Floral shape predicts bee-parasite transmission potential

The spread of parasites is one of the primary drivers of population
decline of both managed and wild bees. Several bee parasites are
transmitted by the shared use of flowers, turning floral resources into
potential disease hotspots. However, we know little about how floral
morphology and floral species identity affect different steps of the
transmission process. Here, we used the gut parasite Crithidia bombi and
its primary host, bumble bees (Bombus spp.), to examine whether floral
traits or species identity better predict three basic steps of parasite
transmission on flowers: feces deposition on flowers, survival of the
parasite on flowers, and acquisition by a new host. We also identified
which traits and/or species were most strongly associated with each step
in the transmission process. We found that both trait- and species-based
models fit the data on deposition of feces and survival of C. bombi on
flowers, but that species-based models provided a better fit than
trait-based ones. However, trait-based models were better at predicting
the acquisition of C. bombi on flowers. While different species tended to
support higher fecal deposition or parasite survival, we found that floral
shape provided explanatory power for each of the transmission steps. When
we assessed overall transmission potential, floral shape had the largest
explanatory effect, with wider, shorter flowers promoting higher
transmission. Taken together, our results highlight the importance of
flower species identity and floral traits in disease transmission dynamics
of bee parasites, and floral shape as an important predictor of overall
transmission potential. Identifying traits associated with transmission
potential may help us create seed mix that presents lower parasite
transmission risk for bees for use in pollinator habitat.