Toxicity and Residue of Mexican mint (Plectranthus amboinicus Lour.) essential oil as a bio-insecticide on cowpea beetle (Callosobruchus maculatus Fabricius)

Abstract

"Cowpea (Vigna unguiculata L.) (Walp)) is one of the legumes grown worldwide due to its high value in protein. However, the crop faces insects` attack from the field to storage especially Callosobruchus maculatus (cowpea grain beetle) which has brought huge economic losses in storage. Insect pests of cowpea had been controlled with various synthetic insecticides but with hazardous effects on human health and the environment. This study assessed plant essential oil which could be used as an alternative to control stored cowpea grains beetles. Therefore, the objectives of this study were: To identify the compounds present in Mexican mint as a bio-insecticide, to determine the residue of the Mexican mint essential oil in treated cowpea grains and to determine the proximate analysis of treated cowpea grains. The study was conducted at Entomology Research Laboratory Department of Crop Protection and Environmental Biology, University of Ibadan using a susceptible cowpea variety Ife Brown cowpea grains. Six treatments: 0.75, 1.25, 1.75 and 2.25 mL/g of Mexican mint essential oil, Phostoxin (0.01g recommended) and without treatment (control) were replicated four times and laid out in a completely randomized design. The essential oil extraction, the essential oil residue in cowpea grains and proximate analysis were determined using standard procedures. Data were collected on mortality of insects corrected with Abbott’s formula and analysed using ANOVA at p<0.05. Results revealed 26 compounds with two prominent compounds identified; monoterpenes (0.22-6.16%) and terpenes (0.28-4.16%). Toxicity of Mexican mint essential oil concentrations on C. maculatus gave a 93.75% mortality rate at 1.25, 1.75 and 2.25 mL/g, respectively which compared favorably with phostoxin (100%), while the control gave highly significant reduction of 12.50%. Compounds found in the cowpea grains residues showed no trace of toxicity which were: alpha. -Pinene (144.98-346.79 mg/L); tau. -Muurolol and Linalool (0.22-0.54mg/L), Di- epi-1,10-cubenol (15.38mg/L) and alpha.-Cadinol (0.35mg/L), respectively from 1.25, 1.75 and 2.25 mL/g. Proximate analysis of treated cowpea grains revealed essential oil at 0.75 mL/g having highest value of 29.75% in crude protein, highest (91.00%) dry matter was obtained from 2.25 mL/g, similar values of 10.75 and 11.14% moisture content was recorded from 0.75 and 1.25mL/g, respectively. Highest similar values ranging from 54.16-54.56% of carbohydrate were obtained from 1.25, 1.75 and 2.25 mL/g, respectively. This study revealed that Mexican mint essential oil at 1.25, 1.75 and 2.25 mL/g contained important insecticidal components, which enhanced high toxicity on Callosobruchus maculatus on stored cowpea grains, with no toxic residue of the essential oil in the cowpea grains and no harmful effects on the nutritional components. Therefore, Mexican mint essential oil could give food quality and safety when used as a bio- insecticide to protect cowpea grains in storage.