Each time they alight and attempt to dig, hundreds of third stage larvae are discharged or exit through the anal opening of the bumblebee. These larvae molt during July and into August developing through the fourth larval stage to adults. During this same period infected bees die from the infestation. The adult nematodes mate and the impregnated females become the infective stage. In autumn and fall when healthy queens are seeking hibernation sites, they are attacked by the infective female nematodes.
Infective females penetrate the queens during hibernation.
Normally only one female is found per bumblebee queen although as high as 74 have been recorded. When the queen emerges in the spring the nematode is a mature female and may already be producing eggs. By June the parasite is matured and third stage larvae are evident in the insect's hemocoel, midgut, and hindgut. A fascinating feature of this nematode is the ability to completely evert the reproductive system which then proceeds to enlarge independently of the original female body. The prolapsed uterus may be from 6 to 30 times the length of the original female and 300 times the volume.