Variable Cuckoo Bumblebee (Bombus variabilis)

The queen of Bombus variabilis invades the nest of a host bumblebee species, where she kills or dominates the resident queen and uses pheromonal manipulation to cause the host workers to raise her offspring. She produces no worker caste of her own, only reproductive males and new queens that emerge to seek out new host colonies.

Bombus variabilis is associated with host species in the subgenus Bombus, though its exact host range across its North American distribution has not been fully documented by researchers. Understanding its specific host relationships is complicated by the bee's growing rarity throughout much of its historical range.

Bombus variabilis has experienced severe population declines and is considered one of the most at-risk bumblebee species in North America, reflecting broader patterns of decline among both cuckoo and non-cuckoo bumblebees across the continent. Researchers have noted that cuckoo bumblebees are particularly vulnerable because their survival depends on the health of their host species' populations.

Historical records document the variable cuckoo bumblebee across broad portions of the eastern and central United States, but confirmed sightings have become extremely rare in recent decades. Its decline has been documented by entomologists studying the broader collapse of native bumblebee populations in North America.