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

Abstract 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 50 decreasing from 2.996 to 0.392 mg kg −1 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.