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

Authors: DE SILVA, L.A.D.M - Rajarata University Of Sri Lanka; PREMATHILAKA, P.A.P.I. - Rajarata University Of Sri Lanka; EGODAWATTA, W.C.P. - Rajarata University Of Sri Lanka; WIJAYARATNE, L.K.W. "WOLLY" - Rajarata University Of Sri Lanka; Morrison Iii, William

Submitted to: Journal of Stored Products Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2025
Publication Date: 10/18/2025
Citation: De Silva, L., Premathilaka, P., Egodawatta, W., Wijayaratne, L., Morrison III, W.R. 2025. Deltamethrin, spinetoram and their combinations alter heat tolerance in Sitophilus oryzae (Coleoptera: Curculionidae) and Tribolium castaneum (Coleoptera: Te-nebrionidae) adults. Journal of Stored Products Research. 115 (2026) 102797. https://doi.org/10.1016/j.jspr.2025.102797.
DOI: https://doi.org/10.1016/j.jspr.2025.102797

Interpretive Summary: Rice weevil and red flour beetle are serious and widespread pests of stored grain throughout the world. Heat treatments can be used for pest management, but efficacy can be low under certain conditions. However, combining heat treatments with other pest management tactics, such as insecticides, could increase their efficacy and reduce the exposure time needed to induce mortality, resulting in reduced energy costs. In this study, we determined the impacts of two insecticide treatments (deltamethrin and a combination of deltamethrin and spinetoram) on the duration of time needed to kill insects during heat treatments. Without exposure to heat, deltamethrin + spinetoram caused higher mortality than deltamethrin alone in rice weevil but not red flour beetle. Heat tolerance was reduced in rice weevil when exposed to either deltamethrin alone or deltamethrin + spinetoram and the duration of heat exposure needed to kill the insects was reduced at a range of insecticide concentrations (1–15 ppm). In contrast, insecticides only reduced heat tolerance of red flour beetle when applied at high concentrations (15 ppm). We conclude that deltamethrin and spinetoram may reduce heat tolerance in some species of stored product insects, allowing for shorter exposure times to heat treatments and thereby also lowering heat energy costs.

Technical Abstract: Sitophilus oryzae and Tribolium castaneum are serious pests of stored food. Insecticides are effective management tools but accompany 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 pre-exposed 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 pre-exposed 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, pre-exposure to deltamethrin + spinetoram synergized mortality and reduced the survival at high temperature compared to deltamethrin alone in S. oryzae but not in T. castaneum. We conclude that pre-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.