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Bombus insularis belongs to the subgenus Psithyrus which was historically treated as a separate genus from the rest of the Bombus due to its parasitic lifestyle (Alford 1975). Michener (2000) considers all species formerly treated as Psithyrus to be Bombus. Psithyrusis a parasitic subgenus, and all species lack a worker caste, cannot produce wax, have a stronger exoskeleton, and have no pollen baskets (Alford 1975). Psithyrus males can be distinguished by an almost straight penis valve shaped like an arrow head; the hind tibia of Psithyrus queens is convex with a hairy outer surface (Williams 2008). Bombus insularis queens have yellow pile covering the mesopleura to the base of legs, dark venter, fourth antennal segments that are much shorter than the third or the fifth, and moderately stained wings (Franklin 1912). Bombus insularis can be distinguished from the closely related Bombus fernaldae by the presence of yellow hair between the antennal bases (Thorp et al. 1983). Queens vary in length from 13.5 mm to 18 mm; in wingspan from 32 mm to 38 mm; and in width of second abdominal segment from 7 mm to 9 mm (Franklin 1912).

Bombus insularis is a cleptoparasite of the nests of other Bombus species. Bombus insularis parasitizes at least 6 other Bombus subgenera (Thorp et al. 1983), an unusually large host range for the Psithyrus subgenus which normally specializes in a single subgenus (Alford 1975). Alford (1975) reviews the life history of B. insularis. Queens of B. insularis emerge later in spring than other Bombus queens and forage on pollen and nectar until a suitable nest to invade is found. Mature nests that were founded earlier in the year appear to be preferred, although entrance to the colony is normally easier if the colony is smaller. The queen enters the nest and will remain motionless with legs drawn in close if she is attacked. Upona successful entrance, the queen will hide for a short time, presumably to acquire the odor of the colony to avoid further attacks by workers. Sometimes B. insularis queens will kill the host queens, but often they are able to cohabitate. She will then construct her own egg cells and quickly lay eggs, sometimes feeding on the host brood. Her eggs and larva are reared by the host workers alongside the host brood. The newly emerged queens and males will immediately leave the nest to acquire mates. The B. insularis queen will then leave and seek out other nests to parasitize.