the 28 species of Psithyrus were considered to constitute a separate genus.

[1][2] They are a specialized socially parasitic lineage which parasitises the nests of 'true' bumblebees, resulting in the loss of the ability to collect pollen and establish their own nests.

Cuckoo bumblebees do not create a worker caste and produce only male and female reproductives.

Before finding and invading a host colony, a Psithyrus female feeds directly from flowers until her ovaries are sufficiently developed, at which time she begins seeking a nest to invade.

She then lays her own eggs, exploiting the host workers to feed her and her developing young through pheromones and/or physical attacks.