Infection dynamics and growth determined by host community context in a bee multi-host pathogen

Biodiversity can support community health by reducing disease transmission and virulence of multi-host pathogens. Several bumble bee species are experiencing sharp declines, with pathogens as a potential contributing factor. Bombus impatiens is a naturally abundant and stable native and commercially-raised species in the eastern US and is embedded in pollinator communities of varying diversity that could mediate pathogen dynamics. A common trypanosome gut pathogen Crithidia bombi infects a diversity of pollinators and has negative effects on its primary hosts, bumble bees. We hypothesize that the potential host community diversity of this pathogen impacts its growth and infectivity in a focal host, B. impatiens, due to ecological differences in parasite transmission and short-term evolutionary dynamics. We collected and isolated 25 C. bombi strains from wild B. impatiens workers across 14 sites differing in pollinator diversity. We calculated population growth parameters (time of the lag phase before growth is detectable, maximum growth rate and carrying capacity) to characterize differences between strains in pure culture. We then conducted infection trials in commercial B. impatiens workers to determine variation in infectivity between strains. We predict that C. bombi collected from more diverse host communities will show slower maximum growth rate, longer lag time and decreased carrying capacity. We also predict these strains will decrease infection success and intensity in inoculated B. impatiens workers. Advancing our understanding of how pollinator communities affect pathogen growth parameters and infectivity provides key insights on the consequences of diversity loss and inform management practices supporting pollinator diversity.