Patterns of parasite infection in bumble bees ( Bombus spp.) of N orthern V irginia

In recent decades, several N orth A merican 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. A season‐long survey of bumble bees in seven grassland meadows of the northern S henandoah V alley and P iedmont regions in V irginia 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. 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 . 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%). 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. 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.