Abstract: The efficacy of thiamethoxam at 0.25, 0.5 and 0.75ppm with or without diatomaceousearth (DE) formulation SilicoSec at the rate of 100ppm was evaluated against Rhyzoperthadominica F. (Coleoptera: Bostrychidae) under laboratory conditions. Three grain types wheat,maize and rice were tested at 30°C and 65% r.h. Adult mortality was recorded after 24 and 48h, 7and 14d post exposure while the progeny production was noted after 60d. The adult mortalityincreased with increasing dose rates and exposure intervals. Thiamethoxam was more effective atlowest dose rates compared with DE alone and more number of dead adults was recorded inwheat. The progeny production was significantly reduced compared with untreated grains. Theresults of the present study revealed the potential of combined treatments of thiamethoxam andDE, but the efficacy varies among grain types.