Resistance reduction and cross-resistance of spinosad-resistant Rhyzopertha dominica (Coleoptera: Bostrichidae) and the association between spinosad resistance and maltase activity.

Background: The lesser grain borer, Rhyzopertha dominica, is a serious stored-products pest mainly controlled by insecticides. Spinosad, an environmentally friendly biological insecticide with low mammalian toxicity, is considered a suitable candidate for R. dominica management. Given the structural similarity of spinosad and maltose, both containing a neutral sugar structure, it was hypothesized that maltase may hydrolyze spinosad, rendering R. dominica resistant to spinosad. In this study, multiple spinosad-resistant strains (RdSR) were used to test the hypothesis.

Results: The resistance reduction examination of a spinosad-resistant strain RdSR-1 showed a significant decrease in resistance level over 1 year with the LC decreasing from 2.996 to 0.392 mg kg indicating an 86.92% reduction. Cross-resistance investigations using the resistant strain RdSR-2 revealed a correlation between spinosad and spinetoram, while no such cross-resistance was observed with other insecticides. The expression levels of four maltase genes were significantly higher in the resistant strain RdSR-1 than the susceptible one. Furthermore, 3,5-Dinitrosalicylic acid (DNS) assays suggested increased spinosad hydrolysis in the resistant strain RdSR-3 compared to the susceptible one. The maltase inhibitor, acarbose, was applied to susceptible R. dominica, resulting in a significant increase in mortality among individuals exposed to both acarbose and spinosad. These findings imply that acarbose can synergize the efficacy of spinosad in R. dominica.

Conclusion: The study suggests maltase as a potential resistance mechanism in R. dominica against spinosad. This novel mechanism combined with reduction and cross-resistance results provide valuable insights for control strategy development. © 2025 Society of Chemical Industry.