Seed coat texture, host species and time of application affect the efficacy of essential oils applied for the control of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) in stored pulses

Abstract

The effects of the texture of seed coats of three pulse cultivars, of which two, TVu12349 (wrinkled cowpea) and Borno brown (rough cowpea) belonged to Vigna unguiculata, and one, Bunmonu (smooth bambara groundnut) belonged to Vigna subterranea, as well as pre-and post-oviposition application on the efficacy of the essential oils of clove (Syzgium aromaticum), West African black pepper (WABP) (Piper guineense) and ginger (Zingiber officinale), against oviposition and development of Callosobruchus maculatus, were investigated under prevailing storage conditions (32-35°C and 40-50% RH) and 24 h darkness in Maiduguri, Nigeria. Seed coat texture influenced the bioactivity of the essential oils, especially clove oil, against oviposition and the proportion of C. maculatus adults that emerged significantly more than host species: the mean numbers of eggs laid on the smooth-seeded Bunmonu, the rough-seeded Borno brown and the wrinkled TVu12349 were 16.9, 16.7 and 24.2%, respectively; adult emergence was 31.1% in the wrinkled TVu12349, 45.3% in the rough-seeded Borno brownand 10.2% in the smooth-seeded Bunmonu. The smooth-seeded Bunmonu reduced oviposition in seeds treated with clove oil by 91.4% and 98.4% relative to the mean number of eggs laid in the rough-seeded and the wrinkled Tvu12349, respectively; comparable figures of oviposition reduction by Bunmonu in untreated seeds were 4.8 and 15.9% relative to the mean number of eggs laid in Borno brown and Tvu12349, respectively. Both pre-and post-oviposition application of the essential oils significantly suppressed oviposition and/or adult emergence. In the pre-oviposition application, treatment of bambaranut seeds with clove, WABP and ginger oils at the rate of 1 mg/5 g seed reduced the mean number of eggs laid by C. maculatus by 70.7, 98.7 and 86.2%, respectively, relative to the number of eggs laid in untreated seeds. Post-oviposition treatments were, however, more effective on average, than pre-oviposition treatments in reducing the proportion of C. maculatus adults that emerged. In the post-oviposition application, no adult C. maculatus emerged in bambaranut seeds treated with clove oil at the rate of 0.5 or 1 mg/5 g seed.