Potential of Olax zeylanica leaves in the suppression of Sitophilus oryzae L. infestations

In spite of the growth of paddy production, significant losses occur globally during post harvest storage of rice, due to the rice weevil Sitophilus oryzae attack, which is considered the major insect pest of stored rice. In order to develop green alternative to synthetic insecticides, the repellent and toxic properties of Olax zeylanica leaf powder, solvent extracts and essential oil were tested against S. oryzae under laboratory conditions. Laboratory reared one week old adults were used to determine the efficacy of plant products. Repellent effect was evaluated within an hour of exposure using a contact-repellency chamber by admixing leaf powder with 30g of rice grains at 4 doses (1, 3, 5 and 7 g). Repellency of leaf-solvent extracts (methanol, ethyl acetate, hexane and water) was investigated at 10%, 50% and 100% (v/v) while the repellency of essential oil was assessed at same concentrations using area-preference method and all the observations were recorded after 30 minutes. Observations on contact and fumiga-tion toxicity of the essential oil were made 30 and 60 minutes after exposure to weevils. All tested plant products elicited significantly very high responses in S. oryzae when compared with the controls (0.00%). Leaf powder of O. zeylanica evoked 100% repellency at the highest dose (23.33% w/w) and the repellent rate increased with the in-crease in dose. In repellency assays using leaf extracts, methanol extract was found to be the most effective causing 96% repellency in the weevils, followed by ethyl acetate (95%) and hexane (89%) whereas the aqueous extract was least effective (70%) indicating the lowest level of repellency. Furthermore, three concentrations of the essen-tial oil evoked extremely high repellent action in weevils (94%, 99%, and 100% respectively) within 30 minutes. Both contact and fumigant toxicity effects with the essential oil were dose dependent and 100% adult mortalities were observed at the highest concentration after one hour of exposure. The overall results of the present study indicate that, O. zeylanica leaves could be utilized as a very successful candidate in the development of bio-insecticides in controlling S. oryzae infestations.