Considering the theme of the study workshop, I would like to emphasize that both integrated control or pest management and host plant resistance date back to the earliest days of agriculture. By its very nature, agriculture is one of the most integrated activities of man. A successful farmer has many skills, which he or she (let us not forget that many farmers are women!) uses in an optimal combination to obtain food and income for the family. Integrated agriculture, control or management are therefore not a recent novelty of agricultural scientists, but rather a necessary response to scientific specialization and subsequent isolation, which has led to some degree of disintegration in modern agriculture. This disintegration is characterized by a tendency for simplification of the agricultural enterprise to the extreme of but one or a few crops, one or a few uniform cultivars and standard fertilizer and pesticide applications. It has been recognized that this development might result in an increased vulnerability and instability of this basic sector of human productivity, especially in parts of the world with a weak infrastructure, where local farmers are not able to respond in time to sudden adversities and are left with unexpected yield losses. Stable and ecologically balanced agricultural production systems are therefore especially essential to small farmers in the so-called Third World. They will benefit more from yield optimization rather than from yield maximization, because the latter is often attended by many disruptive forces.

A keen appreciation of the types and mechanisms of resistance must be attained in order to fully understand and utilize resistant cultivars as effective tools in today's pest management schemes. A historical account of plant resistance terminology is presented. Resistance can be classified as to intensities: immunity; high, moderate and low resistance; or types of resistance: vertical or horizontal resistance. The mechanisms of resistance include: non-preference, antibiosis and tolerance. Examples of the effects of both non-preference and antibiosis resistance on insect numbers are given.

The soybean plant introduction (PI)227687 is highly resistant to 16 different insect pests of soybean in Australia, Brazil, Taiwan and the United States. In most of the pest Lepidoptera, resistance is expressed as greatly increased mortality in the later larval instars. The soybean looper, Pseudoplusia includens (Walker) is most severely affected in the final larval instar. Antibiotic effects include reduced weight gain, growth rate and increased mortality. These effects are enhanced by mechanically wounding foliage 24 hr prior to larval consumption. Antibiosis appears to be due to the combined effects of a feeding deterrent and a growth inhibitor. Resistance is location-specific on (PI)227687 plants and occurs abruptly on the third fully-expanded leaf from the plant apex. Larval growth on leaves of the susceptible variety ‘Davis’ is similar to or greater than that of the upper, susceptible leaves of (PI)227687. Resistance appears to be chemically based, since larvae fed artificial diets containing leaf extracts exhibit antibiotic symptoms similar to those fed fresh (PI)227687 leaves.

The differential attraction by 2 pigeonpea cultivars has been tested through the oviposition behaviour of Heliothis armigera females in a standardized laboratory assay. Seedlings of a susceptible variety (PPE-50) are under choice conditions significantly more attractive at a distance, and without contact allowed other than those of the resistant variety (PPE-45). The volatile signal is contained in the hexane extract from PPE-50 leaves, as shown in the oviposition assay, and after vacuum distillation is present in the more volatile fractions of the mono- and sesquiterpenoids. These compounds, which are also contained in the hexane extracts of 2 more pigeonpea cultivars, a check of medium susceptibility (T-21) and a wild relative (Atylosia scarabaeoides), were compared by capillary gas chromatography. The 4 pigeonpea varieties were characterized by their specific aromagrams, a method which may also advantageously be used in pigeonpea breeding programs.

Suitable winter wheat cultivars of different degree of resistance to cereal aphids, Sitobion avenae F. and Rhopalosiphum pali L. have been studied in the field conditions. The differences in the chemical composition of possible attractants and repellants have been analysed. The results confirm the inter-pendence between contents of yellow and orange plant pigments in the leaves of the studied varieties and their preference by the aphids. The leaves of the susceptible cultivars also had the highest level of sucrose. The number of the aphids was directly proportional to the total free amino acids content. The higher degree of resistance of the cultivars was related to the contents free phenols in their tissues and was also associated with a high value of ‘toxicity index’, which is the ratio of free phenols content to free amino acids content. Also the contents of structural polysaccharides (cellulose, hemicelluloses and pectins) of the investigated cultivars were inversely proportional to the degree of their infestation.

Ethylacetate-soluble fraction of methanol extract of the stems of two cowpea cultivars TVu 946 and Vita 1 were examined by high performance liquid chromatography (HPLC) and tested for antifeedant properties for larvae of Maruca testulalis by applying the extract on the leaf discs from the susceptible cowpea variety Vita 1. Extracts derived from TVu 946 showed significantly greater feeding inhibition than extracts from Vita 1 in all the tests. HPLC analysis gave different elution profiles for the two extracts. Together, these observations implicate allelochemical factors in general, and feeding deterrents in particular, as a basis of TVu 946 stem resistance to Maruca.

Plants express evolutionary adaptations in response to overall environmental stresses (selection pressures), of which insect infestation usually represents only a minor component. The majority of insect-host plant relationships involves adaptations of insect species to existing plants. Over time, plants have evolved a variety of physical and chemical defence mechanisms against environmental stresses. Only some of these are active against insects, and then not against all species. Plants rarely seem to evolve defences specifically against insects. Parameters of plant resistance to parasites are classified as constitutive or inducible. Soybean resistance to agromyzid stem-miners involves both constitutive and inducible components. Expression of such resistance parameters involves controls ranging from single genes (e.g. for trichome traits) to various combinations of genes (e.g. for the metabolic pathway for phenyl-propanoids). Environmental stresses (e.g. high illumination) especially affect the expression of resistance parameters which involve dynamic metabolic pathways. Soybean (Glycine soja) resistance to beanflies is an example of ‘horizontal’ resistance. Thus, the expected stability is long-term. Such stability is attributable to its major parameters being fundamental components of the soybean's primary and secondary growth and differentiation. These kinds of parameters are major products of the soybean's successful evolutionary experience at survival.

Resistant rice varieties have proved to be highly effective against the brown planthopper Nilaparvata lugens (Stål), but their long-term stability is threatened because of the evolution of prolific biotypes. At present, identification of biotypes is based principally on the differential reactions of host rice varieties to pest in greenhouse screenings and in field plantings of test nurseries at many locations. So far, three N. lugens biotypes that attack different rice varieties have been identified in the Philippines. Biotype 1 can survive on and damage only those varieties that do not carry any genes for resistance. Biotype 2 can thrive on varieties carrying Bph 1 resistance gene and on those susceptible to biotype 1. Biotype 3 can infest and destroy varieties having bph 2 resistance gene and those susceptible to biotype 1. However, none of these biotypes is capable of damaging varieties with Bph 3 and bph 4 genes for resistance, and the variety Ptb 33 carrying two unidentified genes is resistant to almost all known biotypes. Recently, a population of N. lugens was found thriving on the common weed grass Leersia hexandra Swartz on the IRRI farm and in some locations in Luzon, Philippines. This biotype does not survive on rice.

Several varieties which are resistant in the Philippines are susceptible in India and Sri Lanka because the South Asian biotypes of N. lugens are more virulent than the Southeast Asian biotypes. To monitor the pest biotypes in different geographical regions and to identify new sources of resistance, an International Rice Brown Planthopper Nursery (IRBPHN) has been established in many cooperating Asian countries.

Biotypes can also be differentiated by studying their behavioural and physiological responses to plants of known genotypes. Distinctive differences can be observed in insect feeding, growth, longevity, fecundity, oviposition and population increase on differential varieties and relative susceptibility or tolerance for hosts' allelochemics. Recent investigations of the existence of subtle morphological, cytological, and electrophoretic differences among N. lugens biotypes and results of hybridization experiments strongly complement other methods of biotype identification.

The evolution of biotypes is a complex process that is governed by the interactions of the genetic and other biological characteristics of the pest populations and the genetic makeup of cultivated varieties. While the strategy of sequential release of varieties with major resistance genes has been fairly successful, the monogenic nature of those varieties makes them vulnerable to the development of virulent pest biotypes. Therefore, present breeding endeavors envisage use of both major and minor resistance genes for effective pest suppression.

The marked differences in the suitability of different Eucalyptus spp. for growth of the larvae of Uraba lugens was further investigated. It is suggested that non-iarval food-plants and poor larval food-plants regularly selected as ‘egg hosts’ may provide a substrate for the development of resistant strains or biotypes assuming the occurrence of matings between individuals that survive and genetic basis for survival in such difficult food-plants.

Differences in the susceptibility or resistance of different plants to an insect are reflected in the magnitude of its population established on them and the resulting damage. The population of an insect on a plant is determined by an interaction between its responses and the plant's characters. As explained previously, the following six main types of responses, operating in as many stages, are involved in the establishment of an insect on a plant: (1) orientation, determining the insect's arrival on or avoidance of a plant in response to its attractant or repellent stimuli; (2) feeding responses, determining the quantity of food ingested from the plant; (3) metabolic responses involving the utilisation of the ingested food and determining the insect's nutrition; (4) development of the insect, if in the larval stage, determined by the quantitative food-intake and nutrition; (5) egg-production in the adult stage, determined by the quantitative food-intake and nutrition; and (6) oviposition.

The first, second and last of the above mentioned responses are behavioural which determine the initial selection or rejection of a plant by an insect. In order, therefore, to understand the principles governing the susceptibility/resistance of plants to an insect species, it is necessary to compare the above mentioned responses to susceptible and resistant plants and, next, to examine the role of the plant characters in determining these responses.

On the basis of the existing literature on the subject, the role of different behavioural responses of insects to plants and their characters in determining their susceptibility or resistance to their insect pests has been considered in this paper.

Factors contributing to host plant resistance in sorghum to Chilo partellus (Swinhoe) were studied. Oviposition on the plants in the field started 3 weeks after germination and peaked 2–6 weeks later. Egg batches (96.1%) were laid on the leaf blades compared with 3.9% on the leaf sheaths. On the leaf blades, significantly more egg batches were laid on the upper (57.2%) than on the lower leaf surfaces. In young plants, leaves attached to the upper and lower segments were preferred for oviposition, while leaves in the upper segments more or less maintained their share of egg batches over time; there was a shift from the leaves in the lower to those in the middle segments with the plant age. Differences among sorghum lines for oviposition were significant and these lines were divided into three categories based on their levels of oviposition.

Sorghum plants reacted to insect feeding in the stem by production of a red staining compound which did not significantly affect the development of the insect. Four damage symptoms were observed and these were related to different plant ages. Plants compensated damage by the production of extra tillers, some of which produced panicles that were ready by approximately the time of harvest. Plants infested 2 weeks after germination produced maximum tillers and juvenile panicles; and also had the least effective tillers, plant heights and grain yields. Grain yields were depressed in the infested plants for the cultivars tested, except in LC 119/80–3 where there were actual grain yield gains.

Leaf-feeding and ‘dead heart’ scores, which were related to reduced plant heights and yields in some of the cultivars tested, are proposed as rational indices for susceptibility ratings and monitoring of crop losses.

The influence of resistance or susceptibility of certain maize genotypes on Chilo partellus colonization was studied. The aspects studied were: oviposition in relation to date of planting, plant age and certain morphological and biochemical characteristics. The pattern of oviposition was similar among the maize genotypes and among the different planting dates. However, the intensity of oviposition differed among the maize genotypes and also with plant age. Plants in the whorl stage of development were the most preferred for oviposition. On the whorl-stage plants, ca 92% of all the eggs were deposited on the lower leaves (1–4 from the ground). As the lower leaves senesced with plant age, the upper leaves were selected for oviposition. On all leaves, the lower surfaces were more preferred. The plant characters that appeared to influence oviposition included plant exudates (as the moth diet) and leaf surface trichomes. The quality of exudates from the different genotypes appeared to influence moth fecundity and longevity, while the trichomes appeared to influence the selection of oviposition sites. The possibility of interactions between these factors and oviposition is discussed.

The ovipositional responses of the stem-borer, Chilo partellus, to the susceptible maize genotype, inbred A, are higher than to the resistant genotype, ICZ1-CM. These responses are elicited by certain characters of the plants that are perceivable prior to the moths' arrival on the plants (distance-perceivable) as well as those perceivable after arrival on the plant (contact-perceivable). Distance-perceivable characters were not found to be responsible for the differences in the oviposition on the two genotypes tested.

The contact-perceivable characters of the two genotypes differ markedly in eliciting oviposition by moths. Contact with the resistant ICZ1-CM elicits much less oviposition than with the other genotype.

This paper discusses insect behaviour and host plant resistance under single-plant species resistance and plant population resistance. Under single-plant species resistance, the effects of repellents, surface texture, shape and colour to insect behaviour are briefly reviewed and discussed. The effects of plant species diversity and plant density on insect behaviour are discussed under plant population resistance. Where possible, this discussion is illustrated with examples from recent behavioural studies on the weevil, Sitona lineatus L.

Electrophysiological tests were done on larval and adult stages of Chilo partellus. The first tarsomere on the forelegs of the moths has three pairs of sensilla; two of them have similar ultrastructural details and they are innervated by gustatory cells which respond equally well to maize leaf exudate but differ in their sensitivity to sucrose. Although the third pair has morphological features resembling those of contact chemoreceptor, it responds to mechanical stimulation only. Taste sensilla on the ovipositor are more sensitive to NaCl than sucrose and they respond to the unknown components in the maize leaf exudate. On the larval maxillae the lateral styloconica sensilla are very sensitive to sucrose and the aqueous extracts from susceptible and resistant maize cultivars; the medial styloconica sensilla are mainly sensitive to NaCl and the aqueous exudate from maize stems. Preliminary tests showed that treated maize piths can provide a suitable medium for the bioassay of larval feeding behaviour.

Insect pests are major yield reducers in sorghum under traditional farming practices in Africa. The use of resistant varieties in an integrated pest management scheme provides the most cost effective and practical means of sorghum insect control. Corneous grain sorghums are more weevil resistant than their softer endosperm counterparts. A wide range of sources of resistance to shootfiy, stem-borer and greenbug are known. Breeding for resistance to sorghum insects has been practised with varying degrees of success. Greenbug resistance breeding in sorghum has been successful in the U.S.A. Stem-borers are perhaps the most serious group of sorghum insects in Eastern Africa and further efforts are needed in breeding for resistance to these pests. In an effective pest resistance breeding work, the breeder and the entomologist must work as a team to make the fastest possible progress. Where several pests are of concern in an area, genotypes with multiple resistance in good agronomic backgrounds are needed.

Genetic variation of resistance and tolerance to stem-borer, Chilo partellus, was studied in three sorghum crosses. Parents, F1 and F2 progenies were evaluated for per cent foliar damage and stem tunnelling as a measure of resistance or susceptibility. The parental cultivars involved in the crosses represented a fairly distinct range of character expression for plant height, foliar damage, stem tunnelling, stem lodging and grain yield. Means, degrees of dominance in F1 and inbreeding depression in F2 suggested that susceptibility was dominant over resistance in crosses susceptible x resistant and susceptible × tolerant, while resistance was dominant over susceptibility in the cross tolerant × resistant. The variation patterns of foliar damage and stem tunnelling in parents, F1 and F2 progenies suggested that they are different and are independently inherited. Both resistance and tolerance mechanisms are operating in sorghum resistance to stem-borers and these mechanisms are independently inherited. Estimates of heritability (broad-sense), genetic coefficient of variation and expected genetic advance, indicated that the cross Serena (tolerant) x IS 2146 (resistant) provides maximum chances of selecting resistant and tolerant plants in the segregating populations. Recurrent selection (population breeding) is suggested to improve the level of stem-borer resistance and tolerance and yield in sorghum.

Pigeon pea (Cajanus cajan L. Millsp.) and cowpea (Vigna unguiculata L. Walp) are the major grain legumes grown in the semi-arid areas of Kenya. Heavy losses in grain yield occur due to their vulnerability to a large number of insect pests in the field as well as damage by weevil during storage. Aphids, thrips, pod-borers and pod-sucking bugs are common insect pests to both pigeon pea and cowpea. Pod fly mostly attacks pigeon pea, and apion beetle is a serious pest of cowpea. Use of chemical control in the semi-arid areas is expensive and beyond the reach of the small-scale farmers. Work on development of less susceptible or tolerant crop varieties has been initiated at the National Dryland Farming Research Station (NDFRS) at Katumani. Results of preliminary screening against major insect pests are reported in this paper.

The sources of resistance against aphids and thrips in cowpea, were received from the International Institute of Tropical Agriculture (UTA), Nigeria, and information on their reaction to these insects under field conditions has been discussed. In pigeon pea, resistance against pod-sucking bug, which appear to be of the antibiosis type, has been identified in the early maturing group of the local germplasm. Implications of breeding for insect resistance and pest management in the two crops are discussed.

The information available on the genetics, breeding, and utilization of host plant resistance to 14 of the most important species of maize stem-borers complex, their biology, and the damage done to the crop is presented.

The components necessary for implementing a programme for breeding for resistance to stem-borers in maize include: a colony of the pest(s), the capability to mass rear them, uniform artificial infestation methods, an array of the available maize germplasm or related species, rating scales and evaluation methods that permit identification of genetic resistance, knowledge of screening and breeding methods, and the personnel and resources to execute the programme.

The paper having emphasized the importance of each component, concludes with a discussion of several HPR programmes and the breeding methods being used in various areas of the world, to develop and utilize resistance to one or more maize pests, as a proven method for gaining increased management control over these pests.