Testing the multiple stressor hypothesis: chlorothalonil exposure alters transmission potential of a bumblebee pathogen but not individual host health

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.