Patterns of parasite infection in bumble bees (Bombus spp.) of the northern Shenandoah Valley in Virginia

Background/Question/Methods

Within the last two decades, notable shifts have taken place within the bumble bee (Bombus spp.) communities of North America. Of approximately fifty species native to the continent, six bumble bees have experienced substantial, rapid declines, and one is believed to have gone recently extinct. Given that recent declines have been synchronous and have occurred primarily among closely related species, it is suspected that a parasite or pathogen may be responsible.  Though bumble bees are host to a wide array of parasitic organisms, little is known about the role parasites play in regulating bumble bee populations. In this study, we examined bumble bees of the Northern Shenandoah Valley in Virginia for the prevalence of parasitism by (1) parasitoid conopid flies, (2) the intestinal trypanosome parasite Crithidia bombi, and (3) the intracellular microsporidian parasite Nosema bombi. Logistic regression models were used to determine whether the probability of being parasitized was related to (i) species, (ii) body size, (iii) or time. 

Results/Conclusions

A total of 835 bumble bees were collected from seven grassland meadows throughout the study region between June 1 and July 31 of 2011, with each site receiving two visits.  Using visual detection methods, we determined that a total of 25% of bees were infected with parasitoid larvae (35% at peak infection), 17.4% of bees captured were infected with Crithidia (25% at peak infection), and only 7.3% were infected with Nosema. Logistic regression analyses revealed that body size and time significantly influenced the probability of parasitism by parasitoid flies, and that species identity significantly influenced the probability of parasitism by Crithidia, with infections being more frequent in abundant species. Although too few detectable incidences of Nosema were found to run a full model, it is interesting to note that that the only heavy infections were found in the two least common species (B. auricomus and B. fervidus). Parasitoid larvae were more likely to occur in larger bees. However, no individuals of the largest bee species (B. auricomus) were infected by conopid larvae, suggesting that there may be a threshold body size targeted by conopid flies. This study illuminates the current relative importance of parasites in local bumble bee populations and provides important baseline data for better understanding the degree of correspondence between parasite load and population trajectories over time.