Potency of Dioscorea sansibarensis (Dioscoreaceae) Leaf Eluates on Callosobruchus chinensis, Linnaeus 1758 (Coleoptera: Bruchidae) in the Protection of Phaseolus vulgaris (Fabaceae)

Abstract

Post-harvest losses of stored Phaseolus vulgaris to the bean weevil Callosobruchus chinensis have reached levels of significant concern. Governments and health organisations propose the discovery of reliable,  healthy and biodegradable pesticides with higher selectivity and a broad spectrum. This study presents investigations on the activities of Dioscorea sansibarensis leaf extracts on the mortality of C. chinensis and reduction of their egg-laying ability. Laboratory experiments under Completely Randomized Design (CRD) were carried out to determine the mortality and anti-oviposition activity of the vacuum liquid chromatography (VLC) eluates of n-hexane (HE), chloroform (CE), ethyl acetate (EE), n-butanol (BE) and methanol (BE) at different concentrations. Bioassay data were subjected to nonparametric statistical analysis and a generalized linear model at p = 0.05. Statistical results showed that the VLC eluates had a mortality activity of 88.01% (R² = 0.8801). Treatment by 0.025 g of HE and 0.075 g of CE had 9.60 and 11.50, respectively at p = 0.181. These mortality records were high as to 0.1 g of ME, 0.05 g of EE and 0.075 g of BE that recorded 8.55, 8.45 and 7.80, respectively. Treatments by 0.05 g of CE, 0.025 g of HE, 0.05 g of EE and the positive control recorded mortality of 10.50, 9.60, 8.45 and 8.35, respectively. The highest mortality was observed in the treatment by 0.075 g of HE and 0.1 g of HE with 12.85 and 13.70, respectively at p = 0.377. The VLC eluates had an anti-oviposition activity of 24.98% (R² = 0.2498) on the C. chinensis. The generalized linear model reported Wald Chi-Square values of 4.363; p = 0.037, 0.711; p = 0.399, 9.125; p = 0.003, 4.363; p = 0.037 on the treatment by 0.025 g of CE, 0.05 g of EE, 0.075 g of BE and 0.1 g of ME, respectively. At p = 0.051, 0.1 g of CE and the positive control attained oviposition of 89.25 and 96.75 respectively. The study presents the first documentation of the lethal activity of D. sansibarensis on the C. chinensis pulse beetle. This could help in the development of Integrated Pest Management (IPM) and could help in the elimination or suppression of the infestation.

Keywords: anti-oviposition, Callosobruchus chinensis, Dioscorea sansibarensis leaf, mortality, Phaseolus vulgaris, vacuum liquid chromatography (VLC) eluates.