Susceptibility of unprotected seeds and seeds of local bambara groundnut cultivars protected with insecticidal essential oils to infestation by Callosobruchus maculatus (F.) (Coleoptera: Bruchidae).

Ten local cultivars of bambara groundnut, Vigna subterranea (L.) Verdcourt obtained directly from farmers in Potiskum, Nigeria and from the Institute for Agricultural Research, Samaru, Nigeria were compared with three improved varieties developed at the International Institute of Tropical Agriculture, Ibadan, Nigeria for their susceptibility to infestation by Callosobruchus maculatus (F.). Three cultivars (Maifarinhaneh, Angale and Bunmonu) with a susceptibility index (SI) of 3.06-3.71 were identified as slightly susceptible to C. maculatus; four cultivars (Bagantere, Bakingangala, Ole and Bakiyawa 1) and one improved variety (TVSU 1061) with an SI of 4.39-4.93 as moderately susceptible; and three cultivars (Bidi, Uzu and Dadinkowa 1) and two improved varieties (TVSU 702 and TVSU 751) with an SI of 5.00-5.34 as susceptible. Five of the cultivars were used to examine the ability of beetle populations to overcome varietal resistance over six successive generations. Development time was significantly longer but percentage of adults that emerged and susceptibility of bambara groundnuts were significantly lower in F(4), F(5) or F(6) generations than in the F(1) or F(2) generation. The efficacy of combining insecticidal essential oils obtained from clove, Syzgium aromaticum, West African black pepper (WABP), Piper guineense, and ginger, Zingiber officinale applied at the rate of 2 mg/20 g seed and six of the local bambara groundnut cultivars (Angale, Maifarinhaneh, Bakingangala, Bagantere, Bunmonu and Bidi) with differing susceptibilities to C. maculatus (F.) was also assessed during a 3-month storage period. The three essential oils significantly reduced the percentage of C. maculatus adults that emerged from the bambara groundnut cultivars in the F(1) generation and the number of adult offspring that developed in the cultivars during the 3-month storage period. The mean number of progeny that developed in untreated seeds and seeds treated with clove, WABP and ginger oils during the study period were 73.0, 0.0, 0.1 and 0.2, respectively. No adult C. maculatus, however, developed in slightly and moderately susceptible cultivars treated with essential oils. Treatment of seeds of Bidi (a susceptible cultivar) with the essential oils reduced the percentage of adults that emerged in the F(1) generation from 26.8% in untreated seeds to 0.0, 0.1 and 0.4% in seeds treated with clove, WABP and ginger oils, respectively; and reduced loss in seed weight after three months' storage from 34% to 0.0, 0.01 and 0.1%, respectively.