The aim of this study was to evaluate the effects of silicon application and administration of the phytohormone gibberellic acid on resistance of the corn plants to the fall armyworm (FAW), Spodoptera frugiperda, and their vegetative characteristics. We evaluated larval and pupal duration, survival and biomass, and adult longevity, malformation and fecundity of S. frugiperda after feeding on plant matter treated with silicon and/or gibberellic acid. The feeding preference of FAW first-instar larvae, the total leaf area consumed by the insects, and the vegetative parameters of corn plants were also evaluated. No significant differences were observed in the measured parameters of larval and pupal stages of S. frugiperda in response to silicon or gibberellic acid. In adult stage insects, the number of eggs per female was significantly reduced in insects derived from larvae fed plants treated with silicon or gibberellic acid. In a non-preference test, 48 h after release, caterpillars preferred control untreated plants and consumed less matter from plants that had received hormonal treatment (gibberellic acid). Gibberellic acid also altered the vegetative characteristics of plants, by increasing their height, shoot fresh and dry mass, and silicon content. We conclude that gibberellic acid can alter the vegetative characteristics and silicon uptake of corn plants, leading to a reduction in their consumption by S. frugiperda larvae and a decrease in female insect oviposition.

Cowpea, Vigna unguiculata (L.) Walp., is susceptible to a wide spectrum of insect pests from seedling to harvest as well as in storage. Insecticide use has been the most commonly adopted method for the control of cowpea pests. However, due to the deleterious effects of insecticides on man, livestock and the environment, research over the years has been focused mainly on developing resistant/tolerant cultivars as a vital component in the integrated management of cowpea pests. This approach has, however, recorded varying degrees of success, due in part to the multiple and often overlapping pest complex attacking at different phenological growth stages of the crop.

In spite of the progress in the utilisation of insect-resistant cowpea varieties, the use of resistance as a control tactic has by no means been fully realised. There are many important cowpea insect pests for which resistant varieties are not yet available for commercial cultivation. Where resistant varieties are available for a given insect, these varieties may not be widely planted because of their unsuitability to local conditions.

In this paper, the status of host plant resistance research in cowpea is reviewed and some novel approaches aimed at solving its seemingly intractable pest problem are proposed, based on the state-of-the-art scientific and crop biotechnological development. The proposed framework for orientation of cowpea resistance research should ensure yield stability and sustainability for the resource-poor, smallscale farmers in Africa.

Eight desi chickpea genotypes of mid-maturity group were screened by using an open-field screening technique. The genotypes ICC-3137, K-850 and ICC-1403 were found to be more susceptible than the others. More eggs were laid and more larvae were recorded on more susceptible genotypes. A low amount of acidity in the leaf extracts of genotypes was found to be associated with susceptibility to gram pod borer, Heliothis armigera (Hubner).

The field experiments were conducted during the summer of 1985 to determine the degree of winter wheat resistance to the grain aphid Sitobion avenae. Asta, Grana, and Saga cultivars were generally more resistant than Dana, Liwilla, and Emika. The resistance mechanism involved both antixenosis and antibiosis. Grana was least preferred during aphid migration. Asta, Saga, and, to a lesser extent, Grana had an antibiotic effect on growth and development of the apterous aphids (longer prereproductive period, lower fecundity and lower values of the intrinsic rate of increase, rm, during the first days of reproduction). Moreover, var. Asta showed a “secondary antixenosis” during colonization of ears due to the presence of awns.

Sixty-four sorghum accessions were screened for resistance to the grain midge, Contarinia sorghicola Coq., under natural infestation conditions at Tamil Nadu Agricultural University, Coimbatore during 1982. Absolute resistance was not found in any of them. Low grain damage was recorded in PVK 50 (6.61%), PVK 55 (11.58%), AF 28 (17.03%) and DJ 6514 (17.05%) while the susceptible accession, SPV 234 recorded high grain damage (92.72%) in the screening trial. The ovipositional preference studies at insectary revealed significantly reduced grain damage in the resistant accessions. Among them, the lowest damage (8.33%) was recorded in PVK 50 followed by AF 28, DJ 6514 and PVK 55 while the susceptible accession, SPV 234 recorded 21.57% grain damage. In the choice experiments, SPV 234 had 11.22–35.36% more infestation than in the resistant accessions evaluated along with it. Studies on development and biology of the midge revealed that antibiosis is one of the mechanisms of resistance in the lines, PVK 50, DJ 6514, AF 28 and PVK 55.

The shootfly, Atherigona soccata Rondani, is a major seedling pest of sorghum in many areas of tropical Asia, Africa and Mediterranean Europe. A chronological review of the progress made in various programmes on the screening techniques (natural and artificial) with artificial diets, sources, stability and mechanisms of resistance, bases (physico-morphological and biochemical factors) of resistance, inheritance and breeding for resistance is made. Many resistant varieties have been identified and sources of resistance utilized in breeding programmes to transfer resistance in high yielding background are listed. In addition, the glossy and trichome traits identified is sorghum cultivars are also reported. Glossy leaf surface and trichomes are frequently associated with shootfly resistance.

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.

Sorghum shootfly, Atherigona soccata, oviposition and infestation on eight resistant sorghum cultivars (IS nos: 2122,2123, 2205, 2291, 4660, 5092, 5480 and 18551) and two susceptible cultivars (Serena and CSH-1), were studied under field conditions (natural infestation). The eight resistant cultivars and CSH-1 were also tested for preference for oviposition in a two-choice situation in the screenhouse. The eight resistant cultivars received considerably less shootfly eggs and showed fewer “dead hearts” than the susceptible cultivars. The results indicate that non-preference for oviposition is an important mechanism for resistance to shootfly in eight test cultivars. There was also evidence for a certain degree of antibiosis in IS 2291.

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.

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 effect of phenolic compounds on the preference for winter wheat cultivars as hosts by cereal aphids and the effect of the phenolics, reported in the leaves of cereals, on feeding intensity of Rhopalosiphum padi (L.) larvae was investigated. Preference for the studied winter wheat cultivars was associated with the level of phenolic compounds naturally occurring in the plants. The tested dihydroxyphenols showed highest inhibition of the feeding of R. padi larvae.

Field observations on Chilo infestation of 100 promising sorghum lines showed that there are six levels of relationship affecting plant colonization and damage level by stem-borers: non-preference for oviposition (E 302, E 303, IS 1044, IS 1151, IS 2162, IS 4660, IS 17739, IS 18328, IS 18349, IS 18479, and IS 18489); feeding of the first larval instars on young leaves (reduced on E 302, E 303, IS 1044, IS 2162, IS 2209, IS 18349 and IS 18427); causing of ‘dead hearts’ by young larval instars due to extensive feeding in stems of young plants (lower for IS 2162, IS 2263, IS 18328, IS 18349); tunnelling (extent and location) of older instars in stems (low in IS 4660, IS 18427 and IS 18479); tolerance of plants to leaf damage (in spite of high damage, plants form panicles) (IS 2205); and tolerance of plants to larval feeding in stems (in spite of high tunnelling, the plants form seeds). The effect of a number of cultivars showing different levels and mechanisms of resistance on the Chilo behaviour and biology was studied in depth under field conditions.

Control of cowpea insect pests has been almost entirely achieved through the use of chemicals by progressive farmers. However, the peasant farmer has traditionally tended to rely on chance and has often been unable to obtain yields that would be profitable. This paper examines the farmer's dilemma and the jrossibility of using host plant resistance (HPR) as an adjunct to other control methods in developing a sound integrated pest management of cowpea pests. The status of HPR in cowpea is reviewed with a view to identifying the areas where progress has been made with respect to the measurement and identification of resistance to formulate an effective integrated pest control program. The difficulties in achieving this are discussed in the light of our present data bank and the diversity of cowpea pests.

Three cowpea cultivars, TVu 946, Ife Brown and Vita 1 were examined for resistance to the legume pod-borer, Maruca testulalis (Geyer). TVu 946 was found to be the most resistant in both the flowering and pre-flowering stages, followed by Ife Brown and Vita 1, respectively. Field and screen-house experiments show that Maruca larvae caused significantly less damage on TVu 946 than on Ife Brown and Vita 1. In addition, larval survival and development on TVu 946 was the most negatively affected. Screenhouse experiments in a choice situation clearly show non-preference of TVu 946 for oviposition by adult Maruca females, and biochemical studies show evidence of antibiosis due to nutritional and antibiotic factor(s).

The results of five experiments on maize resistance to Chilo partellus, suggest that there are at least five levels of relationships affecting plant colonization, larval survival and damage level: non-acceptance for oviposition shown by Inbred D and G and CIMMYT lines 125, 33, 324 and 178 (s3). Preferred Inbred A was oviposited 2–3 times higher. Feeding of young larval instars on young leaves or leaf sheaths, reduced on CIMMYT lines 324, 125 and 33; concentration of feeding of young larval instars on the upper growing part of plant, extensive on the highly susceptible Inbred A and reduced on all resistant lines tested; penetration of young larval instars into stem, reduced on CIMMYT lines 34 and 125 and Inbred G; and feeding of older larval instars in stem, reduced on CIMMYT lines 324, 178, 125 and Inbred G.

Sorghum (Sorghum bicolor) is an important food crop, especially of the semi-arid areas of Asia and Africa. One of the major constraints affecting the popularity of high-yielding hybrids and varieties is their extremely high susceptibility to insect pests.

Painter (1951) has reviewed in detail the work done on resistance in sorghum to the chinch bug (Blissus leucopterus), corn leaf aphid (Aphis maidis) and sorghum midge (Contarinia sorghicola). In the last decade and a half considerable work has been done on resistance to the shootfly (Atherigona seccata), stem borer (Chilo partellus) and sorghum midge (Contarinia sorghicola). By conducting multilo-cation screenings of world germplasm under natural and artificial infestation conditions, not only have promising sources of resistance been identified but a number of them have been utilised in crossing programmes to develop high-yielding resistant cultivars. Some sources with multiple resistance have also been identified.

The mechanisms of resistance in sorghum to the major pests have been investigated. It is seen that the mechanisms are generally complex and the observed field resistance is the total reaction of several components. There are several leads available which justify further intensive work, especially on identifying naturally occurring plant products associated with antibiosis mechanism.