Cuckoo Bumblebee

Bombus vestalis — and the broader subgenus Psithyrus within genus Bombus — are the cuckoo bumblebees, one of the most extraordinary social parasite groups in the insect world. Cuckoo bumblebees are TRUE BUMBLEBEES (genus Bombus, same as familiar buff-tailed and red-tailed bumblebees) that have COMPLETELY ABANDONED the eusocial worker-and-colony lifestyle that defines the rest of their genus. Cuckoo bumblebees have no workers, no nests, no foragers, no brood production of their own. Instead, the entire genus has evolved a parasitic strategy: a single queen cuckoo bumblebee emerges from overwintering in spring, locates the established nest of a host bumblebee species (each cuckoo bumblebee species has 1-3 specific host species — Bombus vestalis parasitizes only Bombus terrestris, the buff-tailed bumblebee), INVADES the nest, KILLS the resident host queen (typically by stinging or biting her), takes over the colony, and uses her parasitic pheromones to suppress the host workers' own reproduction and force them to raise the cuckoo's brood. The host workers continue foraging for nectar and pollen, feeding the cuckoo's brood as if it were their own. The cuckoo's offspring (all reproductive males and queens — no workers) emerge in late summer, leave the nest to mate, and the new queens overwinter to repeat the cycle. The strategy is the same brood parasitism as the European cuckoo bird's (Cuculus canorus) — independently evolved in birds and bumblebees, with strikingly similar behavioral outcomes. Cuckoo bumblebees are anatomically distinguished from non-parasitic bumblebees: they have thicker cuticle (resistant to host defensive stings), darker more sclerotized bodies, no pollen-collecting baskets on the hind legs (no foraging needed), and reduced hair density. The species are a major topic in evolutionary biology of social parasitism research — and the cuckoo bumblebee subgenus Psithyrus contains approximately 30 species worldwide, all parasitic.