The impact of fungal symbionts on the health and reproduction of bumble bees

Bees often interact with microbes, in associations that range from mutualistic to parasitic. Fungi are common inhabitants of this microbial community, with diverse effects on bee health. Some taxa, like the bee-specialist genus Ascosphaera and the opportunistic pathogen Aspergillus flavus, negatively impact colony health, infecting and killing developing larvae. However, other fungi can have beneficial impacts on bee health. Yeasts commonly inhabit colony food stores like honey, and can improve colony development, worker survival, and reproductive output when present. The mechanisms through which these yeasts enhance bee health are not fully understood, but could be due to suppression of pathogenic microbes present in bee hives. To test this, we reared larvae of the common eastern bumble bee, Bombus impatiens, and exposed them to two treatments in a fully factorial design. Larvae were given the common nest yeast Starmerella followed by the bee pathogen Ascosphaera apis, in their food supply. They were monitored for three weeks, over which time their development and survival were tracked. These results will enhance our understanding of the mechanisms underlying the impacts of these fungi on bee health, and how they interact with other members of the nest microbial community.