Bionomic variation among populations of the southern cowpea weevil, Callosobruchus maculatus, and their responses to different varieties of the primary host

Abstract

Geographical variation among three "strains" of Callosobruchus maculatus F. (Coleoptera : Bruchidae), a cosmopolitan pest of stored legume seeds, has been studied. The fecundity, fertility and adult productivity of the strains were measured on seeds of the primary host, the cowpea. The number of eggs laid by females of each strain depended on the number of cowpeas to which she had access. Females of the Nigerian strain laid more eggs on a small number of cowpeas than females of the strains from Yemen and Brazil. The Yemen strain produced fewer adults from individual seeds than the other two strains when similar numbers of hatched eggs were present. Experiments were performed to examine this difference. The results indicated that Yemen individuals were heavier, on average, and consumed a larger quantity of cowpea during development than individuals of the Nigerian or Brazilian strains.

All three strains crossed to produce viable and fertile progeny. Hybrids of crosses between the Yemen strain and either of the two smaller strains were of intermediate size and were produced in numbers which were intermediate to those of the two strains involved.

The response of the three strains to a cowpea variety (TVu 2027) known to exhibit resistance to bruchid attack was measured. The Brazilian strain suffered much higher mortality on TVu 2027 than the other two strains. The Yemen strain suffered slightly higher mortality than the Nigerian strain, but the mean development period of the Yemen strain was shorter. Progeny of inter-strain crosses suffered an intermediate level of mortality on TVu 2027. The proportion of Yemen and Nigerian individuals which could survive on TVu 2027 increased when successive generations were bred on the resistant variety. The consequences of geographical variation in this beetle pest, for the success of new cowpea lines which incorporate the resistant properties of TVu 2027, are discussed.