Understanding how different pesticides influence bee health is inhibited by a limited knowledge about the interactions between different compounds to which bees are simultaneously exposed. Although research has demonstrated synergistic effects of some sterol biosynthesis inhibiting (SBI) fungicides on the toxicity of certain insecticides to bees, a high degree of variability exists in the relatively few SBI fungicide-insecticide interactions tested. Furthermore, most research has been conducted on honey bees, Apis mellifera , limiting our understanding of pesticide synergisms in native wild bees. We tested the effects of the SBI fungicides difenoconazole, myclobutanil, and fenhexamid on acute contact toxicity of the insecticides thiamethoxam (neonicotinoid) and bifenthrin (pyrethroid) to the common eastern bumble bee, Bombus impatiens. Based on range-finding trials, we selected a single dosage of each pesticide, with insecticides approximating LD20 values and fungicides approximating their maximum sublethal levels. We found that the triazole SBI fungicide difenoconazole interacted synergistically with bifenthrin, with a maximum synergy ratio of 1.48, while the triazole myclobutanil interacted synergistically with both bifenthrin and thiamethoxam, with maximum synergy ratios of 11.0 and 2.38, respectively. However, the hydroxyanilide SBI fungicide fenhexamid did not exhibit a synergistic effect on either insecticide. Our results suggest that certain SBI fungicides, and potentially those from particular SBI classes, may be particularly harmful to bumble bee health when co-exposure to insecticides occurs, despite their low toxicity when experienced in isolation.

In response to anthropogenic environmental change, the cues that animals use throughout their lifecycle to optimize fitness may become unreliable, resulting in an ecological trap. Here we investigated whether commercial bumble bee Bombus impatiens colonies managed for early spring crop pollination act as ecological traps for wild nest‐searching Bombus queens by subverting their natural nest usurpation behaviour. An average of 10 dead wild queens were recovered from each standard colony during the 2‐week period of the experiment, but colonies with queen excluders were successful in preventing wild queen deaths. The use of queen excluders did not impact colony performance in terms of resident queen survival, colony reproduction, colony weight gain or worker body size. Sites where wild nest‐searching queens were small had higher rates of failed usurpation, suggesting smaller‐sized queens are disproportionately at risk from failed usurpation. Furthermore, sites where commercial colonies without queen excluders were introduced for spring crop pollination had fewer bumble bee visits to a later‐blooming crop compared to sites without commercial colonies. Synthesis and applications. Our findings reveal a novel mechanism by which commercial colonies can negatively impact wild bumble bee populations and their pollination services. At the same time, we demonstrate a simple and inexpensive risk mitigation tool—a queen excluder—was 100% effective at eliminating this risk without compromising colony performance. Commercial colonies used outdoors during the bumble bee nest‐searching period should be fit with queen excluders to prevent negative impacts on wild pollinator communities and their services.

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