Disease dynamics of Crithidia bombi infections in bumble bees as influenced by exposure to fungicides

The decline of bumble bees across the globe is attributed to a variety of stressors, including parasite infection and exposure to pesticides. There is a gap in the literature describing the interactions of these stressors on bee health and pollination services. Crithidia bombi is a common gut parasite of bumble bees, and while it is not thought to increase mortality in healthy individuals, it may have more extreme negative effects when coupled with exposure to other stressors. In a lab setting, we investigated the effect of the fungicide Pristine® on the virulence of C. bombi infections in common eastern bumble bee (Bombus impatiens) microcolonies of 5 workers. Following inoculation of C. bombi cells, we used PCR methods to test for the presence or absence of infection across two weeks, and qPCR methods to quantify the level of within-host-pathogen success. We found that bees inoculated with C. bombi generally remained infected for 14-day periods, whether simultaneously exposed to fungicide-contaminated pollen or not. We also collected data on microcolony reproduction and found that microcolonies who were both infected with C. bombi and fed pollen contaminated with Pristine® produced the fewest brood cells overall. These findings suggest that fungicide consumption coupled with infection of a common gut parasite may create negative synergistic effects on bumble bee colony fecundity and health.