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 environmental 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 the toxins and toxicants in floral nectar and pollen take many forms, we explored the genomes of core bumblebee microbes (Using RAST) for their potential to detoxify cadmium, copper, selenium, 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 spiked into their diet and identified the direct effects on bee survival. Using field realistic doses, we further explored indirect effects on bee microbiomes. We found multiple genes in core gut microbes that have the potential to 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 environmental chemicals found in floral nectar and pollen can directly cause bee mortality, and at field-realistic levels, likely have indirect, deleterious effects on bee health via their influence on the bee microbiome.