Effects of live yeasts and their metabolic products on bumble bee microcolony development

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 to Bombus impatiens diets 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 pathogen Aspergillus showed reduced reproduction, but yeast treatments reduced Aspergillus prevalence. 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 including Aspergillus . 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.