Environmental conditions affect the response of West African Callosobruchus maculatus (Coleoptera: Bruchidae) populations to susceptible and resistant cowpeas

Abstract

Development and release of resistant cowpea varieties represents an attractive alternative to conventional chemical methods for control of Callosobruchus maculatus (F.), a major pest of stored cowpea, Vigna unguiculata (L.) Walpers, across sub-Saharan West Africa. However, laboratory studies on geographically distinct populations have repeatedly highlighted significant intraspecific variation in performance on resistant cowpeas, which could interfere with their effectiveness as a control method. In this study, the effect of different environmental conditions on population performance on susceptible and resistant cowpeas was investigated to determine whether they could also affect the degree of control achieved in different areas. Seven Nigerian populations were screened against cowpeas under two environmental regimes (Condition A—26°C, 75±5% relative humidity (r.h.), LD11.5:12.5 h, and Condition B—30:15°C (11.5:12.5 h), 30±5% r.h., LD11.5:12.5 h), designed to simulate field conditions during the storage period in different regions of Nigeria. Independent of environmental conditions and seed variety, significant differences among populations were expressed in terms of embryonic mortality, post-embryonic mortality, development time and adult weight. Resistant seeds delayed development, resulted in protracted adult emergences and increased post-embryonic mortality. Environmental conditions are shown for the first time to have a significant effect on population performance on resistant seeds, principally on development time and post-embryonic mortality. Effects on these parameters were not the same; rapid development was favoured under Condition A, although mortality was higher under this regime. The effects of resistant seeds and environmental conditions on performance could not be generalised across biological traits. Furthermore, significant interactions between environment and seed variety and between environment and population, were identified in certain parameters, suggesting performance on a resistant variety may vary significantly across populations, with seed variety and in response to differences in environmental conditions. It is suggested that the only reliable way of predicting the effectiveness of resistant varieties in the field would be to test them against local populations in the areas targeted for release, under local environmental conditions.