Are microsporidia involved in bumble bee decline?

Several species of bumble bees, predominantly those of the subgenus Bombus sensu stricto, are rapidly declining in different geographical areas of the United States. The reasons for decline are not yet clear, and recent reports that pathogen spillover from commercial colonies are involved is difficult to substantiate because there is so little baseline data available on the pathogen complex of U.S. Bombus populations. We are providing such a data set by conducting comparative surveys and screening for six Bombus species, two declining and four stable and abundant species in 153 sites in 25 western and midwestern states. We focused on the microsporidia as a potential exotic invader and show that Nosema bombi remains the only microsporidian species recovered from bumble bees worldwide and has a broad ecological host range in North America, as it does in Europe. Our data suggest that N. bombi occurs more frequently and at higher prevalence in species of the genus Bombus sensu stricto than in species from other subgenera. Bombus occidentalis, Bombus pensylvanicus (species of concern) and Bombus mixtus are the most commonly infected species. Variability in disease occurrence is strongly correlated with site. We are currently conducting experiments on comparative susceptibility of several species.