Fungicide consumption exacerbates the negative effects of a common gut parasite in bumble bee microcolonies

Bumble bees face numerous environmental stressors, including gut-parasite infection and exposure to agricultural fungicides, which can negatively impact colony health. This study evaluates the interactive effects of these stressors on bumble bee (<i>Bombus impatiens</i>) microcolonies, focusing on colony development, worker survival and parasite infection dynamics. Our aim in evaluating these interactions was to determine if bees would experience synergistic negative health outcomes compared to single- stressor exposures. We reared 40 queenless bumble bee microcolonies, and treated them with either fungicide-contaminated pollen, inoculation with a gut parasite, both, or neither. Contrary to original expectations, we did not observe significant synergistic interactions between the two stressors; however, we found that consumption of fungicide was associated with higher likelihood of gut-parasite infection, and delayed recovery from infection. Fungicide consumption was also connected to smaller workers, and smaller male offspring. We also found that gut-parasite infection was correlated with decreased pollen consumption overall, decreased worker survival and fewer developed pupae. This study provides insights into the impacts of co-occurring stressors affecting bumble bees and emphasizes the importance of sublethal effects on pollinator health.