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

Yeasts are common symbionts of bumble bees and their nests, sometimes providing health benefits to their bee hosts. The mechanisms underlying these yeast-derived benefits remain unknown, though hypotheses include direct nutrition from ingestion of fungal cells, resource supplementation, or pathogen protection. We tested how adding living yeasts or their metabolic products to Bombus impatiens diets 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 had minimal impacts on colony performance. While yeasts slightly accelerated offspring production, some treatments also interacted with Aspergillus to reduce reproductive output. Yeast treatments slightly altered chemical composition of nectar, but most distinguishing compounds were unidentified. Our results suggest limited effects of yeasts via direct nutrition, resource supplementation, or modification of diets, instead suggesting that the effects of yeast supplementation are context-dependent, and more research is necessary to better understand the mechanisms underlying their impacts on bee hosts.