Flowering plants mediating transmission of a common bumble bee pathogen
Due to mounting concerns about pollinator declines, there has been a recent surge of interest in assessing factors that affect bee populations, including habitat loss, pesticide exposure, and pathogens. While much has been learned about factors that influence individual or colony resistance to pathogens and the consequences of infection, we still know remarkably little about where and how bees acquire new infections in the field. The common trypanosome gut pathogen Crithidia bombi is transmitted to its Bombus sp. hosts via consumption of spores shed in the feces. Such transmission can occur through shared flower foraging, but whether the likelihood of transmission differs with plant species, and the role of floral traits in mediating such transmission in unknown. Identifying plant species traits that influence probability of transmission would be an important step in our ability to predict transmission dynamics based on knowledge of flowering plant abundance and community composition. To assess the role of plant species identity in horizontal pathogen transmission, we inoculated clean inflorescences of 11 plant species with known amounts of C. bombi, allowed healthy experimental B. impatiens to forage, and then assessed pathogen acquisition.
We found that plant species differed significantly in their ability to transmit C. bombi to B. impatiens hosts. Transmission was lowest in Helianthus annuus, Digitalis purpurea and Thymus vulgaris, and highest in Asclepias syriaca. Continued analysis will identify the role of floral morphological and chemical traits and bee behavior in shaping transmission patterns. Furthermore, by controlling the amount of inoculum per inflorescence, this experiment isolated the direct role of floral traits on transmission but did not consider how traits influence attraction of foraging bumble bees, which could dramatically affect the amount of inoculum deposited on flowering plants. Future work will incorporate bumble bee attraction by allowing wild bees to forage on inflorescences before experimental bee foraging. Taken together, this work will address the role of plant traits and communities in shaping bee-pathogen transmission dynamics.