The term cuckoo bee is used for a variety of different bee lineages which have evolved the cleptoparasitic habit of laying their eggs in the nests of other bees, reminiscent of the behavior of cuckoo birds. The name is technically best applied to the apid subfamily Nomadinae. Females of cuckoo bees can be easily recognized in almost all cases, as they lack pollen collecting structures (the scopa) and do not construct their own nests. They often have reduced body hair, abnormally thick and/or heavily sculptured exoskeleton, and saber-like mandibles, though this is not universally true, and other less visible changes are common, as well.
They typically enter the nests of pollen-collecting species, and lay their eggs in cells provisioned by the host bee. When the cuckoo bee larva hatches it consumes the host larva's pollen ball, and, if the female cleptoparasite has not already done so, kills and eats the host larva. In a few cases where the hosts are social species (e.g., the subgenus Psithyrus, which are parasitic bumble bees that infiltrate nests of species in the subgenus Bombus), the cleptoparasite remains in the host nest and lays many eggs, sometimes even killing the host queen and replacing her - such species are often called social parasites, though a few of them are also what are referred to as "brood parasites."
Many cuckoo bees are closely related to their hosts, and may bear similarities in appearance reflecting this relationship. This common pattern gave rise to the ecological principle known as "Emery's Rule". Others parasitize bees in different families, like Townsendiella, a nomadine apid, one species of which is a cleptoparasite of the melittid genus Hesperapis, while the other species in the same genus attack halictid bees.
The number of times cleptoparasitic behavior has independently evolved within the bees is remarkable; C. D. Michener (2000) lists 16 lineages in which parasitism of social species has evolved (mostly in the family Apidae), and 31 lineages parasitizing solitary hosts (mostly in Apidae, Megachilidae, and Halictidae), collectively representing several thousand species, and therefore a very large proportion of overall bee diversity. There are no cuckoo bees in the families Andrenidae, Melittidae, or Stenotritidae, and possibly the Colletidae (there are only unconfirmed suspicions that one group of Hawaiian hylaeine species may be parasitic).
- The Bees of the World, C. D. Michener (2000)