Vector-borne diseases such as malaria, lymphatic filariases, schistosomiasis, leishmaniasis, onchocerciasis, trypanosomiasis, dengue and dengue haemorrhagic fever (DHF), yellow fever and Japanese encephalitis, globally account for suffering amongst more than one billion people, mostly from the developing world. Geographical distribution and prevalence of these diseases have been discussed. In addition to the mortality caused by these diseases, millions of people lead a hopeless life and are socially unable to achieve self-realization and be economically productive, as a result of these diseases. Examples of such economic impact and the costs of these diseases and methods of control have been given. Economic benefits of control of some of these diseases have been discussed. These concepts are of utility when resource allocation for control of these diseases is under consideration.

Recent developments in the assessment and collection of yield loss data due to pests are discussed. The information is needed for decision-making on resource allocation, on pest management, crop production planning and crop improvement. The difficulties in accurate loss assessment include lack of information on the distribution of pests in space and time, the difference between research and farmers yields, how much control has been used, and changes in crop resistance and in pest status. Sources of data are described and examples of losses in a number of tropical crops. The difficulties met in using losses to justify research and crop improvement projects, with particular reference to banana pests in Tanzania and cassava pests in Ghana, include the absence of reliable data, the selection of a base-line yield and the effects of other causes of loss.

Tropical environments provide optimum conditions for insect growth and multiplication in the ideal medium of food stored by humans. Some insects begin infestation in the field and continue attack in storage. Other species diapause and remain in cracks and crevices in granaries or feed on grain residues while awaiting the new harvest. Since most of the tropical produce is for subsistence, the losses resulting from severe insect attack contribute to malnutrition and famine. In central stores and warehouses commodity management is poor and large amounts are lost to insects. In countries where heavy insecticide and fumigant pressure is maintained to preserve food stocks, insects rapidly build up resistance. The solutions for the future are to reduce dependence on pesticides and utilize in an integrated way the cultural, biological and physical techniques developed in recent years for safer storage and the successful preservation of strategic reserves for food security.

Yield losses from insect pests were determined by the insecticide check method in 60 crops in the Philippines from 1976–1986 in three rice environments with traditional and modern cultivars in 10 sites. Plot size of 100 m2 minimized interplot interference from insecticide protection on untreated plots. Low and high crop losses were recorded across sites in drylands (2–39%, = 25%), rainfed wetlands (13–24%, = 21%), and irrigated wetlands (5–40%, = 18%). Weighted crop loss for the Philippines based on production by environment and cultivar type averaged 18.3% or 0.57 t/ha per crop. Consistently greater yield loss occurred in the vegetative (50%) than reproductive (30%) or ripening (20%) stages across environments and cultivars. No epidemics affected the trials therefore crop losses were those from chronic pests. The major chronic pests in the wetlands were stem borers, leaffolders, whorl maggot, and rice bug; and in the drylands were ants, seedling maggots, white grubs, leaffolders, stem borers and rice bug. Yield loss variability was greater between fields than crops or sites, therefore a control strategy based on corrective actions when pest numbers reach threshold levels is more efficient than prophylactic actions. Yield loss was more in early maturing cultivars stressing the importance of compensation in tolerating insect damage. Insect stresses perhaps can be compensated by total crop management to increase the plant's ability to tolerate insect damage.

This paper, which discusses the current status of conventional insecticides in the management of stored product insect pests in the tropics, shows that insect infestation in stored products is high in the tropics and may lead to 100% damage in severe cases. Conventional insecticides used in their management, such as permethrin, lindane, pirimiphos-methyl, phostoxin, methyl bromide and io-dofenphos, and their methods of application are also discussed.

Methods of storing grains and other grain products are inadequate and in most cases reduce the efficacy of insecticides thereby enhancing the multiplication of the insects. The problems associated with insecticide usage in disinfesting stored products are highlighted and suggestions are made for their solutions.

DDT and BHC were used prior to 1954 as grain protectant but gradually malathion was adopted as prophylactic spray at 50 mg/m2. However, during the continuous usage, doses were enhanced up to 150 mg/m2 to meet the level of effective kill. Toxicity studies on different surfaces, seeds, moisture content level, different insects and stages and emergence of resistance were carried out with respect to malathion and several newer insecticides. DDVP was incorporated in spray schedule at 100 mg/m2. Toxicity data on 17 insecticides, three formulations, persistence on six surfaces and three seeds, tests on 13 insects have been utilized to evolve a strategy for future developments of a prophylactic insecticides. Demerits of switching to new insecticides without broad-based toxicity data have also been advocated. Insects, such as, larva of Trogoderma granarium Everts and adults of Tribolium castaneum (Hbst.), Rhizopertha dominica Fab., Callosobruchus maculatus Fab., Lasioderma serricorne Fab. and Sitophilus oryzae Linn, among beetles and Ephestia cautella (Wlk.) and Corcyra cephalonica Staint from moths were found least susceptible to most of the insecticides. Etrimfos and deltamethrin were adjudged superior for large scale use specially as wettable powder and adjudged suitable to sub-tropical conditions. The paper also provides information on currently used insecticides in India, methods of grain storage at rural and organized level and various constraints faced in the developing countries.

Reasons for dissatisfaction with insecticidal control of disease vectors include their presumed harmfulness to non-target organisms, their cost and difficulties of organizing effective spraying programmes and physiological and behavioural resistance in vector populations. The validity of these objections and possible ways of resolving them are discussed. The importance of assessing the actual field impact of cases where resistance has been detected with laboratory tests is stressed. With regard to behavioural resistance, some data are presented on tests for genetic variation in the tendency of mosquitoes to rest outdoors and hence to avoid contact with sprayed walls. Most of the failures of insecticidal control of vectors can be attributed, not to problems of resistance, but to organizational difficulties. The impregnation of mosquito bed nets with pyrethroids, which householders can do for themselves, is therefore being field tested in several parts of the world. Such impregnation makes nets effective even when they are torn. This method will probably be particularly advantageous against mosquitoes which enter houses to feed, but do not rest on walls.

The development of resistance to insecticides in insect pests is an important and increasing problem. As insecticide use increases in tropical areas, the numbers of resistant insect strains will increase.

The adverse effects of insecticide resistance include crop losses, increased production costs, increased environmental hazards, and socio-economic problems. The increasing difficulties and costs of discovering and developing new insecticides together with the widespread occurrence of resistant strains of insect pests means that insecticides are a limited resource that must be conserved. Methods of conserving insecticides include sensible management of their use and the implementation of IPM programmes.

The problems of insecticide resistance and strategies for minimizing and managing resistance in tropical areas are discussed using examples of insect pests of stored products, cotton, rice and bananas.

A diverse number of insects cause damage and loss to crops in the tropics. Timely and appropriate control measures are necessary if crop losses and hardship to both farmers and the population at large are to be avoided.

To many farmers one of the most effective control measures is often the application of insecticides. The efficiency of insect control with insecticides is largely influenced by a judicious choice of the insecticide and the application equipment. There is therefore a need for the development of appropriate and evaluation of new and existing spray application equipment. Developments of some spray equipments since the late 1960s and their efficacy in the field are discussed. The main advantages of ultra-low volume (ULV) over some electrostatic sprayers are the application of a wide range of insecticides and formulations and better penetration to lower plant parts. The main advantages of electrostatic sprayers are the application of insecticides at extremely low volumes and low energy requirements with some sprayers. The advantages of the hydraulic nozzle sprayers are the even deposition of spray over all plant parts, although logistically the system is inefficient compared to the other systems.

All systems contaminate spray operators during spraying but the exact level of contamination will depend on the concentration, droplet size and formulation of the insecticide.

Use of plant derivatives for insect pest control was common in the tropics before the advent of synthetic pesticides. Plants such as neem, Azadirachta indica A. Juss, chinaberry, Melia azedarach L., and Warburgia spp., have been known for insect repellent and antifeedant properties in addition to other plants possessing insecticidal and growth regulating properties. Practical usefulness of antifeedants derived from plants for management of insect pests in selected field crops and stored grains is discussed. These products would, however, require some definitions of quality control and standardization of biological properties for large-scale use.

Pheromones have been successfully used on a large scale for control of pink bollworm, Pectinophora gossypiella, by mating disruption in Egypt and Pakistan using various slow release formulations. Similar formulations are being developed for control of other cotton bollworms, Earias insulana and E. vittella. Small-scale trials are in progress to evaluate pheromone formulations for control of sugar-cane borers, Chilo sacchariphagus and C. auricilius in Mauritius and India, and the tropical warehouse moth, Ephestia camelia, as a pest of stored cocoa in Brazil. Traps baited with the pheromone of the African armyworm, Spodoptera exempta, have been extensively used in Eastern and Southern Africa as the basis of a forecasting system of pest incidence.

Traps baited with the synthetic aggregation pheromone of the larger grain borer, Prostephanus truncatus, have been developed and are in use in E and W Africa for detection and monitoring of this pest of stored grain.

Despite the growing acceptance and adoption of the IPM concept, up until now no “optimal” or pragmatic IPM programme has been realized. This is apparently because most of these programmes are still in need of new types of pesticides, with no or negligible adverse impact on the environment including the non-target beneficial arthropods. From a practical perspective, such pesticides are still rather far reaching. Most, if not all, pesticides in current use as components of IPM programmes are so far belonging, in one way or another, to the conventional pesticides. It is admitted that in designing IPM programmes, several efforts should be made to minimize not only the frequency of pesticide applications but also their negative consequences on beneficial arthropods. Whilst the first goal has been achieved in several situations, the other one appeared hardly accessible. A number of methodologies are currently being suggested to cause conventional pesticides more selective, but apart from having many technical and other constraints, they might at best have fallen short of providing the hopeful solution. These together with other problems facing the development and practical utilization of juvenoids and related compounds are discussed mainly in view of their possible contribution to tropical pest management.

Available pieces of information on Cicadulina sp. leafhopper biology, with specific references to the suitability of various species for mass rearing and resistance screening for maize streak virus (MSV) are critically reviewed. The following are summarized: (a) effect of distribution pattern and species composition on the efficiency of collection of live specimens for establishing a colony for mass rearing; (b) species biology and fecundity as affected by temperature and humidity; (c) species specificity in MSV transmission; (d) host plant preference and adaptation to mass rearing conditions. All these parameters affect the suitability of various Cicadulina species used as vectors in maize resistance screening and breeding to MSV disease.

Insect pests are major constraints in cowpea production. Breeding for insect resistance is a major objective of cowpea improvement programme at IITA. Resistance to leafhoppers and aphids has been incorporated in advanced breeding lines thus eliminating the need for other control measures. Similarly, a moderate level of resistance to thrips and cowpea storage weevil has also been incorporated thus reducing the damage due to these two pests. A moderate level of resistance to legume pod-borer and pod sucking bugs has also been identified and is being incorporated in advanced breeding lines. The long term strategy is to develop cowpea cultivars with multiple resistance to insect and diseases with superior agronomic characters and local adaptability.

Major rice stem borers in W. Africa were discussed and scientific investigations relating to varietal resistance were reviewed. Impediments militating against a faster rate of development of this area of research in W. Africa were highlighted. A call was made to accord greater emphasis and priority to research on varietal resistance since in the long run it will become the most important rice crop management tactic feasible for the majority of West African small scale rice farmers.

In the tropics, insect pests cause considerable damage to soybean, Glycine max and mungbean, Vigna radiata from germination until harvest and even in storage. Agromyzid flies in the seedling stage, lepidopterous and coleopterous defoliators in the vegetative and early reproductive stages, pentatomid and coreid bugs and lepidopterous pod-borers in the reproductive stage, and bruchids in storage all cause significant damage. Sources of resistance to agromyzids in mungbean (AVRDC accession V 4281), defoliators (PI 171451, PI 227687, PI 229358) and stink bugs (PI 227687 and PI 171444) in soybean, and bruchids in mungbean (V 2164) are available. In most cases, antibiosis appears to be the major mechanism of resistance. These sources are being exploited in the development of agromyzid-resistant mungbean, defoliator- and stink bug-resistant soybean, and bruchid-resistant mungbean at the Asian Vegetable Research and Development Center (AVRDC). Diversity in insect species within a pest complex and, in certain cases, biotypes of single pest at different locations as well as pest ecology are likely to influence the resistance reaction and, in practical terms, the utilization of pest resistance as a tool in the management of insect pests of soybean and mungbean. Greater efforts are required at local level to fully utilize insect pest resistant germplasm.

Tomatoes, an important tropical crop, suffer greatly from various plant viruses which are transmitted by the tobacco (sweetpotato) whitefly, Bemisia tabaci. Since these viruses cannot be controlled directly, the only path available to the grower is to prevent their transmission. The shortcomings of chemical control are well known and the use of alternative control methods is limited. Consequently, resistant cultivars would be the best solution for the grower. Breeding for resistance to the whiteflies as vectors of viruses has some clear advantages compared with breeding plants resistant to the virus. In search for sources of resistance among wild tomatoes, the two most resistant accessions of Lycopersicon pennella, L. hirsutum and L. hirsutum f. glabratum were selected. Although these accessions are susceptible to the virus, in the field they were only slightly, or not, infested. The resistance of L. pennella was found to be based entirely on the sticky material which is exuded by glandular trichomes located on the leaves and stems. The rate of resistance depends partially on environmental factors like photoperiod and light intensity. A reliable method for quantitative detection of resistance in L. pennella was developed.

Classical biological control has been very effective against many introduced pests in the tropics and because it is permanent should be preferred as a control strategy for these pests. However, many pests are native or long established and of uncertain origin. Although classical biological control of these is more difficult, it has been achieved on a number of occasions but in many instances satisfactory control has not been achieved despite considerable effort. Reassessment of control options is needed for these pests based on more thorough quantitative ecological studies than are available at present. Only in this way will rational control strategies be developed, maximizing the contribution of native as well as exotic natural enemies. Even so, in many instances, control of a complex of pests on a single crop will require development of integrated pest management including, at least initially, chemical control of some pests. For effective implementation, more and better trained personnel will be required as will the active co-operation of farmers.

Pearl millet, sorghum, maize and cowpea are important food crops in dryland areas of the Sahel. Losses caused by insects on these low input crops are variable but generally substantial. Basic information on the role of natural enemies is scant. An example of a recent applied research which is mainly directed towards the management of millet earhead caterpillar, Raghuva albipunctella Joannis is narrated. Results of a study initiated on conservation and encouragement of Bracon hebetor Say against Raghuva in a traditional village environment are highlighted. These results demonstrate that such an approach of biological control has a place in even the temporary, already fragile agro-ecosystems and in habitats of low durational stability in the drought-hit Sahel.

Extensive additional base field data have been assembled since 1981 on the natural enemies of other pests, including Heliothis armigera (Hb.) in Senegal. Based on these results, the scope of conservation and encouragement on natural enemies in the Sahelian zone is discussed. Future research, development and training needs, for both short and more importantly long-term planning in this important aspect of pest management are briefly mentioned.

A major cause of yield loss in cassava, Manihot esculenta Crantz in tropical Africa is African cassava mosaic virus, transmitted by the whitefly, Bemisia tabaci (Gennadius). Investigations into the epidemiology of the disease and the ecology of the insect vector at the Kenya coast demonstrated that the serious loss of yield caused by the virus could be greatly reduced if simple changes in husbandry practices were adopted. The regular rogueing of infected plants greatly reduced the incidence of African cassava mosaic virus.