Cuckoo Bumble Bees (Bombus (Psithyrus))

There are about 250 species of bumble bees, genus Bombus, of which 30 have evolved into social parasites known as cuckoo bumble bees [1]. All cuckoo bumble bees were at one time placed in their own genus, Psithyrus, which is now treated as a monophyletic subgenus within Bombus [4]. That is, they appear to share a single evolutionary origin from a presumably parasitic ancestor, rather than each species evolving separately from its free-living host species [2].

The mated Psithyrus female emerges from her overwintering site 1–2 months after the free-living bumble bee host emerges. This delay allows her to invade an established nest of the host species, typically containing some tens of workers. She subdues or kills the host queen and starts to lay her own eggs. These are reared by host workers into new Psithyrus sexuals (males and females), since all Psithyrus species lack a worker caste. The lack of a worker caste has resulted in one of the most distinguishing features of Psithyrus...