Bumble bees are social insects which live in nests or colonies. The adults are represented by workers which are sterile females, queens and males (drones) which come from unfertilized eggs and usually appear in late summer.
Typically, only inseminated queens overwinter and do so underground. In the spring, the queens of Psithyrus species wait until the Bombus nests are moderate in size and then parasitize them. The Bombus queens select a suitable subterranean cavity or surface grass clump as a nesting site. Then the Bombus queen fashions a honey pot of wax scales near the nest entrance into which she regurgitates nectar. Next she makes a pollen clump on the nest floor and lays 8-10 eggs on it. The queen will periodically add pollen and nectar to the peripheral edges of the clump, and eventually more eggs. Developmental time is 16-25 days, with 4 larval molts. Workers live about 2 weeks. Most first brood workers are small due to nutrition. The queen will increase the number of eggs laid as the number of workers to care for them increases.
During the summer, parasitism may eliminate up to 50% of the colony’s workers each week. However, a mature bumble bee nest ultimately contains about 50-400 bees at any given time; the largest known nest contained 756 bees and 385 brood (larvae and pupae).
In the late summer only males and new queens are reared in the nest. Once these new queens emerge, they mate and find a suitable place to overwinter. The males, workers and old queen and any virgin new queens die with the onset of cold weather.