Comparative toxicity of neem and pyrethrum extracts to rice weevil (Sitophilus oryzae L.)

The rice or black weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) is believed to be the most destructive insect of raw cereals in the world. The objectives of the study were to investigate the use of four bioassay methods for evaluating the effects of neem extract and pyrethrins under different exposure periods, as well as to investigate the long term effect of these insecticides on the population development of rice weevil and damage to grain. Different bioassay methods were used to test the acute, contact and chronic toxicities, residual exposure and fumigant action of neem and pyrethrins extracts. Neem extract has a slower action than pyrethrins and requires a higher concentration to kill S. oryzae within 1-2 days. In comparison, pyrethrins have a quicker action and comparatively less active ingredient is required to kill S. oryzae. Although both have varying degrees of contact and residual action, neither have any fumigant action which would be a disadvantage in bulk grain. This study has also shown that neem can severely affect the emergence of adults from infested grains, and can limit the damage caused to rice grains. The practical implications of these results and suggestions for further research are briefly discussed.