Psithyrus

Psithyrus is a subgenus of bumblebees, now classified within the genus Bombus. These bees are known as cuckoo bumblebees due to their parasitic behavior, laying their eggs in the nests of other bumblebee species, much like cuckoo birds. Unlike their host bumblebees, Psithyrus bees lack a worker caste and do not have the specialized pollen-collecting structures found on the hind legs of other bumblebees.12

The parasitic females enter the nests of social bumblebees, and sometimes kill the host queen, to lay their eggs. The unsuspecting worker bees then care for the parasitic larvae. There is often a remarkable resemblance between a parasitic Psithyrus species and the host species it targets.1