Contact toxicity of insecticides against rice weevil, Sitophilus oryzae L. and its effect on progeny production

Post-harvest losses caused by insect pests, particularly the rice weevil, Sitophilus oryzae, pose a significant challenge in food storage facilities worldwide. To combat this pest, synthetic insecticides and fumigants are widely used. However, effective contact insecticides are scarce. Hence, the present study explored the alternatives by evaluating the contact toxicity of various insecticides against S. oryzae using glass, jute, and floor tiles as surface substrates and further examining the effect on progeny production of promising candidate insecticides. Among the insecticides tested, malathion exhibited the highest toxicity on glass and jute surfaces regardless of the exposure period. On the other hand, spinetoram proved to be the most effective on tile surface with an 8 h exposure period. Among the alternate insecticides (spinosad, spinetoram, chlorfenapyr and lambda-cyhalothrin), spinetoram was most effective with LC₅₀ values of 155.8 and 116.9 mg/m² for 4 h and 8 h exposure, respectively, on tile surface; 204.6 and 129.0 mg/m² for 4 h and 8 h exposure, respectively, on glass surface; and 271.5 and 199.5 mg/m² for 4 h and 8 h exposure, respectively, on jute surface. Relative toxicity assessments revealed spinetoram to be 2.11 and 2.51 times more effective than deltamethrin on tile surface for 4 and 8 h of exposure, respectively whereas it was 1.14 times more effective than malathion on tile surface at 8 h exposure. Principal component analysis indicated a higher demand for insecticide doses closely associated with the structural properties of surfaces, particularly evident with jute surfaces. Furthermore, the effect on adult mortality and progeny production by malathion, spinetoram, and lambda-cyhalothrin revealed malathion as the most effective insecticide followed by spinetoram. Carboxylesterase, acetylcholinesterase, and Glutathione S-transferase (GST) activities were notably higher in deltamethrin-treated insects compared to other insecticides. The studies concluded that spinetoram can be considered an alternative to conventional insecticides for the management of S. oryzae under different storage conditions.