Bombus citrinus

Bombus citrinus belongs to the parasitic Bombus subgenus Psithyrus. A previous classification had Psithyrus listed as its own genus, and so this species used to be identified as Psithyrus citrinus. Members of Psithyrus are distinguished from other Bombus based on an abdomen with thick tergites, a long stinger, enlarged mandibles, loss of corbiculae, no worker castes, and a reduction in wax glands and its production;[4][5] all of these features are associated with being inquiline parasites that replace host queens.[6][7] Of the other species within this subgenera it is most closely related to Bombus insularis and Bombus variabilis and has sometimes been subgrouped with these as Laboriopsithyrus or Citrinopsithyrus.[8] Psithyrus diverged around 20 million years ago from a clade containing the subgenera of Megabombus, Senexibombus, and Diversobombus.[5][6] Bombus citrinus is believed to have originated from this line around 2 million years ago in the Eastern Nearctic region.[6] While they have developed specializations toward their specific host species, in a coevolutionary relationship,[7] some of their modifications were likely to have remained broad enough to enable these Psithyrus species to diversify and disperse following a small range of hosts in their time and area.[6] Bombus citrinus may have coevolved with its host species as they emerged within the same time frame and area; around 13 to 5 million years ago for Bombus vagans and around 2 million years ago for Bombus impatiens.[6]

Like all cuckoo bumble bees, the outer tibial surface of the hind leg is convex and densely hairy, rather than adapted for pollen transport. Their heads are more teardrop shaped and the thorax is predominantly yellow including the lower sides and sometimes down to segment T4 or 5. Hair on the face is usually black with some yellow hairs, there are usually no black hairs on the thorax and would not form a band between the wing bases. Metastomal hairs are short and even and longer on T5 than the other joints and matte with dense punctures on T6.[9] The wings are slightly opaque ranging from a reddish brown to brownish black and yellowish during pubescence. They are sometimes interspersed with black hairs.[10] The male genitalia contain some of the most distinguishing characteristics when differentiating bumble bee species. The gonostylus are lined with long hairs, hairs on the yellow volsella are much shorter. The penis valve is long and thin and relatively straight compared to other bumble bee species.[11]

Map showing the distribution of Bombus citrinus

Found in parts of the Northeastern area of North America. Their distribution includes eastern to central parts of Canada, Northeastern U.S states and parts of the mid-western and southeast regions of the U.S.[3]