• Bombus;
  • bumble bee;
  • Conopidae;
  • Crithidia;
  • Nosema;
  • parasitism
  1. In recent decades, several North American bumble bee (Bombus spp.) species have undergone precipitous declines. It is suspected that a parasite or pathogen may be responsible, yet few studies have examined the extent of parasitism and the ecology of host–parasite relationships in U.S. bumble bee populations.
  2. A season-long survey of bumble bees in seven grassland meadows of the northern Shenandoah Valley and Piedmont regions in Virginia was conducted in 2011 to ascertain the local prevalence and predictors of parasitism by the internal parasites Nosema and Crithidia, and by parasitoid conopid flies.
  3. In total, 835 bumble bees representing six species were examined. Using visual detection methods, we determined that 25% of bees were infected with parasitoid larvae, 17.4% with Crithidia, and 7.3% with Nosema.
  4. Nosema infections were more prevalent and intense in locally rare than locally common species, with the two rarest bumble bees [B. fervidus (Fabricius) and B. auricomus (Robertson)], newly suspected to be in decline, having the highest frequencies of infection (11–17.8%).
  5. Crithidia was generally more prevalent in common bumble bee species (11–35%). With fewer than 5% of individuals infected, the two rarest species had the lowest frequencies of Crithidia. Conopid fly larvae were more prevalent in common species.
  6. Body size significantly influenced the probability of parasitism by conopids and Crithidia. Smaller bees were more likely to be parasitised by Crithidia. Larger bees were more likely to be parasitised by conopid flies, although the largest bee species (B. auricomus) was not infected by conopids in this study.