1. Bumblebees are important pollinators in North America and are attacked by a range of parasites that impact their fitness; however, few studies have investigated the extent or causes of parasitism in North America.
2. This study used a 2-year multi-site survey of bumblebee parasitism to ask: (i) how common are parasitoid conopid flies and the internal parasites Crithidia bombi and Nosema bombi in Massachusetts; and (ii) what factors are correlated with parasitism?
3. Infection rates by all three parasites were higher in this study than previously documented in North America. Overall, conopids infected 0–73% of bees in each sample, C. bombi infected 0–82% of bees, and N. bombi infected 0–32%.
4. Conopid flies infected female bees more than males and intermediate-sized bees more than large or small bees. Crithidia bombi infection rates were higher in certain bee species and sites, and exhibited a unimodal pattern of prevalence over time. Nosema bombi parasitism was higher in male than female bees.
5. Infection by N. bombi in two rare bumblebee species was higher than expected based on parasitism rates of common bee species but C. bombi infection was lower.
If high prevalence of N. bombi in these bumblebee species is common, parasitism may be a potential cause of their decline.
6. Given the documented effects of these parasites, the high levels of infection may affect bee populations in Massachusetts and threaten the stability of their valuable ecosystem services.