Gamma irradiation as a tool to produce cowpea ( Vigna unguiculate (L.) Walp.) genotypes resistant to aphid pests

Cowpea aphid ( Aphis craccivora Koch.) is an important pest of cowpea. This study aimed to identify aphid resistant cowpea ( Vigna unguiculate (L.) Walp.) genotypes derived from three susceptible varieties widely grown in Zambia (Bubebe; Lutembwe and Msandile) after mutagenesis by gamma radiation. Eleven genotypes derived in this way were evaluated: six (BB3, BB7, BB8, BB10, BB14 and BBV) from Bubebe, three (LT3, LT4 and LT11) from Lutembwe and two (MS1 and MS10) from Msandile. Aphid resistance was evaluated by recording aphid colony growth, mean relative growth rate (MRGR), intrinsic rate of natural increase ( r m ), doubling time (DT) and feeding behaviour when reared on each genotype. Where colony growth was recorded, significantly lower numbers of aphids were recorded on genotypes BB7, LT3, LT4 and LT11 compared to their parents (Bubebe and Lutembwe). Genotypes LT3, LT4 and LT11 also resulted in lower aphid MRGRs, r m and DT compared to the parent. Slower colony growth, MRGRs, r m and DT on genotypes LT3, LT4 and LT11 and slower colony growth only on genotype BB7 suggests the presence of mutation derived resistance to cowpea aphid. Characterisation of feeding behaviour on LT3, LT4 and LT11 using electrical penetration graph recording showed that resistance to cowpea aphid is mediated by epidermal and mesophyll-based resistance factors. BB7, LT3, LT4 and LT11 are therefore promising genotypes that should be evaluated further for genetic improvement of cowpea against the cowpea aphid. This study highlights the potential contribution of induced mutagenesis in the integrated management of aphid pests.