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Bombus flavifrons belongs to the diverse subgenus Pyrobombus Dalla Torre which is characterized by a malar space of medium length but longer than its apical width and antennal flagellum 2.5 to 3x the length of the scape. The penis valves of the males are usually hook shaped (Thorp et al., 1983). The robust body of B. flavifrons is densely covered in coarse yellow and black pile. The face and head are primarily yellow (Franklin 1912) and mixed with black on the anterior scutum (Thorp et al. 1983). Abdominal segments 3 and 4 are typically black but males may have yellow pile and females, reddish pile on the apical portion (Thorp et al. 1983). Pile at the base of the legs is light and the wings subhyaline (Franklin 1912). Body size and wingspan varies between castes: queens are 13 to 16 mm with wingspans of 27 to 34 mm, workers range between 9 to 12 mm with wingspans of 19 to 27 mm, and males are 11 to 12 mm with wingspans of 25 to 26 mm (Franklin 1912). The male genitalia is very similar to B. centralis with smoothly rounded, sickle shaped penis valves, narrow valsellae and a weakly trilobate sternite 8 that is apically membraneous (Thorp et al. 1983, Franklin 1912).

Bombus flavifrons has an annual colony cycle. Queens emerge in late March from shallow hibernacula dug into the soil to forage and find suitable nest sites, often in abandoned mouse nests. Pollen is collected and manipulated by the founding queen into a ball. Eggs are laid in vertical rows on the top of the ball and covered over with pollen and wax. A nectar pot previously constructed allows the queen to feed while incubating the brood clump at 30-32° C. Larvae hatch after 4-5 days and begin to feed on the pollen mass. The queen continues foraging and regurgitates nectar to the larvae through openings on the top of the brood cells. After 4 molts, larvae spin loose silk cocoons and pupate. The queen now lays a second and third batch of eggs on top of the pupal cocoons using the pollen and wax from the first batch. Female workers emerge 4-5 weeks after the first eggs are laid and take over foraging and nest construction activities. The queen now exclusively constructs egg cells and lays eggs. As the colony expands upwards and outwards and workers increase in number, fertilized eggs become young queens and males emerge from unfertilized eggs. Caste differences are physiological and large numbers of workers are able to provide the food necessary to rear queens. Males are often produced before the new queens and will leave the colony almost immediately after emergence. Young queens may perform both nest and foraging duties prior to mating. Both sexes mate multiple times. Males will mount the queens in the air and continue coitus for several minutes on a nearby surface until kicked off by the female. Once mated, queens prepare for hibernation by eating and increasing vital fat body reserves. The colony declines in late August; workers, males, and the original queen die. The newly mated queens overwinter in small cells in the soil in preparation for spring. (Adapted from Alford 1975 and Thorp et al., 1983)

Polylectic, adults consume nectar and pollen from a variety of 61 plant families, primarily Compositae, Leguminosae, Saxifragaceae, and Labiatae in California with Cirsium, Epilobium, and Mentha receiving the most visits (Thorp et al., 1983).