Neem oil, a product from the neem tree (Azadirachta indica A. Juss) was evaluated for its insecticidal properties (ovipositional deterrent and egg developmental inhibitor) against the adult of the maize weevil, S. zeamais. Significantly fewer weevils (22%, p<0.0011 were found on kernels treated with 10,000 ppm neem oil than in the control (78%). Mineral oil had no deterrent effect on weevil distribution at any dose. Although, 1,000 ppm did not deter insects from corn kernels, it significantly affected their oviposition. Neern oil at 10,000 ppm level not only reduced weevil oviposition in the treated kernels but also inhibited their oviposition on untreated kernels in the same area. In no-choice test similar effect on oviposition was also observed. Weevils held for 7 days on corn kernels treated with 1,000 and 10,000 ppm neem oil significantly laid fewer eggs, but when the exposure increased to 16 days, oviposition in the treatment and in the control was the same. All the eggs hatched normally.

Abstract:Crude neem oil diluted in acetone was tested for its toxic effect on the reproductive ability of the S. zeamais and its parasitoid A. calandrae. Neem oil at 100, 1,000 and 10,000 ppm on corn kernels did not affect the survival of adult weevils. Although, the oviposition rate was greatly reduced with 1,000 and 10,000 ppm neem oil, the eggs hatching and the F1 progeny reproduced normally. Neem oil at all doses (100-10,000 ppm ) did not show any detectable effect on the biology of A. calandrae. However, at 10,000 ppm neem oil showed a significant effect on the reprodutive potential of the parasitoid as measured by adult emergence and normal reproduction by F1 progeny.

The Oviposition rate of the maize Weevils was greatly reduced when they were held with corn kernels treated with 1,000 and 10,000 ppm neem oil in acetone. The cleteramental effect of the 1,000 ppm neem oil treatment was lost by 30 days after treatment, but the 10,000 ppm retained its effect for 60 days. Doses of 1,000 and 10,000 ppm of neem oil repelled weevils from corn kernels treated 24 h before the test, however, repellency disappeared after 15 days in 1,000 ppm, but not the 10,000 ppm.