Bumblebees exposed to a novel ‘bee-safe’ insecticide have impaired alloparental care and reproductive output

For many species, juvenile survival is determined not only by factors that directly affect mortality, but also indirectly via the quality of care that they receive. Social insects like bumblebees are useful for studying these interactions since larvae are exposed both directly to stressors such as pesticides and also indirectly via impairment to adult care. Flupyradifurone (FPF) is a novel insecticide used globally. While FPF has minimal effects on adult mortality at field-realistic concentrations, past work suggests it negatively impacts larval development, though the mechanism remains unclear. Here we tested two non-mutually exclusive hypotheses concerning how pesticides impact bumblebee (Bombus impatiens) larvae: First, that FPF is directly toxic to larvae. Second, that FPF indirectly harms larvae via impaired brood care from adults. We tested the first hypothesis by hand-feeding larvae field-realistic pesticide doses and measuring mortality, and we then assessed both hypotheses in a cross-fostering experiment, where we measured the number and size of larvae present. We found that only the insecticide exposure status of adult caretakers affected larval mass, and that there were no differences in survival across groups when larvae were directly hand-fed four pesticide doses. Together, these results point to larval impairment being driven primarily by effects on parental care rather than direct toxicity from pesticides. More broadly, our results highlight a need for comprehensive risk assessment, beyond adult mortality, that evaluates indirect effects on brood care, in order to provide a more comprehensive understanding of how stressors impact bee populations.