Psithyrus

Cuckoo bumblebees are members of the subgenus Psithyrus in the bumblebee genus Bombus. Until recently, the 29 species of Psithyrus were considered to constitute a separate genus.[1] They are a specialized lineage which has lost the ability to collect pollen and to rear their brood. They have lost the worker caste and produce only sexuals, male and female. They are inquilines in the colonies of other bumblebees. Before finding and invading a host colony, a Psithyrus female feeds directly from flowers. Once she has infiltrated a host colony, the Psithyrus female usurps the nest: she kills or subdues the queen of that colony and forcibly (using pheromones and/or physical attacks) "enslaves" the workers of that colony to feed her and her developing young. When the young emerge, they leave the colony to mate, and the females seek out other nests to attack.

Bombus (Psithyrus)
Bombus bohemicus
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Apidae
Genus:	Bombus
Subgenus:	Psithyrus
Species
See text

Female cuckoo bumblebees aggressively attack host colony members, and sting the host queen, but ignore other animals (including humans) unless disturbed.