Bombus bifarius has a relatively small body size ranging from 8 to 14 mm (0.31 to 0.55 in) for workers and 15–19 mm (0.59–0.75 in) for queens, with short, even hair covering their bodies. B. bifarius individuals express multiple color polymorphisms; however, many similarities exist between these color variants. Hair on the faces of B. bifarius individuals is usually yellow or white in color and sometimes exhibits black coloration on the top of the head. In at least the lowermost third of the thorax, there is also black coloration. The hindlegs and pollen baskets can be a brownish-orange or black, depending on whether metasomal tergite (abdominal segment) 3 is black or not. In the nominate color variant, metasomal tergites 2 and 3 are red, while in the nearcticus color variant, metasomal tergites 2 and 3 are black.
Sexual dimorphism Males are similar in size to female workers, ranging from 8 to 13 mm (0.31 to 0.51 in). Their eyes are also similar in size and shape to those of their female counterparts, which is in direct contrast to other species of bumblebee. Colorations on their bodies are similar to those of workers and the queen; however, T3 and T6 are most frequently black in males and can vary between black, red, and yellow in workers and queens.
Nests Bombus bifarius nests are made underground or on the surface of the ground. These nests are often small and are made up of one singular open chamber. B. bifarius can also make their nests in abandoned rodent nests.
Diet Bombus bifarius forages for pollen and nectar from the following plants: Aster, Centaurea, Chrysothamnus, Cirsium, Epilobium, Ericameria, Haplopappus, Helenium, Lupinus, Melilotus, Monardella, Penstemon, Ribes, Senecio, Solidago, and Symphoricarpos.