The prevalence, distribution and hosts of Crithidia bombi in wild bumble bee populations.

Several species of bumble bees, predominantly those of the subgenera Bombus sensu strict and Fervidobombus, are declining across the United States. While the causes of these declines are as yet unknown, pathogen spillover from commercial colonies has been suggested as a possible precipitating factor. One hindrance to further investigation is the paucity of baseline data available on the pathogen complex of U.S. bumble bee populations. The present study targets six Bombus species, two declining and four abundant, in 153 sites in 25 western and Midwestern states. Here we report the prevalence of a trypanosome protozoa, Crithidia bombi in native bumble bee populations. Our data suggest that C. bombi occurs more frequently and at higher prevalence in species of the subgenus Pyrobombus than in species from other subgenera; however, there is also a strong spacial component to the prevalence of C. bombi infection. Bombus mixtus, Bombus vosnesenskii and Bombus bifarius are the most commonly infected species in the western US, while Bombus impatiens is the most commonly infected species in the Midwest. Variability in disease occurrence is strongly correlated with site and cross-infestation of multiple species at a single sight is common.