Responses of three populations of the seed beetle, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae), to seed resistance in selected varieties of cowpea, Vigna unguiculata (L.) Walp.

Abstract

The seed beetle, Callosobruchus maculatus (F.), is the major pest of stored cowpea seeds. It is currently known to exist as geographically distinct populations (biotypes) which vary in many biological parameters. Numerous varieties of cowpea which exhibit significant levels of resistance to a single Nigerian strain of the beetle have been bred at the International Institute of Tropical Agriculture. In this study, 11 purportedly resistant varieties as well as two susceptible ones were evaluated against three populations of the beetle originating from Cameroon, Brazil and Burkina Faso, adopting a stringent experimental protocol. For each biotype, the resistant cowpeas were distinguished from the susceptible ones in terms of larval survival, developmental period, adult weight at emergence and lifetime female fecundity. These different variables were significantly correlated with each other. Larval survival was poorer on the resistant cowpeas but estimation of this and other parameters is subject to variation due to differences in the period of observation of infested seeds. Food consumption by each larva was generally similar in susceptible as well as the resistant cowpeas. Slower development of surviving larvae produced smaller adults which were less fecund in some resistant cowpeas but not in others. The Campinas population appears to be more adversely affected by the resistant cowpeas than the populations from Cameroon or Burkina Faso. It is demonstrated that the design of assays for resistance needs great care and can affect conclusions. The responses of populations (biotypes) of C. maculatus to cowpea varieties show such variability that studies using single populations are of limited or no practical value.