Components of Cowpea Resistance to the Seed Beetle Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae).

Abstract

Cowpea, Vigna unguiculata (L.) Walp., serves as a major source of dietary protein in many tropical and subtropical regions around the world. To identify loci associated with agronomically desirable traits, eight elite cowpea cultivars were systematically inter-crossed for eight generations to yield 305 recombinant inbred lines. Here, we investigated whether these founder parents also possess resistance to the seed beetle Callosobruchus maculatus (F.), a highly destructive post-harvest pest. We estimated larval survival in seeds, egg-to-adult development time, adult mass at emergence, and seed acceptance for oviposition. Survival varied significantly among cowpea cultivars, but the pattern was complicated by an unexpected source of mortality; on three cultivars, mature larvae in a substantial fraction of seeds (20-36%) exited seeds prematurely, and consequently failed to molt into viable adults. Even if such seeds were eliminated from the analysis, survival in the remaining seeds varied from 49 to 92% across the eight parents. Development time and body mass also differed among hosts, with particularly slow larval development on three closely related cultivars. Egg-laying females readily accepted all cultivars except one with a moderately rugose seed coat. Overall, suitability ranks of the eight cultivars depended on beetle trait; a cultivar that received the most eggs (IT82E-18) also conferred low survival. However, one cultivar (IT93K-503-1) was a relatively poor host for all traits. Given the magnitude of variation among parental cultivars, future assays of genotyped recombinant progeny can identify genomic regions and candidate genes associated with resistance to seed beetles.