible for developing nerve gases; they are amongst the most toxic chemicals known to man. The widespread use of these compounds in agriculture is due to their high insecticidal and acaricidal activity, broad spectrum and rapidity of action (acute toxicity) on pests, low stability in biological media, decomposition with the formation of non-toxic products to humans and animals, relatively rapid metabolism in animals, low rates of use per unit of treated area, rapid decomposition in soil and water, and their moderate toxicity to fish. Among the negative features of organophosphates are their high toxicity to humans and animals, and the relatively rapid appearance of resistant pest populations after prolonged use.
Modern organophosphorous insecticides and acaricides are compounds of pentavalent phosphorous of the general formula:

Where R1 and R2 are alkoxyl (CH3O), alkyl (CH3), or aryl (C6H5) radicals combined with alkoxyl ones, or dimethylamine groups and X is a residue of a weak acid. The bond of X with the phosphorous has an anhydride nature, and the compound itself- the properties of a phosphorylating agent. Any compound with a similar structure has the ability to phosphorylate very important substrate in an organism. It has been established that the enzyme contained in nerve tissues- acetylcholinesterase, which plays an important role in the transmission of nerve impulses, is such a substrate. The fundamental structural elements of the insect nervous system are the nerve cells called neurons, whose designation is to receive, interpret and transmit information in the form of electrical signals (impulses). The short and numerous branches of a neuron (dendrites) gather information, which is transmitted along the axon, a thin tube. When the neuron is at rest, the axon maintains a chemical balance in the neuron by keeping more potassium ions inside the cell and more sodium ions outside the cell. When an electrical impulse is transmitted, the membrane surrounding the axon is stimulated at the Node of Ranvier to permit the different ions to leak through the axon membrane. Potassium and sodium ions change places to create an electrical signal which travels along the axon membrane. The space between two neurons is called a synapse or synaptic cleft. The cleft is filled with a gel-like substance that has a high electrical capacitance, as a result of which electrical signals cannot pass through it. When an electrical impulse reaches the synapse, structures known as synaptic vesicles discharge neurotransmitters into the synapse to lower its electrical capacitance, so that an electrical impulse can pass to the other neuron or a muscular fibre. Acetylcholine and noradrenalin are the two most widespread neurotransmitters. Synapses where a nerve impulse is transmitted with the aid of acetylcholine are cholinergic, and those where noradrenalin is the mediator are called adrenergic. In insects as in some animals, the impulse probably crosses the synaptic cleft by the quick release of the chemical acetylcholine. This transmitter substance is quickly destroyed by the enzyme acetylcholinesterase, and synthesized again through the acetylation of choline. Acetylcholinesterase plays an important role in this cycle, because the inhibition of its activity leads to the accumulation of free acetylcholine in the synaptic cleft, as a result of which the normal transmission of nerve impulses is disrupted.
The active site of acetylcholinesterase consists of two sections: (i) the anionic section (A) containing an ionized carboxyl of asparagine and glutamic acids, and (ii) the esterase section containing a hydroxyl of serine (E).

Acetylcholine decomposes in three stages: 1. Sorption of acetylcholine on the anionic section of the enzyme

2. Acetylation of the enzyme with the formation of choline

3. Desorption of choline and hydrolysis of the acetylated enzyme. Here, the enzyme is regenerated and acetic acid is evolved. Regenerated acetic acid reacts with choline to form acetylcholine.

All organophosphorus insecticides react with acetylcholinesterase when they enter an organism.

Phosphorylation of the enzyme occurs with the detachment of the acid residue X. The nature of X does not affect the structure of the inhibited enzyme, but acts strongly on the process of its inhibition. Unlike the acetylated enzyme, phosphori lized cholinesterase hydrolyzes very slowly (in several hours and even days). Consequently, the acetylcholinesterase is withdrawn from the sphere of action for a prolonged period, and acetylcholine accumulates in the synaptic cleft, thereby causing a sharp disruption of the functions of all organs having cholinergic innervation, resulting in poisoning of the organism.
Insects and mites develop specific resistance to organophosphates when such chemicals are used for many years. Specific resistance of insects develops in species which produce several generations in a season. Resistance is known to have appeared in aphids, houseflies, cockroaches, mosquitoes, and some other insect species. The main way of dealing with acquired resistance and preventing it is to alternate the use of insecticides and acaricides with different mechanisms of action. Examples of the commonly used organophosphates in the tropics include Demeton-S-methyl (Metasystox 55), Diazinon (Basudin), Fenthion (Lebaycid), Malathion, Melvinphos (Phosdrin), Monocrotophos (Nuvacron, Azodrin), Parathion (Bladan), Phosphamidon (Dimecron, Dicron), Pirimiphos-ethyl and-methyl, Tetrachlovinphos (Gardona), Trichlorphon (Dipterex).

CARBAMIC ACID DERIVATIVES
The compounds comprising this group include highly active insecticides, fungicides and herbicides. However, only the esters N-alkyl carbamic acids have insecticidal properties. Most insecticides from this group do not exhibit any systemic activity, although they can penetrate the leaves and root system of plants. No adverse effects on plant growth and development as well as on crop yield have been recorded through the use of carbamate insecticides. However, high doses of carbamates may inhibit the process of cell division, thereby causing the death of root hairs. Carbamic acid derivatives are highly active contact and stomach insecticides with a broad spectrum of action. The mechanism of these substances on insects and mammals consists in the inhibition of the enzyme, acetylcholinesterase in the nerve tissues. The commonly used carbamates in the tropics are carbaryl (Carbaryl 85), carbofuran (Furadan), Pirimicarb, Methomyl (Lannate), Propoxur (Baygon, Blattarex, Unden), Promecarb (Carbamult), and methiocarb (Mesurol, Draza).
FORMAMIDINES
The insecticides comprising this group are comparatively new and effective for organophosphorus- and carbamate-resistant pests. They inhibit monoamine oxidase, allowing biogenic amine to accumulate, which then act chemical transmitters at synapses, causing continuous transmission. Important members of this group include chlordimeform (Galecron® or Fundal®), which are valued as ovicides and larvicides. Progress in the development of this group has been hampered by suspected carcinogenic properties.

NATURAL ORGANIC COMPOUNDS OR BOTANICAL INSECTICIDES
Many plants contain natural insect toxins, and the so-called botanical insecticides are derived from them, or are their synthesized analogues. These compounds have the advantage of being able to break down rapidly in biological media and are therefore environmentally safe. Notable among the naturally occurring insecticides are pyrethrins, nicotine and rotenone.
Pyrethrins are extracted from the flower heads of two types of chrysanthemums (Pyrethrin cineraefolium). The varieties grown in the Kenyan highlands yield the highest percentage of active ingredients. Pyrethrins cause insect paralysis, and, because of their low mammalian toxicity, are used as ‘knock-down’ agents in aerosols. They are unstable to sunlight and are rapidly hydrolyzed by alkalis with a loss of insecticidal of properties. The loss of insecticidal properties may be delayed by the addition of synergists, such as piperonyl butoxide, sesamin, etc. However, recently-produced synthetic analogues, called pyrethroids, are more photostable. Examples are permethrin (Ambush) and cypermethrin (Cymbush), which are active against a wide range of insect pests, particularly Lepidoptera on cotton, fruits and vegetables.
Nicotine is a non-persistent, non-systemic, contact insecticide with ovicidal properties. It can be used as a fumigant in closed spaces, and the water-insoluble salts (nicotine sulphate) have been used as stomach insecticides. Nicotine is prepared from tobacco (Nicotiana tabacum) by steam distillation or solvent extraction. It is effective against aphids, capsids, leaf miners and thrips on a wide range of horticultural crops.
Rotenone (Derris) is the name given to the main insecticidal compound of certain Derris spp. and Longocarpus spp., known for many years to be effective as a fish poison and an insecticide. It is a selective non-systemic insecticide with some acaricidal properties; non-phytotoxic, and readily oxidized in sunlight and by alkali to less insecticidal products. It is usually formulated as dusts of the ground root with a non-alkali carrier; dusts may be stabilized by the addition of a small quantity of a strong acid, such as phosphoric acid. It is effective against aphids, caterpillars, thrips, some beetles, and red spider mites.

FUNGICIDES
Fungicides are chemicals utilized in the control of plant diseases caused mainly by fungal pathogens. Basically, there are only three approaches to the fungicidal control of plant diseases: (a) the protection of the healthy host (prophylaxis); (b) the cure or therapy of the diseased host (chemotherapy), and (c) the destruction of spores or pathogen propagules on the host surface, e.g. seeds (disinfestation). Thus, there are three categories of fungicides used in plant disease control, namely, the protectant, the eradicant, and the disinfectant. There may be large differences between the three types of fungicides based on molecular structure and constitution. There are also fundamental differences in time and place of application. Protectant fungicides are used to prevent plant infections. To be efficient, they must be present in a high enough concentration of the active principle when the inoculum arrives. They should also possess the properties of stability, initial retention and adherence such that the entire plant surface is adequately protected against air-borne spores of the pathogen. Eradicant fungicides are used to kill the fungus after infection has occurred. Also included in this category are fungicides capable of inhibiting the growth of the fungus. Based on the effects a fungicide can manifest on fungal growth, it can be described as fungicidal, when the fungicide kills the fungus, fungistatic, when the growth of the fungus is inhibited, and genestatic, when sporulation of the fungus is inhibited. Chemical disinfectants are often applied to the environment immediately surrounding the host plant, but this is not a practicable strategy for the control of cereal diseases. They may also be applied to the plants themselves, or more specifically to seed, in an attempt to inactivate contaminating pathogens. Seed treatments using popular seed dressing agents, such as Fernasan D, Apron plus, Luxan, etc. can be considered in this category. Fungicides may also be classified into systemic and non-systemic based on the nature of their mode of action. Systemic fungicides or their decomposition products can penetrate the plant cuticle and then be translocated within the plant as potential therapeutants. A chemical that is absorbed but not translocated can only confer fungitoxcity at the sites of application. Such chemicals are termed topical therapeutants or more often, eradicant fungicides. Systemic fungicides in present-day use include the benzimidazoles, ergosterol biosynthesis inhibitors, morpholines, hydroxypyrimidines, organic phosphates, carboxamides and guanidines. Non-systemic fungicides do not penetrate into plants but remain on their surface and act on the pathogens upon direct contact with them. This group includes inorganic compounds of copper, sulphur, and derivatives of dithiocarbamic acid and mercury.

SYSTEMIC FUNGICIDES IN PRESENT-DAY USE
The systemic fungicides in current use have been divided into seven different groups based on the nature of their mode of action. 1. Benzimidazoles- The benzimidazoles belong to the group of heterocyclic organic compounds containing two or more hetero-atoms. They are particularly effective against Ascomycetes, the Basidiomycetes and Phycomycetes are generally insensitive. Members of this group are also referred to as ‘MBC generators’ because many of them owe their activity to methyl benzimidazol-1-yl carbamate (MBC) to which they decompose within the plant or in water in vitro. Their primary site of action is the nucleus where they inhibit or disrupt mitosis by preventing the formation of mitotic spindle fibres. Resistance to the benzimidazoles has occurred in the field and, in some cases, has led to the breakdown of control measures. The benzimidazoles used as fungicides include benomyl, carbendazim, fuberidazole, thiophanate-methyl. 2. Ergosterol biosynthesis inhibitors- The compounds in this group disrupt the synthesis and function of cell membranes by the inhibition of ergosterol biosynthesis. This group is represented by the following fungicides in general usage: imazalil, prochloraz, triforine, propiconazole, triadimefon, triadimenol. 3. Morpholines- These fungicides also belong to the group of heterocyclic compounds containing two or more hetero-atoms. Currently used compounds are: fenpropimorph, tridemorph. 4. Hydroxypyrimidines- These are highly selective against the powdery mildews. The representatives of this group include dimethirimol, ethirimol. Resistance to the hydroxypyrimidines has been shown to develop rapidly in both the field and laboratory. 5. Organic phosphates- Some organophosphorous compounds have been screened and found to have fungicidal properties. Representatives of this group include include ditalimfos and kitazin. Ditalimfos is used against barley powdery mildew. It is not certain as to its mode of action, but it is likely that it is a site-specific inhibitor. Kitazin is used for the control of rice-blast pathogen, Pyricularia oryzae. 6. Carboxamides- The representatives of this group include carboxin and oxycarboxin. Carboxin is widely used as a seed treatment, especially against barley loose smut (Ustilago nuda). Oxycarboxin was initially used quite extensively against cereal rusts, especially yellow rust of wheat (Puccinia striiformis) but has now been replaced by more recent systemics, such as propiconazole, fenpropimorph and triadimefon. The primary site of action for the carboxamides is in the mitochondria where they interfere with respiration by blocking succinate dehydrogenation. 7. Guanidines- These compounds are related to urea. One member of this group, guazatine, is used for the control of seed-borne fungal diseases of cereals.

MAJOR GROUPS OF NON-SYTEMIC AGRICULTURAL FUNGICIDES
Copper- Copper compounds were among the first to be used in chemical control of plant disease. Among the copper compounds, Bordeaux mixture and copper oxychloride are extensively utilized in the treatment of plant vegetative parts. These compounds derive toxicity through their ability to precipitate proteins and cause coagulation of cell cytoplasm.
Sulphur- Sulphur was the earliest recorded fungicide and has probably been more widely used than any other. It is best known for its effectiveness in the control of powdery mildews. Originally, sulphur was used as a finely divided dust or dispersible powder. The more finely divided the sulphur, the greater is its fungicidal properties. Although the fungicidal action of sulphur is poorly understood, a generally accepted hypothesis suggests that it acts as a hydrogen acceptor and thus interferes in the normal process of hydrogenation and dehydrogenation. Lime sulphur is a widely utilized sulphur compound. It is prepared by boiling sulphur and lime-water. Lime-sulphur consists mainly of calcium polysulphides which decompose to release the toxic elemental sulphur.
Mercury fungicides- The organic and inorganic preparations used in agriculture are highly effective fungicides and bactericides. All preparations are extremely toxic to man, animals and birds, and, as mercury will accumulate in the body and therefore proceed up the food chain, there is much public opposition to their use. In many countries, for example, New Zealand, their use has been banned by legislation and throughout the world there is a major effort in progress to find suitable less-toxic alternatives. Inorganic mercury compounds have generally been superseded by organomercurials for plant disease control. All organomercurials have the general formula, R‒Hg‒X
Where R may be either an aryl or alkyl radical and X may be an organic or inorganic acid group. Alkyl compounds have been widely used as seed dressings against seed-borne diseases in Sweden. Mercury compounds, no doubt, are effective against the pathogens but they also contribute, through mercury poisoning, to the death of many wild birds including the birds of prey which live on the seed-eating smaller birds.
Organic fungicides- The early fungicides were all inorganic chemicals, but following the discovery of organo-mercury preparations as fungicides by Riehm in 1913, research efforts were intensified in screening many other organic compounds for fungicide activity. It was not until the discovery and patenting of dithiocarbamates in 1934 by Tisdale and Williams of the du Pont Company that organic compounds for use as foliage fungicides began to be developed. With the shortage of copper and mercury during World War II, there was an added impetus to the introduction of suitable organic fungicides. Sulphur was never scarce during that period and the screening of organic sulphur compounds for use in synthetic rubber production led to the discovery of the fungicidal properties of the derivatives of dithiocarbamic acid which are collectively called the dithiocarbamates. Dithiocarbamic acid
The first of these compounds , tetramethyl thiuram disulphide, now known as TMTD or thiram, was used as an activator in the production of synthetic rubber and was the first to be used commercially as a fungicide both as a foliar protectant and a cereal seed dressing.

Apart from the thiuramdisulphides, the dithiocarbamates in use as fungicides today are derivatives of two main groups. 1. Dimethyl-metal-dithiocarbamates- Although many metallic dithiocarbamates have been synthesized, only iron (Ferbam) and zinc (Ziram) dimethyl dithiocarbamates have been commercially successful.

Ferbam is a black solid used as a protective fungicide against many fungal diseases. It leaves an unsightly and undesirable deposit on the sprayed surface. Although Ferbam is

compatible with most pesticides, copper and mercury compounds or lime may reduce its efficiency.

Ziram is a skin irritant and induces dermatitis in some people. These factors coupled with the discovery of equally good alternative organic fungicides have led to a marked decline in its use.

2. Ethylene-bis-metal-dithiocarbamates- The compounds comprising this group are formed by the joining of two dithiocarbamic acid molecules through the carbon atom.

The sodium salt is called Nabam, the zinc salt, Zineb, and the manganese salt, Maneb. Nabam was used initially as a protective fungicide under the trade name Dithane but it has now been demonstrated to be markedly improved if zinc sulphate (ZnSO4) and lime (Ca (OH)2) are added to the mixture. When zinc sulphate is added, the mixture is referred as Zineb tank mix which is best known for its use against tomato leaf mould caused by Clasdosporium fulvum. Nabam has been largely superseded by zinc ethylenebisdithiocarbamate, zineb and the manganese salt, Maneb. Of late, a combined formulation comprising zinc (2.5%) and maneb (20%) called Mancozeb has been developed. All the metallic bisdithiocarbamates are now being marketed as various types of dithane. Another successful organic fungicide which has rivaled dithiocarbamates is Captan. Captan is a member of the heterocyclic nitrogen group of compounds whose fungicidal activity was discovered by Kittleson in 1952. It is very persistent and widely used as a foliage and fruit protectant.

HERBICIDES
Herbicides are chemicals used for killing or adversely affecting plant growth. Herbicides have been classified into several groups on the basis of (a) when they are applied; (b) where they are applied; (c) how they move in plants; (d) type of plants killed; (e) chemical structure or composition of the active ingredient; and (f) physiological action. The interrelationships between these factors are shown below: Translocated (Systemic), e.g. 2,4-D Foliar applied Non-translocated (Contact), e.g. Propanil

Preemergence, e.g. atrazine Selective Soil applied Preplant incorporated (CIPC)

Translocated (Systemic), e.g. glyphosate Herbicide Foliar applied Non-translocated, e.g.
Paraquat, diquat Temporary sterilants
(fumigants), methyl bromide Non-selective Soil applied Persistent sterilants
(simazine)

(a) Classification based on time of application: Based on time of application, herbicides are classified into preplant, preemergence and postemergence treatments.
A preplant herbicide is applied before the crop is planted. For example, methyl bromide may be used in seed beds to kill most weed seeds and vegetative reproductive parts before planting. However, some herbicides are very volatile and have to be applied and incorporated into the soil before the crop is planted. Herbicides in this group are called preplant incorporated herbicides. They include EPTC, trifluralin and molinate.
A preemergence treatment is any treatment made prior to emergence of a speficied weed or crop. The treatment can be applied preemergence to both weed and crop. Therefore, to correctly specify the timing of treatment, statements such as “preemergence to the weeds, or to both weeds and crop, or to the crop” will have to be made. The treatment is usually applied to the soil surface after the crop is planted but prior to its emergence. Atrazine 80WP and Primextra 500FW are the two most commonly used preemergence herbicides for weed control in maize in southwestern Nigeria.
A postemergence treatment is any treatment made after the crop or weeds have emerged. The chemical may be applied postemegence to the crop, but preemergence to the weeds. Any postemergence herbicide should be selective to the crop in which it is applied. For instance, atrazine 480 FW is applied early postemergence in maize because the crop has the ability to metabolize atrazine into non-toxic products. Examples of postemergence herbicides include 2, 4-D, MCPA, atrazine 480 FW, etc. (b) Classification based on point of application: Herbicides used in terrestrial habitats are applied either to crop foliage or to the soil. All foliar-applied herbicides enter the plant primarily through the foliage and can exert their toxic effects on the foliage or move to other parts of the plant that are most susceptible to them. The activity of water-soluble foliar-applied herbicides is affected by rainfall. If it rains within a couple of hours after application, the herbicide could be washed off the treated leaves. For example, the phytotoxicity of glyphosate is easily reduced if it rains within 6 hours of application. Herbicides applied to the soil kill plants as a result of their inhibition of germinating weed seeds, or their toxic effects on the coleoptiles and plumule of germinating grass weeds or broadleaved weeds, respectively. Since soil-applied herbicides need to go into soil solution prior to uptake, soil moisture conditions play a decisive role in their activity. Herbicidal efficacy is drastically reduced in dry soils. (c) Classification based on herbicide movements in plants: Herbicides may exert their toxic effects at points of contact with the plant, or they may move from the point of application to other parts of the plant where their toxic effects are manifested. Herbicides that kill the tissues they touch are referred to as contact herbicides. Contact herbicides that are in common use in the tropics are paraquat, diquat, propanil and oryzalin. Translocated or systemic herbicides are those that move in treated plants either in the xylem vessels and the interconnected cells (collectively referred to as apoplast) or in the phloem and living tissues of the plant (the symplast). Systemic herbicides include atrazine, dalapon, glyphosate and metobromuron. (d) Classification based on the type of plants killed: All herbicides kill plants selectively or nonselectively. The concentration of the herbicide, the type of plant treated and the age of the treated plant play a major role in how the plant responds to the herbicides. Nonselective herbicides exert their toxic effects on all plants with which they come in contact. Examples are paraquat, diquat, and glyphosate. Selective herbicides are those that will preferentially kill certain plants at recommended rates but will not affect other plants with which they come in contact. Examples include 2, 4-D, MCPA, diclofop-methyl, etc. (e) Classification based on chemical structure: Herbicides can be classified into distinct chemical groups based on structural formula. The majority of herbicides in use today are organic compounds, but there are still a few inorganic herbicides in current use. Apart from these two groups, there is a new group of herbicides known as biological herbicides. These herbicides are derived from specific fungi, and are used for weed control in crops. Chemical classification of herbicides attempts to group herbicides on the basis of molecular structures of their active ingredients. Members of a particular group may differ in their modes of action, but in most cases all members of the group tend to affect the same physiological processes. There are still a group of miscellaneous herbicides that do not belong to any of the major groups. A common example is glyphosate (Round up®). (f) Classification based on physiological action: Herbicides exert their toxic effects by adversely affecting certain physiological processes in the plants. Accordingly, herbicides are classified into mitotic poisons, photosynthetic inhibitors, nitrogen metabolism inhibitors, respiratory poisons, desiccants, growth regulators, etc.

PESTICIDE FORMULATIONS AND USE OF ADJUVANTS
Formulation is a process by which a pure chemical substance (e.g. the active ingredient of a pesticide) is blended with other ingredients and made available in a form that will improve handling, storage, application, efficacy and safety. Pesticides are formulated for various reasons, the most common ones being (a) to reduce the concentration of the active ingredient through dilution in appropriate solvent; (b) to make the pure chemical available in a form that will allow uniform distribution of the pesticide on the target organism; (c) to reduce the degree of hazard and contamination during handling, application and storage, and (d) to improve the efficacy of the pesticide through gradual release of the active ingredient, better protection from degradation and/or greater uptake by the target organism. Pesticides are formulated in many ways in response to various application conditions, application equipment, availability and cost of adjuvants, toxicity of the pesticide and other safety considerations. The formulations available for control of the different categories of pests are manifold and include the following: (1) Salts or water-soluble powders; (2) water-soluble concentrates; (3) wettable powders; (4) flowables; (5) water-dispersible granules; (6) granular formulations; (7) dusts; (8) pellets; (9) microencapsulation; (10) water-miscible liquids; (11) ultra-low volume concentrates; (12) Fumigants; (13) baits; (14) pastes; (15) impregnation solutions; (16) aerosols; (17) ointments and greases.
SALTS OR WATER-SOLUBLE POWDERS (S, SP)
Salts or water-soluble powders are powdered materials that will dissolve in water. Pesticides formulated as solid dry salts must be very soluble in water to ensure that a pesticidal concentration of the solution can readily be made for field application. It is also important, particularly in the tropics, that such salt formulation should not be hygroscopic in order to get a good shelf-life. These formulations dissolve in water to form true solutions and are used for foliar applications. Pesticides formulated as water-soluble powders consist of the active ingredients together possibly with wetting, spreading and sticking agents and often a drying agent to prevent the resulting powder from fusing into hard lumps when they are stored under humid conditions. A major drawback of this type of formulation is that it s readily washed off the leaf surface if rain falls within a short time after application. Examples of salt formulations of pesticides used in the tropics are hexazinone, sodium salt of TCA (trichloroacetic acid) and sodium salts of 2, 4-D and MCPA.
WATER-SOLUBLE CONCENTRATES (WSC, SL)
Water-soluble concentrates are liquid homogenous formulations intended for application as true solutions of the active ingredients after dilution in water. They consist of the active ingredient, water as the diluent, and a surfactant to increase foliar uptake. These formulations share with the water-soluble powders the disadvantage of being easily washed off a leaf by rain falling after the chemical is applied. Examples of water-soluble concentrates are amine formulations of 2, 4-D, bentazon, amitrole, glyphosate and MAMA.
EMULSIFIABLE CONCENTRATES (E, EC)
Emulsifiable concentrates are formulations that form an emulsion when water is added to them. Some active ingredients are not soluble in water, but can readily be dissolved in organic solvents. If they are sufficiently soluble they are formulated with these solvents formulations that consist of the active ingredient of the chemical, the organic solvent (diluent) and surfactant- which may consist of an emulsifier and a wetting agent. The emulsions formed by these chemicals in water are of the O/W type, and only require little or moderate agitation of the solution to prevent the nonpolar chemical droplets dispersed in the water from separating out. Many of the soil-applied herbicides are of the EC formulations. Examples of some EC chemical formulations in use in the tropics are bifenox, butylate, EPTC, metolachlor, pendimethalin, trifluralin and lindane.

WETTABLE POWDERS (W, WP)
Wettable powders are formulations prepared from chemical active ingredients that are neither soluble in water nor sufficiently soluble in organic solvents to be formulated as emulsifiable concentrates. The formulation consists of finely ground solid particles of the active ingredient, solid carrier or diluent, wetting agent and a dispersing agent. The diluent is an inert material of clay mineral or organic matter origin. In order to use the wettable powder it should be first mixed in a little quantity of water in a bucket to make a slurry, which is then poured slowly into a spray tank that already contains water (about one third full). It is necessary to stir the solution repeatedly to avoid separation of the solid particles. Wettable powders should not be poured directly into the spray tank. Examples of chemicals commonly formulated as wettable powders are atrazine, diuron, metribuzin, and DCPA, but many of them are now available as flowables or water-dispersible granules.
FLOWABLES (F, FW, LF)
Flowables, also called liquid flowables are formulations that consist of finely ground solids of the active ingredient of a given pesticide in slurry of water and adjuvants. The solids may settle to the bottom of the jar when the pesticide formulation is in storage, but goes readily into suspension when the jar is shaken vigorously. The oily liquid above the water in the jar is the adjuvant which may consist of oil, emulsifiers, dispersing agents and wetting agents. A flowable formulation should be shaken vigorously to ensure proper mixing of the formulation components before it is poured out. Flowables differ from WSC and ECs by being more viscous, easier to use and measure, and also more effective than wettable powders because the particles of the active ingredient are finer than those of the wettable powders with better distribution on the surface of the target organisms and soil surface. Chemicals currently marketed as flowables include atrazine, linuron, metribuzin, Primextra, and atrazine.

WATER-DISPERSIBLE GRANULES (EDG, SG, DG)
The water-dispersible granules are also known as dry flowables. They are made up of fine granules that have been impregnated with pesticides. The granules also contain dispersing agents and other surfactants which enable them to break up, when poured into water, into a solution of the consistency of wettable powder suspensions but without the separation and settling that is observed when wettable powders are mixed with water. The water-dispersible granules have advantages over wettable powders in that they can be measured by pouring, as is done with liquids. Examples of pesticides formulated as water-dispersible granules are chlorsulfuron (Glean 75DF), DPX-L5300 (Granstar 75DF), metribuzin, and linuron.

GRANULAR FORMULATIONS (G)
A granular pesticide formulation is usually prepared by impregnating (soaking up) a liquid pesticide to granules made from clay, corn cobs, walnut shells, or other porous materials. Granular pesticides are often applied to the soil, but can also be applied over plants from where they fall to the ground since they do not cling to plant foliage. Application of a pesticide formulation as granules is usually safer than liquid or dust formulations of the same pesticide.

DUSTS (D, DS)
A dust formulation consists of a pesticide mixed with finely ground talc, clay, pyrophyllite, powdered nut hulls, or other such inert materials. Dusts must be uniform in particle size. They are used dry and should not be mixed with water. Dust formulations are used to treat seeds and growing plants to manage insects, diseases, or other pests. Dust formulations require special equipment for application, may be wasteful, and more hazardous to use than other pesticide formulations. One disadvantage associated with the use of dusts is that wind, mechanical rubbing, and rain all remove dusts from plant surfaces. A film of water on plant surfaces increases the sticking power of dusts. Thus, it is more efficient to dust at night when leaves are wet with dew or when there is no wind, if this possible. Dusts may have to be reapplied at frequent intervals to provide adequate protection.
PELLETS
Pellets are formulated in the same way as granules but differ from granules by having larger particle sizes. Pellets are applied directly on the target, and the chemical is released as the pellets disintegrate. This formulation is useful for weed control in forestry.

MICROENCAPSULATION
This is a new development in pesticide formulation technology. The active ingredient of the pesticide is encased in an inert microscopic capsule (diameter, 3-10 µm), which may be gelatine or various types of polymers. The capsules are suspended in a liquid and application is made with conventional sprayers. The encapsulated pesticide formulation may be more costly to manufacture, but it has many advantages over conventional formulations. These advantages include: (a) relative ease of handling and safety; (b) the ability of the pesticide to be released more slowly over a longer period than is possible with emulsifiable concentrates, and (c) better protection from loss due to volatilization than with highly volatile pesticides that are formulated as emulsifiable concentrates.

WATER-MISCIBLE LIQUIDS (S, for solution)
Water-miscible liquids are totally miscible in water. The technical grade material may be water-miscible initially, or it may be dissolved in alcohol, making it water-miscible. These formulations resemble emulsifiable concentrates in viscosity and colour, but remain clear when diluted with water.

ULTRA-LOW-VOLUME CONCENTRATES (ULV)
Ultra-low-volume concentrates are normally the technical product in either its original liquid form or its solid form (the original product dissolved in a minimum of solvent). They are frequently applied without further dilution as an extremely fine spray by special aerial or ground spray equipment that limits the volume from half litre to a maximum of 5 litres per hectare. ULV formulations are used where good insect control can be obtained, which allows economizing through the elimination of high spray volumes varying from 30 to 100 litres per hectares. This technique has proved quite useful when insect control is desired over vast areas. However, drift is an inherent problem because of the fineness of the spray droplets.

FUMIGANTS
There are two types of soil-applied pesticides- fumigant and nonfumigant. A fumigant produces a gas, vapour, or fume in the soil that kills not only nematodes but also many bacteria, fungi, insects, rodents, and weeds. Some soil fumigants are applied directly to the soil; others are applied under gas-proof covers of polyethylene or similar materials. Fumigants may be used to control a single pest or a number of pests. All fumigant pesticides are applied as gases or have a gas phase after they are applied. The gases diffuse through the air in the soil water, and enter the bodies of nematodes through their skin. Similarly, these toxic chemicals are absorbed through the cell walls of fungi and bacteria and kill them.

BAITS
Baits consist of small quantities of a pesticide mixed with large quantities of an easily dispersible substance that is attractive to the pest in question. For example, bran is usually mixed with an insecticide, such as BHC (Benzene hexachloride) and used as bait for the control of locusts. Having been prepared, the baits should be scattered among the swarm of insects or laid in a band across their path.

PASTES
Paste formulations usually contain only a low percentage of the active ingredient, together with a filler and some form of liquid medium and often a sticking agent. These pastes are ready for use under rather specialized conditions, e.g. the banding of trees, etc. Other pastes are produced to be equivalent to water-dispersible or wettable powders mixed with a small amount of water. Prior to application, this type of paste has to be stirred into water.

IMPREGNATION SOLUTIONS
Impregnation solutions usually consist of an active chemical dissolved in a solvent of high penetrating power, e.g. BHC dissolved in kerosene, or a high penetrating liquid that gives protection in itself- such as the tar-distillation by-products, for example, creosote. This type of formulation is used when certain raw materials, such as timber, have to be protected against the damage of wood-boring beetles, termites, rots, moulds, lichens, etc. In this case, there is no plant damage to be considered and phytotoxic solvents may be used. However, care must be taken if there is a likelihood of animals licking the material.

AEROSOLS
Some materials, e.g. pyrethrins, are dissolved in a highly volatile liquid and put into a container under pressure to form the common domestic aerosol bomb. On the release of pressure through the opening of a minute aperture, the liquid comes out in the form of a fine mist, carrying with it the toxic substance. After some time these tiny droplets are deposited upon a surface and evaporate to leave a residue of toxic material.

OINTMENTS AND GREASES
Ointments and greases are used for treatment of small number of animals, and for treatment of individual parts of animals. They are also used as a supplement to spraying or dipping in order to treat parts of the animal not easily wetted and penetrated, such as inside the ears and under the tail. One of the oldest materials used against animal parasite, Stockholm tar, still forms a part of some of such preparations as do germicides, lanolin, and petroleum greases.

ADJUVANTS
An adjuvant is any substance in a pesticide formulation, or added to the spray tank to aid the operation or improve the effectiveness of the pesticide or its application characteristics. The term includes such materials as wetting agents, spreaders, emulsifiers, dispersing agents, foaming adjuvants, foam suppressants, penetrants, and correctives. The term is often incorrectly used as a synonym for surfactant. While a surfactant is in fact an adjuvant, the reverse is not necessarily true. A surfactant is a material which improves the emulsifying, dispersing, spreading, wetting or other surface-modifying properties of liquids.

SURFACE-ACTIVE AGENTS
Surface-active agents (surfactants) include wetting agents, emulsifiers, spreaders, dispersing agents, and detergents. Surfactants make it possible for one liquid to be suspended as minute droplets in other liquid, particles of a solid to be dispersed in a liquid, and for a liquid to spread or wet the surface of an object on which it is deposited. For example, water does not mix with oil or oil-like chemicals; one usually repels the other. However, by adding a surfactant, in this case an emulsifying agent to the oil, the chemical can be mixed with water to form an emulsion; as such, it can easily be sprayed through the sprayer. Similarly, water is also repelled by the waxlike cuticle found on plant surfaces. By adding a surfactant, in the form of a wetting agent, the effectiveness of a chemical may be completely changed due to increased absorption by the waxlike cuticle. The molecule of a surfactant has a polar (water-soluble, hydrophilic) segment and a non-polar (oil-soluble, lipophilic) segment. The balance between these two segments is referred to as the hydrophilic-lipophilic balance (HLB), and it affects the action of the surfactant. The HLB of a surfactant molecule or a mixture of two or more surfactant molecules is a quantitative value of their polarity. This concept was developed by Griffin (1949), and is particularly useful when surfactants are used as emulsifiers. It uses a scale of 0 to 20, and the higher the value, the higher the hydrophilic properties. Thus surfactants with HLB values that are nearer 20 are soluble in water, and oil-soluble surfactants have low HLB values. Many wetting agents have HLB values of 7 to 9. When they are added to polar solvents they increase the ability of these solvents to wet lipid surfaces.

TYPES OF SURFACTANTS
Based on their ionization or dissociation in water, surface active agents can be classed as nonionic or ionic. Nonionic surface active agents have no particular charge, whereas ionic surface active agents manifest either a positive or negative charge. Nonionic surfactants ionize little or not at all in water. They are classed as non-electrolytes and are usually chemically inactive in the presence of the usual salts. Thus, they can be mixed with most pesticides and still remain chemically inert. Many emulsifying agents are of this type, and they are usually liquids. Ionic surfactants ionize when in an aqueous medium, some being anionic (-) and others cationic (+). Anionic agents are those in which the anion part of the molecule exerts the predominant influence. Wetting agents, detergents, and some emulsifiers fall in this group. Cationic agents become ionized in water with the cation part of the molecule exerting the predominant influence. They have powerful bactericidal action but are expensive and have been of only limited use in agriculture.
Table 1. Classification of surfactants based on dissociation Type | Common name | Trade name | Chemical composition | Anionic | Allinate | Sipon ES | Lauryl polyoxyethylene salts | | Diocusate | Aerosol OT | Dioctyl sodium sulfosuccinate | | Ligsolate | Polyfon O | Salts of lignosulfonic acid | | Nonfoster | Gafac RM 510 | Polyoxyethylene nonylphenol phosphate esters | | None | Vatsol OT | Dioctyl ester of sodium sulfosuccinate | Cationic | None | AHCO DD 50 | Alkylbenzyl quaternary ammonium halide |

SURFACTANTS CLASSED ACCORDING TO USE
Wetting agents: A wetting agent is a substance, which when added to a liquid, increases its spreading and penetrating power by lowering the surface tension. Effectiveness is measured by the increase in spread of a liquid over a surface area, and by the “contact angle” of the liquid and surface. Many materials are used as wetting agents, including long chain alcohols, petroleum sulfonates, acid sulphates and derivatives, sulfonated aromatic derivates, esters of fatty acids and clays.
Emulsifying agents or emulsifiers: An emulsifying agent is a surface-active substance which stabilizes (reduces the tendency to separate) a suspension of droplets of one liquid in another which otherwise would not mix with the first one. For instance, if oil is added to water and shaken vigourously, the oil is momentarily suspended as small droplets in the water, forming an emulsion. The water is a continuous body and is therefore referred as the continuous phase; however, the oil is referred to as the discontinuous phase. If the mixture is allowed to stand, the oil and water will separate. A third material is added to decrease the tendency to separate or to increase the stability of the emulsion. This is called the emulsifying agent or emulsifier. An example of an emulsion is milk which consists of butterfat dispersed in water with casein acting as the emulsifying agent. This type of emulsion is called an ‘oil-in-water’ (O/W) emulsion. Butter and mayonnaise are emulsions with water in the dispersed phase and fat or oil in the continuous phase. This type of emulsion is called a ‘water-in-oil’ (W/O) emulsion.
Spreaders or spreading agents: A spreading agent is a substance which increases the area that a given volume of liquid will cover on a solid, or another liquid. Spreaders and wetting agents are closely related in that when a wetting agent reduces surface tension, spreading naturally follows.
Dispersing agents: A dispersing agent is a substance that reduces the cohesion between like particles. Some dispersing agents are good wetting agents, but others have little or no effect on the surface tension. Some wetting agents and dispersing agents are incompatible and have a tendency to interfere with each other if used together.
Sticking agents: Sticking agents, as the name implies, are substances that cause the pesticide to adhere to the sprayed surface. Many of the surfactants discussed above may act as sticking agents.

THE CHOICE OF FORMULATIONS
There are a number of factors which need to be considered when choosing a chemical formulation to deal with a crop protection problem. These factors include: (1) the stage of the crop at the time of application; (2) the nature of the crop; (3) physical environment; (4) biological environment; (5) equipment available; (6) type of labour available, and (7) economic factors. 1. The stage of the crop at the time of application- This is a very important factor to consider when deciding what chemical to use and in what formulation to apply it. Although a chemical may control or eradicate a pest insect, weed or disease if it is applied at their susceptible stage, the effect on the crop itself has to be taken into account. Some crops are likely to be damaged when a particular chemical is applied at one stage and not at another. For example, small grains including rice, are very sensitive to 2, 4-D during germination and seedling stages. Consequently, treatment during this time will usually cause many malformations of the head, general stunting of the plant, and reduced yields. 2. The nature of the crop: This factor is important in determining the final selection of the formulation based on the following reasons: (i) different species of plant may exhibit different levels of susceptibility to a chemical in a particular form; (ii) the purely morphological characteristics of a plant may make it advisable to use one type of formulation, and (iii) the method of planting obviously affects the type of treatment that can be adopted. Among crop plants, differences exist in their physiological make-up, hence a formulation that is applied to one crop may not be suitable for another. For example, weeds growing in maize crop can be safely treated with the amine formulations of 2, 4-D provided that the application is made at the right time and in the right concentrations. The same formulation would be quite likely to produce damage symptoms if used in oats. The morphological characteristics of a plant which dictate the type of formulation can be reduced to the height, the type of foliage, the density of the foliage, and the rooting habit. On mature palm trees and rubber trees that usually grow tall, the chemical that will control a pest or disease may have to be applied in the form of a dust or atomized spray because coarse spraying is often impracticable. Some formulations will spread and adhere to one type of leaf surface more than others. Therefore, the size, shape and surface of the individual unit of foliage are also an important factor to be considered. Leaves with smooth waxy surfaces are difficult to wet, and the spray used to protect them must be formulated to contain wetting and spreading agents. Other leaves are rough and hairy, and require for their protection treatment with formulations that have good powers of penetration. The rooting habit of the plant becomes important in modifying the choice of formulations in the early stages of plant growth, when the roots are usually soft and liable to chemical and mechanical injury, especially if they tend to extend themselves along the surface of the ground. In this case, care must be taken to ensure that the treatments are made to the aerial section of the plant so as to preclude any form of injury to the developing root. The growth habit of the plant together with the method of planting also dictates the method of application and the type of formulation that can be chosen. In their early stages, row crops, such as maize, can often be treated with a chemical from between the rows. 3. Physical environment: Environmental factors which determine the type of formulation that may be chosen to protect the crop include temperature, wind, light, rainfall and humidity. The ambient temperature around a crop may affect the choice of formulation in a number of ways. To a certain extent, high temperatures tend to increase the rate of metabolism and development of new tissue. If a crop is to be protected from an attack that affects the newly developed areas of the plant in a hot climate, formulations that are economic to apply frequently may have to be used so that they may protect these areas as they develop. Certain chemicals have critical temperature thresholds, e.g. nicotine formulations due to their fumigant action work much more rapidly at temperatures above 15ºC. Some chemicals are rapidly broken down by high temperatures, and if a lasting effect is desired from the treatment, these materials are unlikely to be of much value under high temperature conditions. Certain chemical formulations might prove quite suitable for foliar applications at low volumes in cold climates, but prove to be disappointing under warmer conditions, e.g. materials formulated to be sprayed in small droplets in cool climates will likely evaporate before the droplets reach the surface to be protected in hot, dry tropical climates. Long periods of bright sunlight such as those experienced in tropical countries can reduce the effectiveness of many formulations. Chemicals like pyrethrins are rapidly broken down and lose their effectiveness under the ultra-violet radiation in bright sunlight, and if a persistent effect is required, formulations of pyrethrins must contain a synergist, e.g. piperonyl butoxide that will inhibit such a rapid breakdown. Wind can help to determine what formulation of a chemical to be applied. Materials formulated as fine dust particles or liquid droplets are obviously unsuitable for use during periods of high wind. However, certain materials are formulated for application in light breezes. Drift dusting and spraying rely on the aid of such breezes. Humidity and rainfall are the important moisture factors that influence the choice of formulations. If formulations have to persist on surfaces subjected to heavy rainfall, they should contain sticking agents, or should possess characteristics which endow them with properties of great tenacity. High relative humidity often favours slow evaporation, and this can be advantageous when formulations which evaporate too quickly are to be applied in dry atmospheres. 4. Biological environment: Although a formulation may be effective against a pest, weed or disease in his crop, a farmer must not select a formulation until he is satisfied that it will not adversely affect some other factor in the natural environment. Undesirable side-effects can follow the application of a wrong formulation. One of the most serious dangers lies in the application of insecticidal formulations which may prove highly effective in controlling a pest in the first instance, but which may also destroy beneficial predatory insects which tend to keep pest population in check. All chemical applications must be considered in the light of their possible effect on cattle, game and other wild and domestic animals; great care must be taken to prevent their grazing the land until the residual amounts of the pesticide have diminished to a level at which no danger is posed to the grazing animals. 5. Equipment available: It is obvious that one cannot choose to apply a dust formulation when one possesses only spraying equipment, but many farmers overlook the effect that the available equipment can have on the choice of formulation. Some spraying machines will distribute effectively only those materials that are in solution and not those that must be applied as a suspension, such as the wettable powders. Certain formulations require the machine to provide positive agitation in the tank, while others are quite satisfactory if they receive only the mild agitation provided by a flow-back from the pump. The type of nozzle available can play an important role in determining whether or not a formulation may be applied through a machine. Specific nozzle types are recommended for spraying insecticides, fungicides, and herbicides. 6. Type of available labour: Certain classes of unskilled labour are unfit to be given the responsibility of handling poisonous formulations. To ensure the safety of the farmer, the crop and the environment, handlers of pesticides must be well trained on pesticide identification, purchase, transportation, storage, use, and safety precautions. If the farmer is left with no alternative than to employ the services of unskilled labourers, then he must see that the application operation is carefully supervised either by him or by reliable personnel. The type of labour available also has an indirect effect on the class of formulation selected, because their stage of advancement will play a part in determining the type of machinery that they can be trusted to handle. 7. Economic factors: All control measures should be designed to reduce crop yield loss below the economic figure (variously quoted at 10-20 percent of expected return) suggested as the tolerance limit before control measures become worthwhile. A good control practice rarely aims at total eradication of a pest because of the bad effect this might have on natural balance. At times it is very difficult to judge whether or not it is worthwhile to apply a chemical formulation, especially when a sudden change in the weather might do the job, e.g. a heavy downpour of rain might wipe out a plague of greenfly or enable the crop to make such a rapid growth that it can overcome the attack. However, for a chemical application to be meaningful, the benefit derived from a treatment should do a little more than pay for the total cost of that treatment. When trying to discriminate between two possible formulations for a crop protection application, it is necessary to consider not only the relative efficiency of the two materials when properly applied, but also the amount of labour required to make the application and the time involved in the application. Certain materials, such as dust formulations are usually cheap to apply, but wasteful of chemicals, while spray formulations can be applied with less waste but at a higher cost.

PESTICIDE APPLICATION
Pesticides are applied in various ways to plants and soils. They can be applied broadcast, as a band, as a directed spray, and as a spot treatment. Broadcast treatment or blanket application is uniform application to an entire area. Band application usually implies treating a narrow strip- usually directly over or in the crop row. The space between the rows is not chemically treated, but is usually cultivated as is the case with weed control in row crops. This method reduces the chemical cost because the treated band is often one-third of the total area with comparable savings in chemical cost. Directed sprays are applied to a particular part of the plant, usually to the lower part of the plant stem or trunk. Trees are often basally treated by directing the spray to the base of the trunk. Spot treatment is treatment of a restricted area, usually to control an infestation of a weed species requiring special treatment. For example, soil sterilants are often used on small areas of serious perennial weeds to prevent their spread.
The choice of a method of pesticide application is influenced by pesticide formulation, the target to be treated and by the type of pest problem to be solved. Pesticide formulation has an overriding influence on method of application. Depending on the type of formulation, a pesticide may be applied by hand (e.g. some granular formulations), sprayed (e.g. formulations that require a liquid carrier for dilution) or applied with other special equipment (e.g. the use of granular applicators for pellets and granules).

PESTICIDE APPLICATION EQUIPMENT
The type of equipment used for pesticide application is dependent to a large extent on the types of pesticide formulations available to the farmer. Most of the pesticides in current use are formulated so that they can be diluted in liquid diluents (carriers). This type of pesticide formulation is applied with sprayers. Granular pesticides or dust formulations do not require a liquid carrier for application, and are therefore applied with granular applicators or dusters.
Sprayers for pesticide application come in various forms, sizes and designs. Inspite of this diversity, all sprayers with the exception of electrodyne sprayers and some with spinning disc nozzles must have a tank, a pump and a nozzle or nozzles. In addition to these, sprayers have a few essential component parts that are necessary in order to apply a chemical successfully and uniformly. These are the pressure regulator, the hose, filter, shut-off valve and spray lance or boom. All other accessories of a sprayer, such as agitators, strainers, pressure gauge, pressure relief valve, recirculation pipe, etc. help to ensure that chemicals are efficiently applied.

Major components of a sprayer
All sprayers used for pesticide application in present-day use agriculture have a tank, filter, pump, pressure regulator, hose, shut-off valve, spray lance or boom, and nozzle (or nozzles) in common. (a) Tank
The main function of the tank is to hold the spray solution. Most modern spray tanks are constructed of plastic, high-density polyethylene and polypropylene materials that are resistant to chemical reactions. The older models of spray tank are constructed of brass, stainless steel or aluminum. Depending on the capacity of the tank, and on the type of sprayer, the tank may be equipped with a device for recirculating the spray solution to prevent separation of the components of the solution. A spray tank without a gauge should be translucent so that the operator can easily estimate the content. This is particularly important in the case of knapsack sprayers. A good tank should have a filter that will serve to minimize the problem of nozzle clogging during pesticide application.

(b) Filter
Filters, also referred to as strainers, are important components of a sprayer. Their main function is to prevent foreign matter, such as twigs, insects, leaves, etc. from getting into the sprayer tank and ultimately plugging the nozzles. This is particularly important in the tropics where farmers have many limitations on the quantity and quality of water available for both household use and pesticide application. A coarse filter should be fitted at the port through which chemicals are poured into the tank, while fine strainers should be fitted at the nozzle housing, and at other location in the system where the presence of foreign matter could interfere with the smooth functioning of the system.

(c) Pump
The main function of the pump is to generate the pressure necessary for forcing the chemical out of the sprayer. Two types of pumps are distinguished based on the type of pressure they generate. These are the air pressure or gas type and the hydraulic type. The air pressure type is commonly used in knapsack sprayers. These pump generate air pressure inside the spray tank, and it is this air pressure that forces the spray solution out through the nozzle whenever the shut-off valve is opened. Sprayers equipped with air pumps always have pressure built up inside the tank in order to expel the solution. There are variations in air pressure sprayers from the Lever Operated Knapsack (LOK) sprayers to small-plot precision sprayers used for research work. The precision sprayers may use compressed air, liquefied air or gases, such as CO2. The hydraulic pressure types of pumps are employed in motorized sprayers. The tanks of these sprayers are not pressurized during use. In order words, they do not depend on building up pressure in their tanks as a means of expelling spray solutions. Instead the pumps are electrically or mechanically operated and the pressures being generated move the solutions from the tank, through the delivery lines and finally out of the nozzles. The following six types of power-operated or hydraulic pumps are commonly used in pesticide sprayers: 1. Piston (reciprocating) - On upstroke, the piston pulls liquid into the pump chamber. On the opposite stroke, the fluid is forced out of the chamber through a surge tank to allow for a continuous and steady flow. 2. Diaphragm – This is akin to a piston pump except the piston is replaced by a flexible diaphragm. 3. Roller-Impeller – Rollers are fitted into slots formed on the periphery of the impeller (a solid wheel fitted onto a shaft). The slots allow the rollers to follow the off-centre contour of the pump housing. Fluid fed into the inlet side is forced out by the squeezing action of the rollers. 4. Flexible-Impeller- It is similar to roller-impeller pump except that it has a series of rubber paddles attached to an off-centre shaft. As the paddles turn, they force the liquid out of the pump. 5. Gear- Fluid enters pump and gear teeth force liquid into discharge outlet. 6. Centrifugal- Liquid enters through the centre of the rotating impeller and is flung outward by centrifugal force and forced out the outlet.

(d) Pressure gauges
Pressure gauges are important because pressure affects the amount of spray applied and droplet sizes. Use the lowest possible pressure for each job because high pressure uses extra power, wears equipment faster and increases drift. Pressure gauges should be handled with care and equipped with a gauge protector when spraying corrosive materials, or when using a piston-type pump. Do not use a gauge under too much pressure.

(e) Pressure regulator
The main function of the pressure regulator is to prevent the pressure generated in the spray tank from exceeding a preset limit considered adequate for the smooth and safe operation of the sprayer. When the desired operating pressure cannot be reached, check the pressure relief valve by observing the flow from the return hose. If there is flow in the return hose before the operating pressure is attained, the relief valve is not seating and must be checked for wear, trash, and broken springs. It is a must that defective relief valves be repaired or replaced immediately.

(f) Cut-off valves/shut-off
The shut-off valve controls the flow of liquid from the spray tank to the nozzle. The pesticide solution in the spray tank flows out of the nozzle whenever the shut-off valve is opened provided there is sufficient pressure in the tank or sprayer system to force out the liquid. The liquid will continue to flow until the valve is shut-off.

(g) Hose
The function of the hose is to conduct the spray solution from the tank to the spray lance. Hoses used in most of the modern sprayers are made of both chemical and oil resistant materials, such as neoprene and plastics.

(h) Boom
A boom bears the nozzles through which the spray solution is discharged. A spray boom may have a single nozzle or multiple nozzles. If the boom has only one nozzle, it is called a spray lance or wand. A tractor-mounted sprayer usually has a boom equipped with multiple nozzles, and a knapsack sprayer usually has only one or two nozzles although some backpack knapsack sprayers using compressed air may be equipped with multiple nozzles.

(i) Nozzles
The function of the spray nozzle is to break down and distribute the liquid evenly over the target. The effective use of agricultural chemicals depends on the selection of nozzles that meet the requirements of each particular spray application. To meet various spray requirements, nozzles are classified based on spray delivery patterns, spray angle, discharge rate, and the materials from which they are made. On the basis of their spray patterns, eight types of nozzles can be distinguished. 1. Flat fan nozzles are used for broadcast or boom spraying. Drift is less with them than it is with standard cone nozzles. Since their spray rate tapers at the edges, these nozzle patterns must overlap 30-50% for even distribution. 2. Even flat fan nozzles are used for band spraying with a uniform spray pattern throughout. They should not be used for broadcast spraying. 3. Whirlchamber (nonclog) nozzles are available as wide angle (120º) hollow cone nozzles. Clogging is minimized and less drift occurs due to the lower boom height and large droplet size. Coverage remains fairly constant with changes in boom height. 4. Flooding nozzles may be used to apply herbicides and fertilizer solutions. They operate at low pressure and have wide patterns (up to 160º). The pattern width varies with pressure and height. 5. Boomless (or cluster) nozzles are used for wide swath spraying. Only one or a cluster of nozzles may be used. The boomless nozzle does not give as uniform coverage as other types, and the spray pattern is more affected is more affected by wind than a boom-type nozzle. 6. Hollow cone nozzles are designed for moderate to high pressures and are used to obtain thorough coverage of crop foliage and very uniform distribution. 7. Disk-core cone nozzles are used to produce hollow or solid cone spray patterns. The combination of disk and core used determines the spray pattern and angle. These nozzles are used for spraying abrasive materials at high pressures. 8. Solid cone nozzles are used for hand spraying, spot spraying, and foliar application of pesticides at moderate pressures.

The correct operating pressure should be selected for each spray job. Since drift is often a problem when spraying herbicides, the pressure should be as low as possible for the nozzle to operate. Insecticides are normally sprayed at moderate pressures, while fungicides are sprayed at high pressures to achieve coverage.

PESTICIDE CALCULATIONS
The efficacy of pesticides in the control of pest organisms depends to a large extent on the concentration of the pesticide and on application conditions. Pesticides may be applied to the soil or crop foliage to control pest organisms. The dose of a pesticide recommended for pest control in a given cropping situation is the result of extensive field evaluation of both pest and crop responses to the pesticide in various environmental and soil conditions. Crop injury and safety of the sprayer operator, improved pesticide efficacy, reduced cost of pest control and environmental safety are the reasons for pesticide calculations. Accurate calculation of the quantity of pesticide to apply on a target is an important step in pesticide use. Failure to calculate the quantity of pesticide correctly leads to wrong measurements, unsatisfactory pest control, crop injury and possibly environmental pollution. A few examples of pesticide calculations will be given to illustrate some of the common problems encountered with pesticide use in the field. These problems are related to: (a) the amount or quantity of the commercial product needed to apply on a target; (b) the rate of application of the active ingredient of a pesticide; (c) the area that can be treated with a given quantity of product; and (d) the concentration of a pesticide when the quantity to be applied is known. Two terms that are frequently encountered in pesticide formulations are ‘active ingredient’ and ‘acid equivalent’. The active ingredient is that part of a chemical formulation which is directly responsible for the pesticidal effects. In some pesticides, the entire molecule is considered to be the ‘active unit’. Therefore, if the chemical was 99% pure, it would be considered 99% active ingredient. In others, the pesticide activity is more accurately calculated on an acid equivalent basis. The acid equivalent refers to that part of a formulation that theoretically can be converted to the acid. In this case, the acid equivalent is given as the active ingredient. The term is frequently used for pesticides formulated as salts or esters of acids. In such formulations, it is only the fraction of a pesticide molecule representing the parent acid that is pesticidal. Hence, it is usual to express the effective doses of these pesticides on the basis of acid equivalents. The proportion of the “active ingredient” representing the acid equivalent is obtained by dividing the molecular weight of the acid form of the pesticide minus one, by the molecular weight of the salt or ester derivative of that pesticide. The acid equivalent is calculated as follows:

CALCULATIONS INVOLVING QUANTITY OF PRODUCT
Problem 1.
Gesaprim 80WP (Atrazine) was applied preemergence in maize planted in a 15 ha forest land. If the herbicide was applied at the rate of 3.0 kg a.i. /ha, what quantity of the formulated product was used?

Solution to Problem 1.
Atrazine in the formulation= 0.8 kg a.i. /kg of product.
Application rate= 3.0 kg a.i. per hectare.
Therefore, the quantity of product used per hectare is 3.0 kg a.i. /ha 0.8 kg a.i. /kg

Problem 2.
The butoxyl ethanol amine salt of 2, 4-D containing 360 g / litre of the ester was applied to 5 ha bush regrowth. If the chemical was applied at the rate of 1.5 kg a.e. /ha, what quantity of the product was used? (Molecular weight of 2, 4-D acid is 221 and that of the ester is 321).

Solution to Problem 2.

USE OF FORMULA FOR SOLVING PROBLEMS
The relationship between pesticide concentration, the rate of application, and the area to be treated is given as follows:

For dry formulations, C is expressed as percentage weight of the formulation. For ease of use, convert C and R to the same units. Formula is valid for calculations involving areas greater than 1 ha.