Essential oil extract from Moringa oleifera roots as cowpea seed protectant against cowpea beetle
Cowpea (Vigna unguiculata (L.) Walpers), is an important food legume in drier regions of the world, which is threatened by cowpea weevil (Callosobruchus maculatus), which is a cosmopolitan field-to-store pest of cowpea with infestation usually starting from the field. Cowpea weevil causes quantitative and qualitative losses, manifested by seed perforation, reductions in seed weight, reduced income to households, reduced market value and low seed germination. The objective of this study was to evaluate the efficacy of Moringa oleifera (Lam.) roots essential oil extract on the survival of Callosobruchus maculatus. Behaviour of Hexane extract of powdered Moringa oleifera roots was obtained by the Soxhlet extraction method. Moringa roots oil extract was applied at dosages of 0.5 l, 1.0 l, 1.5 and 2.0 ml per 20.0 g of Ife Brown Peduncle cowpea (susceptible variety) seeds. Dichlorvos (DDVP: 2,2-dichlorovinyl dimethyl phosphate) was included as a check. It was applied at dosage of 2.0 ml per 20.0 g cowpea seeds. Moringa roots oil extract at 2.0 ml per 20 g cowpea seeds resulted in significantly (P<0.05) higher mortality of 98.1% compared with Dichlorvos (89.9%). Also, 2.0 ml of root extract applied on the dorsum of adult insects (topical application) resulted in the highest mortality (100%) at 72 hours after application, compared with Dichlorvos (99.2%). There were significant reductions in the number of eggs laid in the order of 7.5 (0.5 ml), 8.0 (1.0 ml), 11.8 (1.5 ml) and 6.5 (2.0 ml); compared with Dichlorvos (20.8). Significantly fewer adults emerged from 0.5 ml (20.3), 1.0 ml (15.5), 1.5 ml (11.8) and 2.0 ml (9.0) root extracts than with Dichlorvos (37.0). Similarly, oviposition, contact toxicity and mortality were dose dependent; and higher concentrations significantly protected cowpea seeds against damage by C. maculatus. There was no significant difference in seed weight loss. Also, cowpea seeds coated with moringa roots oil extract germinated normally. The roots oil extract at 2.0 ml per 20.0 g cowpea seeds reduced damage by C. maculatus and was effective in controlling C. maculatus infestation on stored cowpea seeds.
Keywords: 2,2-dichlorovinyl dimethyl phosphate, contact toxicity, Moringa oleifera, oviposition, Vigna unguiculata