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Abstract Bumble bees can benefit from fungi, though the mechanisms underlying these benefits remain unknown and could include nutrition, resource supplementation, or pathogen protection. We tested how adding living yeasts or their metabolic products toBombus impatiensdiets in a factorial experiment affects microcolony performance, including survival, reproduction, and pathogen presence. We additionally assessed effects of yeast treatments on diet (nectar and pollen) chemical composition using untargeted metabolomics. Yeasts impacted microcolony reproduction and survival, but effects depended on source colony. Colonies containing the putative pathogenAspergillusshowed reduced reproduction, but yeast treatments reducedAspergillusprevalence. Yeast treatments altered chemical composition of nectar and pollen, but most distinguishing compounds were unidentified. Our results suggest limited direct effects of yeasts via nutrition, resource supplementation, or modification of diets, instead suggesting that yeasts may benefit bees through interactions with the pathogens includingAspergillus. Overall, the effects of yeast supplementation are context-dependent, and more research is necessary to better understand the factors important in determining their impacts on bee hosts. 

Abstract Bee–fungus associations are common, and while most studies focus on entomopathogens, emerging evidence suggests that bees associate with a variety of symbiotic fungi that can influence bee behavior and health. Here, we review nonpathogenic fungal taxa associated with different bee species and bee-related habitats. We synthesize results of studies examining fungal effects on bee behavior, development, survival, and fitness. We find that fungal communities differ across habitats, with some groups restricted mostly to flowers (Metschnikowia), while others are present almost exclusively in stored provisions (Zygosaccharomyces). Starmerella yeasts are found in multiple habitats in association with many bee species. Bee species differ widely in the abundance and identity of fungi hosted. Functional studies suggest that yeasts affect bee foraging, development, and pathogen interactions, though few bee and fungal taxa have been examined in this context. Rarely, fungi are obligately beneficial symbionts of bees, whereas most are facultative bee associates with unknown or ecologically contextual effects. Fungicides can reduce fungal abundance and alter fungal communities associated with bees, potentially disrupting bee–fungi associations. We recommend that future study focus on fungi associated with non-honeybee species and examine multiple bee life stages to document fungal composition, abundance, and mechanistic effects on bees.