Patterns of parasite infection in bumble bees (Bombusspp.) ofNorthernVirginia

Abstract

In recent decades, severalNorthAmerican bumble bee (Bombusspp.) 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 inU.S. bumble bee populations.A season‐long survey of bumble bees in seven grassland meadows of the northernShenandoahValley andPiedmont regions inVirginia was conducted in 2011 to ascertain the local prevalence and predictors of parasitism by the internal parasitesNosemaandCrithidia, 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% withCrithidia, and 7.3% withNosema.Nosemainfections were more prevalent and intense in locally rare than locally common species, with the two rarest bumble bees [B. fervidus(Fabricius) andB. auricomus(Robertson)], newly suspected to be in decline, having the highest frequencies of infection (11–17.8%).Crithidiawas 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 ofCrithidia. Conopid fly larvae were more prevalent in common species.Body size significantly influenced the probability of parasitism by conopids andCrithidia. Smaller bees were more likely to be parasitised byCrithidia. 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.