Carbohydrate limitation, but not exposure to the insecticide flupyradifurone, impairs wild bumblebee nest initiation.

Loss of floral resources and pesticide exposure are key drivers of wild pollinator declines. Synergistic interactions between pesticides and poor nutrition may exacerbate individual effects, but how these anthropogenic stressors interact remains poorly understood, particularly across understudied life phases. Queen bumblebees in temperate regions emerge from hibernation in spring and attempt to initiate a colony. In agricultural environments, queens typically emerge during (i) a carbohydrate "hunger gap," whereby nectar demand is higher than supply and (ii) during a time of year in which pesticide use is high. Here, we assessed the impact of carbohydrate limitation and exposure to the novel insecticide flupyradifurone on colony initiation of wild-caught bumblebee (Bombus terrestris) queens. Using a fully crossed experimental design, queens were exposed to a field-realistic concentration of flupyradifurone (1.6 ppm) at high (50% (w/w)) or low (15% (w/w)) sucrose concentrations. We found that bees provided with low-sucrose concentrations produced no eggs or larvae compared with 31.6% in the high concentration groups. Queens fed a lower concentration of sucrose also developed smaller ovaries and performed less nest initiation behaviors. We also found that exposure to flupyradifurone increased the activity of bumblebee queens, and that carbohydrate limitation and flupyradifurone synergistically reduced the likelihood of nest initiation behavior. These results demonstrate the importance of carbohydrate intake during the spring when queens are founding colonies, highlighting the need to consider not only floral diversity within environmental land management schemes but also floral quality.