Disease dynamics of synergistic stressors in bumble bee microcolonies

Bumble bees are native North American pollinators, experiencing decline in several species' populations. Causes of this decline are multifold, and it is likely that synergistic interactions between stressors are exasperating the problem. We investigated the combined and independent effects of fungicide consumption and Crithidia bombi infection on the health of queenless common eastern bumble bee microcolonies. We exposed 45 microcolonies of five bumble bee workers to either fungicide-contaminated pollen, Crithidia bombi infection, both, or neither. We collected frass from workers daily, for 14 days following initial C. bombi exposure, and quantified the level of C. bombi infection using qPCR methodology. We measured infection level, infection change over time, and worker mortality. We found that combined stressors were associated with increased worker mortality, and that fungicide consumption was correlated with higher average Crithidia infection levels, slower recovery from infection, and a higher likelihood of infection occurring overall. These results have important implications for bumble bee health in the face of increased urban and agricultural fungicide use, with potential impacts on pollination services and population size.