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Bombus borealis belongs to the subgenus Subterraneobombus in which females can be distinguished by small ocelli at the supraorbital line (Thorp et al. 1983), while males can be distinguished by spoon-shaped penis valvesthat are turned inwardsas well as the presence of a raised longitudinal keel posteriorly on sternum 6 (Williams et al. 2008)B. borealis individuals have white pile on the face between the eyes;the fifth antennal segment is longer than the fourth or third;the first four abdominal segments are covered with yellow pile, while the remaining segments are black;and the outer surface of the male hind tibia is concave (Franklin 1912). The length of the queen varies from 15 mm to 19 mm; her wing spread from 32 mm to 39 mm; and the width of the second abdominal segment 8 mm to 9.5 mm. Workers vary in length from 10 mm to 15 mm; in wing spread from 26 mm to 32 mm; and in width of the second abdominal segment from 6.5 mm to 8 mm. Males range in length from 12 mm to 15 mm; in wing spread from 26 mm to 31 mm; and in width of second abdominal segment from 6 mm to 7.5 mm (Franklin 1912).

Alford (1975) outlines the life history of Bombus borealis. Newly mated B. borealis queens overwinter beneath the soil litter and emerge from their hibernacula in late spring. Queens are transitory for a time, growing in size while collecting pollen and looking for a suitable nest. Once a suitable nest has been found, the queen constructs an apple sized hollow structure within it. The queen deposits her eggs within a mound of pollen on the floor of the structure; she also constructs a honeypot for storing nectar. Newly hatched larvae begin consuming the pollen mound, requiring the queen to continue provisioning it. The queen periodically incubates her brood by sitting upon it and respiring to generate body heat. The larvae spin cocoons in the final instars, as do the pupa; the cocoons may be re-used later for storage of pollen or nectar. Upon pupation, the emerged adults take nectar from the honey pot. Once the nest consists of the new young workers and the queen it can be considered a social unit and is referred to as a colony. Subsequent generations are produced differently from the first: new eggs are laid in clumps in cells atop the pupating first generation of workers, and workers are now responsible for provisioning of the growing larva and the honey pot. The caste differentiation of each generation varies throughout the year, with the first generations containing all workers, followed by a worker/male split, followed by mostly males, followed by a male/queen split, followed by mostly queens. The factor initiating queen production has not been established but it appears the colony must reach a size capable of maintaining nest temperatures and food stores before queens are produced. Young queens remain in the colony and will mate during their first week. Males leave the hive and do not return; they establish a methodical flight path and mate with encountered queens. Hobbs (1966) reported that males of B. borealis will attempt to mate with queens in the nest, a behaviour seen in select other Bombus subgenera. Only the newly mated queens will overwinter in hibernacula; males, founder queens, and all workers perish.