Mechanisms of pathogen spillback in pollinating bees: Implications for disease spread

While some host-pathogen associations demonstrate strict host specificity, many pathogens possess broad host ranges, capable of transmission across diverse taxa. While the mechanisms driving pathogen spillover from primary to alternative hosts are well characterized in some systems, less is known about the ecological determinants facilitating pathogen spillback. For example, Crithidia bombi is a fecal-orally transmitted pathogen that infects bumble bees (Bombus spp.) through shared use of flowers. Recent studies document that C. bombi infects alternative hosts; however, it remains unclear whether, and to what extent, these species spread C. bombi back to bumble bees. We use a mechanistic approach to assess how three steps of C. bombi transmission (deposition of infected feces on flowers, survival of pathogen on flowers, and acquisition of pathogen by primary host) affect the probability of pathogen spillback. We chose hosts that vary in body size, including primary host Bombus impatiens, and alternative hosts Osmia cornifrons and Megachile rotundata. We hypothesize that larger alternative hosts will spillback C. bombi while smaller alternatives will not. Deposition data for both O. cornifrons and M. rotundata demonstrate a significant preference for defecating on the corolla over other floral structures. Fecal droplets of O. cornifrons and M. rotundata have a mean survival time of 16 minutes and 4 minutes, respectively. Overall, we found alternative hosts can spillback C. bombi infection in the lab; however, field experiments are still ongoing. Understanding the mechanisms driving pathogen spillback is essential for understanding disease spread and informing conservation efforts.