Field studies to investigate the effect of intercropping on the relative resistance and susceptibility of cowpea cultivars to Maruca testulalis impure stands and intercropped with maize, were conducted at the International Centre of Insect Physiology and Ecology (ICIPE). It was clearly indicated that intercropping affected the relative resistance and susceptibility of cowpea cultivars. The resistance of TVU 946 to Maruca was reduced when the cultivar was under maize. This was attributed to the phenological changes observed, i.e. when planted with maize there was a significant (P = 0.05) increase in length of the pods and peduncles and significant (P = 0.05) reduction in the number of branches. Weekly mean temperatures and relative humidities indicated that there were significant differences between cropping patterns. Similarly there was a significant reduction in the photosynthetic active radiation incident on cowpea canopy in all the intercropped plots. This suggested that both intercropping and microenvironmental differences observed, reduced resistance in cultivar TVU 946 by probably acting directly on the crop, thus rendering it more susceptible.

Four maize varieties, 8321–21-W, DMR-LSR-W, ACR-83TZUT-W and TZB were simultaneously fed to Sitophilus zeamais in a completely randomized design. Susceptibility was assessed on such parameters as weight loss of grains caused by the weevil infestation, number of progenies at 42 days after infestation and the percentage of damaged kernels. The least susceptible in terms of loss of weight of grains was TZB. Losses in the other three were not significantly different at 5% level (P ≻ 0.05).

The least percentage mortality of adult weevils and the highest number of punctured kernels were observed in var. 8321–21-W (48%), 27 kernels respectively while var. DMR-LSR-W had the highest mortality score (80%) and ranked second in the total number of kernels punctured (13). Details of the description of maize varieties used and the analysis of damage caused by the weevil are discussed in the paper.

Sixteen taxa in the genus Vigna were compared for susceptibility to attack by Callosobruchus rhodesianus (Pic.) under laboratory conditions. Ovipositional response, adult emergence and their development periods differed significantly between the species. Although oviposition took place on all the Vigna taxa at varying levels, the bruchid was unable to develop successfully on four of the 16 taxa Investigated. V. vexillata var. angustifolia, V. vexillata var. vexillata, V. oblongifolia and V. luteola were resistant to attack.

Eighteen cowpea varieties were evaluated for their resistance to cowpea weevil, Callosobruchus rhodesianus (Pic) infestation. The experiments were conducted at 27°C and 65–70% r.h. The varieties showed a differential response to infestation by C. rhodesianus with respect to oviposition, egg hatch, adult emergence, adult survival and development period. The intrinsic rates of increase and the indices of susceptibility of the varieties were estimated. The varieties were categorized into three groups of susceptibility (low, medium and high) to C. rhodesianus infestation based on the adult survival data. IT 82D-1064 and IT 81D-1032 were the least susceptible varieties.

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.

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.

Lepidopterous stem-borers are the most serious and widely distributed insect pests of sorghum. Among the several avenues of their control, the use of resistant varieties holds good prospects. In order to identify the sources of resistance to stem-borers (Chilo partellus, Busseola fusca, Eldana saccharina and Sesamia calamistis), a large number of sorghum lines from Kenya, India and Texas, U.S.A. have been screened under a very heavy natural infestation in Western Kenya. Several promising lines with resistance potential have been identified for the stem-borer complex which include: IS 10370, L-2 (TX 2780), IS 10364, IS 1044, S-178, IS 3962, IS 4213, IS 12447, IS 18479, IS 18326, L-l (A&B TX 2756), IS 4405, IS 10711, IS 18676, IS 5613, IS 18517, IS 18323, IS 4881, IS 1151 and IS 18427.