Numerous threats to pollinator health are putting pollinator populations and their essential ecosystem services in jeopardy. Although individual threats are widely studied, their co-occurrence may exacerbate negative effects, as posited by the multiple stressor hypothesis. A prominent branch of this hypothesis concerns pesticide-pathogen co-exposure. A landscape analysis in bumble bees (Bombus spp.) demonstrated a positive association between local use of the fungicide chlorothalonil and prevalence of a microsporidian pathogen Nosema bombi in declining species. This is suggestive of an interaction, but causation needs establishing. We tested the multiple stressor hypothesis with field-realistic chlorothalonil and N. bombi exposures in worker-produced B. impatiens microcolonies. Chlorothalonil was not avoided in preference assays, setting the stage for co-exposure. However, contrary to the multiple stressor hypothesis, pathogen and pesticide co-exposure did not affect survival. Bees in this study showed a surprising level of tolerance to N. bombi infection, which also did not differ between chlorothalonil and control treatments. However, transmission-ready spore loads were higher in infected bees previously exposed to the fungicide. Thus, co-exposure could have consequences for pathogen dynamics in host communities. This underlines the importance of considering both within- and between-host processes when addressing the multiple stressor hypothesis in relation to pathogens.

See the uploaded README file (READ_ME_Calhoun_et_al_Chlorothalonil_Nosema.txt).