CHEMICAL COMPOSITION AND INSECTICIDAL ACTIVITY OF INDIAN BORAGE ESSENTIAL OIL AGAINST MAIZE WEEVIL

Biopesticide has become more important in stored product pest management, since the use of synthetic insecticides causes adverse effects to human health and the environment. The aim of this research was to determine the chemical composition and toxicity of essential oil from Indian borage, Plectranthus amboinicu (Lour.), leaves against adults of maize weevil, Sitophilus zeamais Motschulsky. The essential oil was investigated by Gas Chromatograph-Mass Spectrometer (GC-MS) technique. The efficacy of this essential oil against S. zeamais using impregnated filter paper and vapor-phase tests. Experiments were performed under Completely Randomized Design (CRD) with 4 replications. Contact toxicity towards maize was investigated at 6 concentrations of essential oil 0 5,000 10,000 15,000 20,000 and 25,000 ppm and fumigant activity was performed using bioassay at 6 concentrations including 0 50 100 150 200 and 250 mu L/L air. Data were recorded as number of deaths maize weevil from 24 to 168 h. The principal compounds in the oil were carvacrol (40.49%), caryophyllene (16.76%), C-terpinene (11.61%), o-Cymene (8.50%), humulene (5.88%), caryophyllene oxide (2.75), p-mentha-1,4(8)-diene (2.17%), and terpinen-4-ol (1.19%). The essential oil of P. amboinicu leaves showed strong contact toxicity at 120 h and fumigant toxicity at 72 h against adults of S. zeamais with 100% adult mortality of 25,000 ppm and 250 mu L/L air, respectively. The results indicated that the essential oil of P. amboinicu leaves showed potent for management of S. zeamais population.