Fungicide effects on the pathogen load of a bumble bee gut parasite
Pesticides can have a range of effects on non-target organisms, such as pollinators that provide beneficial ecosystem services. For example, bumble bees are often exposed to chlorothalonil, a commonly used fungicide. Using the USGS National Pesticide Synthesis Database, predicted chlorothalonil use was associated with increased Crithidia bombi infection in Bombus impatiens across 162 sampling locations. However, the direct effects of chlorothalonil on parasite population growth in bees are unknown. We examined the effect of chlorothalonil on pollinator-parasite relationships using the gut parasite C. bombi, which commonly infects bumble bees. C. bombi impairs foraging abilities, hinders colony growth and increases bee mortality when food is scarce. We predicted that consumption of chlorothalonil would increase C. bombi infection within hosts. We inoculated individual Bombus impatiens workers with a standardized dose of C. bombi and then fed bees one of four artificial nectar treatments in 30% sucrose solution for one week: control, high chlorothalonil (100 ppb), low chlorothalonil (50 ppb) or a pulse treatment that alternated between high chlorothalonil and control solutions daily to mimic fluctuations in chemical exposure encountered in the wild.
We found no statistically significant effect of chlorothalonil on C. bombi infection level (P = 0.11). However, colonies differed in infection levels (P = 0.047), suggesting that bee genotype affects individual response to parasite infection, but there was no interaction between colony and treatment. Bees are social insects and testing the effect of chlorothalonil at the colony level would provide greater insight into the ecological effects these chemicals can impose on bumble bees. Future work testing higher concentrations, to which bees are often exposed, and combinations with other pesticides may help to reconcile our laboratory findings with landscape-level patterns suggesting links between chlorothalonil use and C. bombi parasite loads.