The total number of eggs laid by Callosobruchus rhodesianus (Pic.) and Callosobruchus maculatus (F.) females in the presence of single and mixed species populations of males at different densities were counted. Increasing male density significantly reduced the realised fecundity of C. maculatus but not C. rhodesianus females. In the presence of C. maculatus males, however, the number of eggs laid by C. rhodesianus was greatly reduced. The reduction in egg laying and observations on mating behaviour indicates direct physical interference between weevils and competition for mates. An adequate number of beans were provided so competition for oviposition sites by females was unlikely. The ability of C. maculatus to inhibit oviposition of C. rhodesianus is suggested as one reason for its superiority over the latter when the two species compete for limited resources.

The fecundity and duration of oviposition in three strains of Callosobruchus rhodesianus from the highveld, middleveld and lowveld regions of Zimbabwe and a laboratory strain were recorded on cowpea and greengram. Within-legume variability for both parameters was high among the four strains. The lowveld strain was the least fecund and had the longest duration of oviposition. The middleveld strain showed the highest fecundity on cowpea, but did not differ significantly from the other three strains on greengram.

The oviposition behaviour of the cowpea weevils, Callosobruchus rhodesianus (Pic.) and Callosobruchus maculatus (F.) on clean and egg-laden adzuki beans was examined in choice and no-choice experiments. The oviposition rate of C. maculatus was greater than C. rhodesianus in both experiments. Both weevil species were able to distinguish between clean and egg-laden beans. Discrimination by both species was greater against seeds with 12 eggs than with 6, regardless of which species had previously laid eggs. There was a weak indication that the presence of C. maculatus eggs at high densities might reduce the oviposition rate of C. rhodesianus.

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.