Evaluation of F3 generations of the hybrid crosses between vegetable and field cowpeas and their parents for resistance to the cowpea bruchids (<i>Callosobruchus maculatus</i> (F))

Abstract

An evaluation of crosses between vegetable and field cowpea for resistance to the cowpea Bruchids, [Callosobruchus maculatus (F)], was carried out at the Enugu State University of Science and Technology, Enugu southeastern Nigeria. There was a significant difference in the field infestation of the cowpea lines by C. maculatus. AD-36-WB had the highest percentage field infestation by the insect pest, whereas, AE—36-C X AN-16-D, and AN-14-D were free from the field infestation by the pest. Vegetable cowpea lines tested had some levels of resistance to the cowpea Bruchid and the gene responsible for the resistance were successfully transferred from vegetable cowpea to the susceptible field cowpea by cross-breeding techniques. No vegetable cowpea tested had complete resistance to the cowpea bruchid, however, they had lower susceptibility than the field cowpea tested. Reduction in the relative susceptibility of the cowpea to C. maculatus was achieved by cross breeding field cowpea with vegetable types. For instance, in crosses between AD-36-W (field cowpea) and AE-36-C (vegetable cowpea), AD-36-WB (field cowpea) and AN-14-D (vegetable cowpea), AD-36-W (field cowpea) and AE-36-F (vegetable cowpea), AD-36-WB (field cowpea) and AE-36-C (vegetable cowpea), AN-14-D and AE-36-F reduced susceptibility of AD-36-W and AD-36-WB to C. maculatus from 63.59% to 51.09%, 68.46% to 59.09%, 63.59% to 47.38% and 68.48% to 38.84% respectively. Also, gene responsible for susceptibility to the storage insect pest were successfully transferred from field cowpea to the vegetable cowpea through cross-breeding techniques. A cross between two vegetable types namely; AE-36-C and AN16-D had the least susceptibility level of 24.32%. There was a significant difference in mean number of eggs laid per cowpea line with AD-36-WB recording the highest mean number of 134.75 eggs and AE-36-F recording the least mean number of 4.75 eggs. There was also a significant difference in the percentage adults emergence per cowpea line with AD-36WB having the highest percentage of 68.46% adults, and AE-36-C X AN-16-D having the least percentage of 24.32% adults. The result of the experiment also showed a significant difference in mean longevity of female adults reared from cowpea lines with those reared from AD-36WB X An-14-D, and AE-36-C having the highest mean longevity of 14 days each whereas those from AD-36WXAE-36-C had the least mean longevity of 5.6 days. Also, there was a significant difference in mean testa thickness of the seeds tested with AE-36-C having the highest mean testa thickness of 0.20mm, whereas AD-36-WB had the least mean testa thickness of 0.02mm. However, there was no correlation between either testa thickness or moisture content and levels of infestation by the storage insect pest. Again, there was a significant difference between mean weight loss of seeds tested after infestation by C. maculatus, with AD-36WB recording the highest mean weight loss of 0.38g, whereas AE-36-C and AE-36-F, had the least mean weight loss of 0.07g. There was a positive correlation between the weight loss of the adult C. maculatus and the weight (g) of the seeds from which the insect pest was reared (r = 0.03).

Key words: Cowpea lines, cross-breeding, Callosobruchus maculatus, resistance, susceptibility