The relative contributions of host density and genetic diversity on prevalence of a multi-host parasite in bumblebees

The role of population and demographic factors in influencing the transmission and establishment parasites infecting multiple hosts is poorly understood. We assessed the effects of these factors on parasite prevalence in a model system - the intestinal protozoan Crithidia bombi (Trypanosomatidae) infecting a range of bumblebee species (Bombus spp.). We used microsatellite markers and sibship reconstruction to infer genetic diversity and the density of host populations to infer their relative contributions to parasite prevalence. We established the prevalence, type (single-vs. multiple-strain infection) and intensity of C. bombi infections in workers and males of three common bumblebee species (B. terrestris, B. lapidarius and B. pascuorum) at 30 locations across Germany. We found evidence that colony density promoted prevalence, while increased genetic diversity lowered prevalence in B. terrestris. The effect size for genetic diversity was much larger than for colony density. Thus, genetic factors affected the prevalence, while demographic and life-history traits (e.g. population density and seasonal cycle of development) were additional factors shaping the spread and establishment of a multi-host parasite. Bombus lapidarius possessed characteristics that indicate it might act as a reservoir species spreading disease to other species.