Abstract
Pesticides are important drivers of global insect declines, and their detrimental effects can be amplified by synergistic interactions with other stressors. While there is growing evidence of adverse impacts of the next-generation insecticide flupyradifurone (FPF) on pollinators, its effects on other insects and interactions with other stressors remain poorly understood. We investigated the effects of chronic exposure to a range of FPF concentrations, alone and in combination with the entomopathogenic fungus Metarhizium brunneum, in the non-target ant species Lasius niger. Twenty-day exposure to FPF concentrations ≥ 100 ppm significantly increased worker mortality, whereas lower concentrations (≤ 50 ppm) had no direct lethal effect. Furthermore, worker survival following acute exposure to M. brunneum was altered by prior pesticide exposure: compared to unexposed controls (0 ppm), workers exposed to sublethal FPF concentrations (5 and 50 ppm) for 10 days before fungal challenge were more susceptible to infection, with hazard ratios for fungal-induced mortality rising from 1.69 in controls to more than 3.5 in pesticide-exposed individuals. This demonstrates an indirect sublethal effect of FPF via synergy with the pathogen, raising further concerns about the environmental impact of novel pesticides and their implications for insect conservation.