Bumble bees are an important group of insects that provide essential pollination services as a byproduct of their foraging behaviors. These pollination services are driven, in part, by energetic exchanges between flowering plants and individual bees. Thus, it is important to examine bumble bee energy metabolism and explore how it might be influenced by external stressors, which contribute to declines in global pollinator populations. Two stressors that are commonly encountered by bees include insecticides and nutritional stress. Our study examines the effects of neonicotinoid insecticide exposure alone, and in combination with nutritional stress, on bumble bee metabolism using a novel metabolomic approach. We hypothesized that exposure to the insecticide imidacloprid would disrupt bumble bee energy metabolism, leading to changes in key metabolites involved in energy metabolism. We exposed Bombus impatiens workers to imidacloprid according to one of three exposure paradigms designed to explore how sustained versus more limited imidacloprid exposure influences energy metabolites. After bees were exposed to imidacloprid, they were subjected to artificial nectar starvation. Our results showed that the strongest effects of imidacloprid were observed when treated bees also experienced artificial nectar starvation, suggesting a combinatorial effect of neonicotinoids and nutritional stress on energy metabolism. Overall, this study provides important insights into the mechanisms underlying the impact of neonicotinoid insecticides on bumble bees and underscores the need for further investigation into the complex interactions between environmental stressors and bee metabolism.

In this study, we used a metabolomic approach to examine how sublethal, field-realistic exposure (5 ppb) to the neonicotinoid insecticide imidacloprid, on its own and alongside starvation stress, influences energy metabolism in the common eastern bumble bee Bombus impatiens. Individual worker bumble bees (n = 195) were assigned to one of four insecticide treatment groups: 1) sustained, 2) limited-early, 3) limited-late, and 4) untreated (control). All bees first underwent a seven-day exposure period wherein they were provided with either treated (sustained, limited-early, limited-late) or untreated sucrose solution (untreated; control group) according to their treatment group. After the seven-day exposure period, all bees underwent an experimental starvation period. A subset of bees from each treatment group was immediately collected (0-hour group) to determine how imidacloprid exposure alone impacts bee metabolite levels in the absence of starvation. We then subsequently collected subsets of bees from each treatment group at one of three additional starvation time points, separated by eight-hour intervals, at 8, 16, or 24 hours following the refeeding period. Bees were collected in liquid nitrogen and sent to the UC Riverside Metabolomics Core Facility for metabolomic analyses targeting central carbon metabolism to assess how metabolites involved in sugar energy metabolism are influenced by insecticide exposure.