Psithyrus

Cuckoo bumblebees are members of the subgenus Psithyrus in the bumblebee genus Bombus. Until the 1990s, Psithyrus was considered to constitute a separate genus. They are a specialized socially parasitic lineage which parasitises the nests of 'true' bumblebees; they do not collect pollen or establish their own nests. Cuckoo bumblebees do not create a worker caste and produce only male and female reproductives. They are considered inquilines in the colonies of 'true' bumblebees.