Bee-associated fungi mediate effects of fungicides on bumble bees

Bumble bees are important pollinators that face threats from multiple
sources, including agrochemical application. Declining bumble bee
populations have been linked to fungicide application, which could
directly affect the fungi often found in the stored food and
gastrointestinal (GI) tract of healthy bumble bees. We test the hypothesis
that fungicides impact bee health by disrupting bee–fungi interactions. We
examined the interactive effects of the fungicide propiconazole and fungal
supplementation on the survival, reproduction and microbiome composition
of microcolonies (queenless colonies) using two species, Bombus
vosnesenskii and B. impatiens. We found that
in B. vosnesenskii, fungicide exposure decreased survival, while
fungal supplementation mitigated fungicide effects. For B.
impatiens, fungicide application had no effect, but fungal supplementation
improved survival and offspring production. Fungicides reduced fungal
abundance in B. vosnesenskii microcolonies, but not
in B. impatiens, where instead fungal addition decreased fungal
abundance (ITS copy number). Fungal composition varied between treatments
but differently between bee species. In B. impatiens, fungal
addition increased microbiome diversity. In B. vosnesenskii, the
abundance of the pathogen Ascosphaera was negatively
associated with survival, while the
yeast Zygosaccharomyces was positively associated with
survival. Our results highlight that bumble bee species differ in response
to fungicides and in the nature of bee‐fungi associations. Fungicides can
alter bee–fungi interactions with consequences for bee survival and
reproduction, and exploring the mechanisms of such interactions, including
interactions among fungi in the bee GI tract, may offer insights into
bumble bee biology and conservation strategies.