Evaluation of relative resistance of four cowpea (Vigna unguiculata (L.) Walp) varieties to the black cowpea aphid (Aphis Craccivora) in South-eastern Botswana

Abstract

Cowpea (Vigna unguiculala (L.) Walp) is an important food legume in many tropical countries, but its production is seriously constrained by insect pests. Aphis craccivora is one of the major pests of cowpea worldwide. Abundance of the pest was assessed on four cowpea varieties, Black eye, B359, IT18 and Tswana in field cages in order to determine their relative resistance to the pest in southeastern Botswana. The data was analysed using MSTAT-C programme package. Aphid abundance varied significantly between varieties (P < 0.05). Overall abundance was greatest (95.4 aphids per plant) on Black eye and least (17.7 aphids per plant) on IT18 plants. Therefore, cowpea variety IT 18 was 5.3 times more resistant to the black cowpea aphid than variety Black eye. Using relative resistance to A. craccivora., the varieties were ranked in the order: IT18 > B359 > Tswana > Black eye. A relatively higher proportion (53%) of alates than apterae was produced on variety IT 18. Variety IT 18 showed a combination of antibiosis and antixenosis mechanisms of resistance against the pest. This variety would be the most suitable for use in breeding programmes aimed at developing more resistant cultivars to the cowpea aphid.