To continue identifying your bee, return to the key, beginning at couplet #[NUMBER??]; to learn more about cuckoo bumble bees, read on:

Cuckoo bumble bees are externally similar to true bumble bees (and often are mistaken for them), but they can be differentiated by the absence of a pollen basket on the hind leg (in females), and by the more sparse covering of hair on the upperside of the abdomen. As in true bumble bees, solitary mated Psithyrus queens hibernate throughout the winter and become active during the spring. The new cuckoo bumble bee queen typically becomes active somewhat later in the year than does the true bumble bee queen, the reason being that, instead of initiating its own colony, the Psithyrus queen needs to find an already-established colony of true bumble bees. Once that is done, she enters the true bumble bee colony, kills all of the eggs, larvae, and pupae of the existing colony, and lays her own eggs. Her brood will be taken care of by the true bumble bee workers; no Psithyrus workers are produced. This parasitic strategy, which is superficially reminiscent of that of the cuckoo bird in the nests of other bird species, is the basis of the common name of this group of bumble bees.

After the colony is parasitized by the cuckoo bumble bee, the true bumble bee queen remains in the nest, but she stops laying eggs, and the colony eventually dies off without producing males and new queens as it usually would. Some species of true bumble bees are more prone than others to parasitization by cuckoo bumble bees. One species, Bombus fervidus, apparently is able to ward off Psithyrus by covering the invading queeen with honey. In uncommon instances, Psithyrus even tries to parasitize species other than true bumble bees. For example, Plath (1927) reported an unsuccessful attempt by a cuckoo bumble bee to enter a honey bee colony.