Cuckoo bumblebees are are members of the subgenus Psithyrus in the bumblebee genus Bombus. They are a specialized lineage which has lost social behavior, and lost the ability to collect pollen.
Before finding and invading a host colony, a Psithyrus female will feed directly from flowers. Once she has infiltrated a host colony, the Psithyrus female will kill or subdue the queen of that colony and forcibly (using pheromones and/or physical attacks) “enslave” 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.