Effects of spinosad and abamectin on different populations of rice weevil Sitophilus oryzae (L.) in treated wheat grain

In laboratory conditions (25+/-1 deg C and 60+/-5% r.h.) effects of natural insecticides spinosad and abamectin on five S. oryzae populations (laboratory, Šid, Gornji Milanovac, Žabari and Novi Pazar) were investigated. Both insecticides for all tested populations were applied to untreated wheat grain at following rates 0.25, 0.5, 1.0 and 2.0 mg AI/kg, subsequently 25 adults were added in each plastic vessels (V=200 cubic cm) containing 50 g of treated wheat, in four replicates, for each population tested. Mortality of weevils was determined after 2-, 7- and 14-days, and the effect on progeny production was determined 8-weeks from parental exposure. Efficacy of spinosad and abamectin after 2-days of weevil exposure for all tested populations and all application rates was less than 15%. After 7-days of exposure, the efficacy was greater than or equal to 95% for weevils from Žabari, in wheat treated with 2 mg/kg of spinosad and abamectin, and for weevils from Gornji Milanovac, only in wheat treated with 2 mg/kg of spinosad. After 14-days of exposure the efficacy greater than or equal to 95% was found for laboratory weevils and weevils from Žabari and Gornji Milanovac, in wheat treated with 1 and 2 mg/kg of spinosad, and for S. oryzae from Novi Pazar and Šid, in wheat treated with 2 mg/kg of spinosad. At the same time for all tested populations abamectin at rates of 0.5, 1.0 and 2.0 mg/kg was 94-100% efficien t. No insecticide achieved total (100%) progeny reduction in tested populations of S. oryzae, while high progeny reduction (greater than or equal to 95%) was found only in weevils which were in contact with wheat treated with 1 and 2 mg/kg of abamectin. The results showed that for highly efficient control of different populations of S. oryzae in wheat grain, greater than or equal to 2 mg/kg of abamectin, and, particularly, spinosad should be applied.