M.T.H.P. Perera and M.M.S.C. Karunaratne
Department of Zoology, University of Sri Jayewardenepura, Sri Lanka
Leaf powders of seven commonly found plant species, Ocimum graticimum, Morinda citrifolia, Aegle marmelos , Annona squamosa, Annona reticulata, Olax zeylanica and Ricinus communis were screened for their insecticidal activity under laboratory conditions (30±1 ºC and 84 – 86% RH) against the maize weevil, Sitophilus zeamais. The powders at the rate of 15g/50g of maize grains were tested separately and the mortality of adult weevils was recorded after 24 hours. The results of this bioassay revealed that O. zeylanica was most active towards S. zeamais eliciting a hundred percent mortality of the weevils. However, the mortality values for the other six plant powders were significantly very low with 20% for Ocimum graticimum, 16.6% for Morinda citrifolia and Aegle marmelos, and 6% for Ricinus communis, A.squamosa and A. reticulata. Leaf powder of O. zeylanica, then, was tested at five dose rates (1.0, 2.5, 5.0, 7.5, 10.0g) against the weevils.
Mortality of adult weevils was recorded after 6, 12 and 24 hours of treatment and LD50 and LD99 values were given at the respective time intervals. It was observed, in this experiment, that the mortality of weevils increased with the increase of the dose as well as the exposure time.Â Also, weevil mortality was found to be 100% at the dose rate of 10g just after 12h of treatment. Results from this study indicated that leaf powder for O. zeylanica was extremely toxic to S. zeamaize weevils with LD50 values of O. zeylanica were 5.84, 2.47 and 1.84g at 6, 12 and 24 hours of exposure respectively. In a similar vein, LD99 values of 38.55, 8.33 and 6.51g were observed for the same exposure times. The present study revealed that Olax zeylanica could be highly promising as a bio-pesticide which can compete effectively with the hazardous synthetic insecticides in the management of maize weevil infestations in storage.