Control of stored grain pest, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) using the essential oil isolated from Plectranthus zeylanicus

The bruchid, Callosobruchus maculatus (F.) causes major losses during the storage of cowpea seeds Vigna unguiculata (L.)Walp.] in Sri Lanka. Essential oil isolated from Plectranthus zeylanicus plant was tested for potential insecticidal activity against C. maculatus. The gas chromatography studies of the essential oil of P. zeylanicus showed that ρ-cymene (3.5%), β-caryophyllene (0.2%), geranyl acetate (9.3%) and geraniol (7.2%) were the major constituents. The adults of C. maculatus were susceptible to both fumigant and contact toxicity of P. zeylanicus plant oil. LC(50) values of 0.927 and 0.010?g?L(-1) were obtained for fumigant toxicity and contact toxicity assays, respectively. Oviposition and F(1) adult emergence were significantly inhibited by P. zeylanicus plant oil at a concentration higher than 0.001?g?L(-1) in both fumigant and contact toxicities. The analysis of olfactometer and choice camber bioassays revealed the repellent effects of the oil of P. zeylanicus plant.