Abstract 1. Phorid (Diptera, Phoridae) and conopid (Diptera, Conopidae) parasitism among four North American bumble-bee (Hymenoptera, Apidae) species was investigated. Male bumble-bees experienced a significantly higher incidence of parasitism by the phorid, Apocephalus borealis Brues, and a significantly lower incidence of parasitism by the conopid, Physocephala texana Williston, than did workers.
2. The incidence of parasitism by A. borealis and P. texana varied between bumble-bee sexes and species in patterns that did not reflect differences in relative host abundance. Differences in foraging behaviour between bumble-bee workers and males, as well as between species, may explain these results.
3. Bumble-bee workers and males parasitised by A. borealis had significantly shorter lifespans than unparasitised bees. Based on previous estimates of bumble-bee mortality, A. borealis parasitism may reduce worker lifespans by up to 70%. In contrast, the mortality rate of bees parasitised by P. texana was not significantly different from that of unparasitised bees.
4. These results contrast with previous work highlighting the importance of conopid parasitism to bumble-bee populations, and suggest that phorid parasitism may impose greater costs to bumble-bees than does conopid parasitism in local populations.