The direct and indirect effects of environmental toxicants on the health of bumble bees and their microbiomes

Bumble bees (Bombus spp.) are important and widespread insect pollinators,
but the act of foraging on flowers can expose them to harmful pesticides
and chemicals such as oxidizers and heavy metals. How these compounds
directly influence bee survival and indirectly affect bee health via the
gut microbiome is largely unknown. As toxicants in floral nectar and
pollen take many forms, we explored the genomes of bee-associated microbes
for their potential to detoxify cadmium, copper, selenate, the
neonicotinoid pesticide imidacloprid, and hydrogen peroxide - which have
all been identified in floral nectar and pollen. We then exposed Bombus
impatiens workers to varying concentrations of these chemicals via their
diet and assayed direct effects on bee survival. Using field-realistic
doses, we further explored indirect effects on bee microbiomes. We found
multiple putative genes in core gut microbes that may aid in detoxifying
harmful chemicals. We also found that while the chemicals are largely
toxic at levels within and above field-realistic concentrations, the
field-realistic concentrations - except for imidacloprid - altered the
composition of the bee microbiome, potentially causing gut dysbiosis.
Overall, our study shows that chemicals found in floral nectar and pollen
can cause bee mortality, and likely have indirect, deleterious effects on
bee health via their influence on the bee microbiome.