Repellent and Insecticidal Properties of Scent Leaf-orange Peel Extract as an Ecofriendly Approach in the Control of Cowpea Weevils (Callosobruchus maculatus)
Esther Ogechi Enechukwu *
Department of Food Chemistry, Centre of Food Technology and Research Rev. Fr Moses Orshio Adasu University, Makurdi Benue State, Nigeria.
Isaac Gbaa Agbidye
Department of Chemistry, Rev. Fr Moses Orshio Adasu University, Makurdi Benue State, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
Cowpea (Vigna unguiculata) is an important food crop in Nigeria, but its storage is threatened by the cowpea weevil (Callosobruchus maculatus). Dependence on synthetic insecticides can result in pesticide residues, pest resistance, environmental contamination, and risks to human health. This study investigated the insecticidal and repellent activities of ethanolic extracts of scent leaf (Ocimum gratissimum) and orange peel (Citrus sinensis), applied individually and in combination against C. maculatus. Orange peels and scent leaves were separately macerated in ethanol for seven days, while the chemical constituents of the extracts were characterised using gas chromatography-mass spectrometry. Extraction yielded 6.67 g of orange-peel extract from 100 g of sample and 17.5 g of scent-leaf extract from 150 g. Both extracts exhibited concentration- and time-dependent insecticidal activity, although scent leaf generally performed better than orange peel when applied alone. At 84 h, mortality reached 76.65% with the highest orange-peel concentration and 79.98% with the highest scent-leaf concentration. The combined extracts produced greater activity: OP4SL4 achieved approximately 99.83% mortality at 48 h and 100% at 72 and 84 h. Repellency was also strongest with the combined extracts, with OP4SL4 reaching approximately 100% by 6 h. At 24 h, the lethal doses for OP4SL(1-4) were LD10 = 2.053 mg/mL, LD30 = 3.078 mg/mL, LD50 = 4.062 mg/mL, LD90 = 8.035 mg/mL, and LD99 = 14.064 mg/mL. The combined treatment produced the lowest lethal-dose estimates, indicating enhanced potency. These findings indicate that the scent leaf-orange peel extract, particularly OP4SL4, has potential as an eco-friendly botanical treatment for protecting cowpea.
Keywords: Cowpea, Callosobruchus maculatus, Ocimum gratissimum, Citrus sinensis, botanical insecticide, ethanolic extract, mortality, repellency, lethal dose, stored-product pest