Deltamethrin, spinetoram and their combinations alter heat tolerance in Sitophilus oryzae (Coleoptera: Curculionidae) and Tribolium castaneum (Coleoptera: Tenebrionidae) adults

Sitophilus oryzae and Tribolium castaneum are serious pests of stored food. Insecticides are effective management tools but have many drawbacks. The exposure to bacterial formulation spinetoram, pyrethroid deltamethrin and high temperature are effective treatments but their effects on certain species when used alone or in combination are poorly understood. This study evaluated the single and combined effects of spinetoram, deltamethrin and heat on the mortality of S. oryzae and T. castaneum adults. The adults were exposed prior to deltamethrin alone or deltamethrin + spinetoram combinations. Subsequently, the insects were held at 42°C and 46°C, respectively. Insect mortality at different durations was performed to calculate a LT₅₀. Without exposure to heat, deltamethrin + spinetoram combination caused higher mortality than deltamethrin alone in S. oryzae but not in T. castaneum. Increased duration at high temperature increased the mortality in both species. The heat tolerance (LT₅₀) was reduced in S. oryzae when exposed prior either to deltamethrin alone or deltamethrin + spinetoram combination, at all the concentrations tested. However, T. castaneum adults exhibited reduced heat tolerance at 15 ppm either with deltamethrin alone or deltamethrin + spinetoram. In general, prior exposure to deltamethrin + spinetoram additively induced mortality and reduced the survival at high temperature compared to deltamethrin alone in S. oryzae but not in T. castaneum. We conclude that prior exposure to deltamethrin and spinetoram may reduce heat tolerance enabling pest management at shorter exposure durations and thereby also lowering heat energy costs. Future experiments should address the limitations identified in this study for improvements to management and to assess the success under field settings.