Mortality and GST enzyme response of saw-toothed grain beetles, Oryzaephilus surinamensis (Coleoptera: Silvanidae) exposed to low insecticide concentrations

The saw-toothed grain beetle, Oryzaephilus surinamensis is a cosmopolitan insect pest that attacks stored grains and dried fruits, causing damage to stored dates in the United Arab Emirates (UAE). Stored dates may contain insecticide residues as a result of insecticide field applications and it is of interest to know the effect of insecticide residues on O. surinamensis. The aim of this study was to evaluate mortality among O. surinamensis adults exposed to residual levels of the insecticides carbosulfan, chlorpyrifos, cypermethrin, imidacloprid, malathion and spinosad which are commonly used on date palm trees in the UAE. Also to evaluate the Glutathione S-Transferase (GST) enzyme activity in the O. surinamensis adults exposed to insecticides. Adults of O. surinamensis were subjected to petri dish insecticide bioassays. Low insecticide concentrations, simulating field residues, were tested and the range of 0.713-3.31 jig mL^-1 caused 50% mortality. Chlorpyrifos was the most toxic insecticide while cypermethrin was the least toxic. In addition to effect on mortality, insecticide exposure caused induction in GST enzyme activity in O. surinamensis adults. Because elevated GST enzyme activity can play a role in the detoxification of insecticides and in the development of insecticide metabolic resistance in insects it is likely to have a similar role in O. surinamensis population.