D3123: Mechanisms of parasite spillback in pollinating bees: Implications for disease spread

While some host-parasite systems can be characterized by a parasite infecting a single host species, in others, a parasite can spread disease across taxonomic groups via a complicated web of species interactions. While the ecological determinants characterizing parasite spillover from primary hosts to alternatives is well-documented in some systems, less is typically known about parasite spillback. For example, Crithidia bombi, a Trypanosomatid parasite, spreads fecal-orally via shared flower use, infecting primary bumble bee hosts and replicating in various alternative bee species. It remains unknown if and how much alternative host bees spread Crithidia back to primary bumble bee hosts. Here we use a mechanistic approach to assess how three steps of parasite transmission (deposition of infected feces on flowers, survival of parasites on flowers, and acquisition by the primary host) affect the likelihood of parasite spillback from alternatives back to primary hosts. We use the primary host Bombus impatiens and four alternative host bee species that vary widely in body size. We hypothesize that the larger sized alternative hosts will be able to spill back Crithidia infections while smaller sized alternatives will not. Preliminary data conducted on the primary host B. impatiens suggest that body size and fecal droplet volume are positively correlated while body size and parasite infection intensity were negatively correlated. Experiments with the alternative hosts and three transmission steps are on-going. Understanding pathogen spillback in webs of host-species interactions are critically important for understanding disease spread and providing valuable insights into the areas of conservation and biodiversity.