Insecticide tolerance shapes performance responses to multiple stressors in <i>Leptinotarsa decemlineata</i>

Insect pests are remarkably successful in evolving resistance to management tactics while facing multiple sources of stress in modern agroecosystems. One possible explanation for this success is that repeated exposure to insecticides may enable pests to tolerate additional stressors through cross-protection. Using the Colorado potato beetle (CPB), Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), we tested whether selection for imidacloprid tolerance influences responses to multiple stressors. We compared imidacloprid-selected and unselected beetles exposed to sublethal imidacloprid (LD₁₀), high temperature (40 °C), or their combination, measuring effects on mobility, herbivory, development, fecundity, and mortality. Compared to unselected beetles, imidacloprid-selected beetles exhibited 35% lower mobility and 30% lower survival to adulthood under combined stress. Under high temperature alone, mobility in selected beetles was 27% lower, and under imidacloprid alone, mobility was 17% lower than in unselected beetles. Herbivory was reduced in both beetle groups under all stressor treatments, though no significant differences were detected between groups for the combined treatment. Development time and reproductive output were unaffected by any stressor treatment. Independent Action model predictions indicated that combined stress caused greater-than-expected reductions in survival in imidacloprid-selected beetles, suggesting synergistic effects. These findings suggest that selection for insecticide tolerance may create vulnerabilities to environmental stress, a dynamic that could inform pest management strategies under climate change.