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.