Recent declines in several North American bumble bee species have highlighted both the need to regularly monitor populations and to gauge potential risk factors across species ranges. The high selectivity of species declines, along with the lack of known species-specific sensitivities to recent landscape changes, climate factors or novel pesticides, has led researchers to suspect that the primary driver of these declines is disease, in particular Nosema bombi. While there is clearly a critical need for epidemiological studies of Nosema in bumble bees in order to understand pathogenicity, there is also a basic need to examine bumble bee species distributions and the frequencies of infection across species and sites in order to understand potential disease reservoirs for sensitive species and potential next declining species if the pathogens are only in the process of acquiring new hosts or increasing virulence. To examine species abundance and distribution, we sampled bumble bees with blue vane traps across 27 sites from 2014-2016. To examine pathogen load, we hand collected five bumble bee species at 12 sites and scored them for Nosema and Crithidia infections by visual means under the microscope.
Using blue vane traps, we collected and identified 17,078 bumble bees, from eleven species. We collected two formerly common species recognized to be in decline: Bombus affinis (1 individual) and Bombus pensylvanicus (24 individuals). To our knowledge, the Bombus affinis specimen represents the only one collected in the state since the 1990s. While Bombus pensylvanicus was detected more frequently, it was usually rare. It occurred at eleven sites, but at eight sites it was the least collected species present. Where captured by net and analyzed for disease, Bombus pensylvanicus showed the highest incidence of Nosema infections (over 50%), but its infection prevalence was not statistically greater than that of its closest relative, Bombus fervidus. These species not only had the greatest incidence of Nosema, but also were the most likely to develop intense infections, with some individuals harboring more than a million spores. Bombus auricomus had an intermediate infection frequency (~12% of individuals) and intensity. The most common species studied for disease, Bombus impatiens and B. griseocollis rarely carried Nosema spores. This study lays the groundwork for regional monitoring of bumble bee species, particularly for additional species shown to be susceptible to infection by the pathogen Nosema bombi.