Does Presence of Parasites Contribute to Virginia Bee Decline?

Background and aims: Bumble bees are an important group of pollinators and are a necessary ecosystem service provider in both agricultural and natural contexts. However, certain species have experienced precipitous population declines in recent decades. Broad surveys across the United States have revealed a pattern of increased infection rates of the declining species by the microsporidian parasite Nosema bombi, implicating it as a cause or contributing factor to bumble bee decline. Although three declining species occur historically in Virginia, none were previously surveyed for infection levels in our region and very few individuals were sampled previously in the entire eastern United States. The purpose of this study was to examine the infection rates of Nosema and another gut microparasite in common and declining bumble bee species in northern Virginia to see whether the pattern detected in other regions of the country is present in our area. Methods: We collected bees belonging to common (B. impatiens, B. griseocollis), newly declining (B. fervidus, B. auricomus), and heavily declining species (B. pensylvanicus) from old field and wildflower meadow sites in northern Virginia. Fecal samples and gut tissue were analyzed for the presence and intensity of infections of the microparasites Nosema bombi and Crithidia bombi using phase-contrast microscopy. Currently, all species have been examined for Crithidia but only B. pensylvanicus and B. impatiens have been analyzed for Nosema. Key results: While there was no significant difference between species or sites in terms of infection rate of Crithidia, the infection rate of Nosema was significantly higher in B. pensylvanicus than in B. impatiens. Conclusions: Our results are consistent with other regions of the country and support the idea that Nosema is playing a role in the decline of certain species through differential susceptibility.