RESPONSES OF 3 POPULATIONS OF THE SEED BEETLE, CALLOSOBRUCHUS-MACULATUS (F) (COLEOPTERA, BRUCHIDAE) TO SEED RESISTANCE IN SELECTED VARIETIES OF COWPEA, VIGNA-UNGUICULATA (L) WALP

The seed beetle, Callosobruchus maculatus (F.), is the major pest of s tored cowpea seeds. It is currently known to exist as geographically d istinct populations (biotypes) which vary in many biological parameter s. Numerous varieties of cowpea which exhibit significant levels of re sistance to a single Nigerian strain of the beetle have been bred at t he International Institute of Tropical Agriculture. In this study, 11 purportedly resistant varieties as well as two susceptible ones were e valuated against three populations of the beetle originating from Came roon, Brazil and Burkina Faso, adopting a stringent experimental proto col. For each biotype, the resistant cowpeas were distinguished from t he susceptible ones in terms of larval survival, developmental period, adult weight at emergence and lifetime female fecundity. These differ ent variables were significantly correlated with each other. Larval su rvival was poorer on the resistant cowpeas but estimation of this and other parameters is subject to variation due to differences in the per iod of observation of infested seeds. Food consumption by each larva w as generally similar in susceptible as well as the resistant cowpeas. Slower development of surviving larvae produced smaller adults which w ere less fecund in some resistant cowpeas but not in others. The Campi nas population appears to be more adversely affected by the resistant cowpeas than the populations from Cameroon or Burkina Faso. It is demo nstrated 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 usin g single populations are of limited or no practical value.