Selection of a Cowpea Weevil (Coleopera: Bruchidae) Biotype Virulent to Cowper Weevil Resistant Landrace TVu 2027

Abstract

Studies of Cowper weevil, Callosobruchus maculatur (F.), larvae in seeds of resistant (Cowper land race) TVu 2027 and susceptible ‘California Black eye No. 5’ (CB5) revealed that 10-15% of the Cowper weevil population develop at the same rate in TVu 2027 as in CB5 seeds. When adults having the same developmental rates on TVu 2027 as CB5 were selected and inter crossed for 8 generations, there was no significant difference in within-seed developmental time between the selected and un selected populations. Therefore, these rapidly developing individuals do not represent an emerging Cowper weevil biotype but may result from a chance avoidance of the resistance factor by individual Cowper weevil larvae, coupled with seed-to-seed variation for the resistance factor. However, we were able to develop a Cowper weevil biotype virulent to TVu 2027 by selecting and inter crossing the most rapidly developing individuals over 53 Cowper weevil generations. After 53 generations of selection the Cowper weevil biotype developed as rapidly in TVu 2027 seeds as did the unselected population in susceptible CB5 seeds. The biotype developed at the same rate on Cowper weevil-resistant KNS and KNW lines as on TVu 2027, confirming that the basis of resistance is the same in all 3 lines. Also, biotype virulence to KNS and KNW suggests that the biotype can be used to identify Cowper lines with the TVu 2027 source of resistence.