Montana Field Guide

For definitions and diagrams of bumble bee morphology please see the Montana State Entomology Collection's Bumble Bee Morphology page. Large sized and long-tongued: queens 22-26 mm, workers 16-19 mm. face long with cheek slightly longer than broad, clypeus smooth and lacking pits or punctures in middle; hind leg tibia flat on outer surface and lacking long hair, but with long fringe on sides forming a pollen basket, mid leg basitarsus with back far corner acutely pointed but not narrow. Hair short and even; hair black usually between bases of wings, hair on face black but upperside of head usually yellow, sides of thorax black, T1 with yellow hairs dominant on sides, T2-3 yellow, T4 black. Males 13-21 mm, compound eyes greatly enlarged (larger than for any female Bombus), hair color pattern similar to queens and workers but upper side of head yellow, antenna short, flagellum 2x longer than the scape (Colla et al. 2011, Williams et al. 2014).

Bombus auricomus is considered a species that emerges at intermediate times (not early or late)(Grixti et al. 2009). Queens active April to October, workers May to October, males June to October (Williams et al. 2014). In Illinois, queens were first observed on 12 May, the first worker on 20 July (Frison 1918). In southern Ontario queens active May to October, workers July to September, males August to September (Colla and Dumesh 2010, Colla et al. 2011); earliest record 5 May.

Please see the Montana State Entomology Collection's Key to Female Bumble Bees in Montana. Combination of hair of face black with upperside of head usually yellow, hair on sides of thorax mostly black, clypeus smooth (not pitted) along midline, T2 yellow, proximal portion of basitarsus of hind leg blunt and distal posterior corner of basitarsus of mid leg acutely pointed but not narrow, and presence of pollen basket help distinguish females of this species from other Bombus. Mostly black hairs along midline of T1, and clypeus with pits uniformly spread across surface (not concentrated in middle), separate this species from B. pensylvanicus; strong black band on thorax between wings, and T6-7 black in males, separate this species from B. nevadensis.

Open farmlands and fields in Ontario, tallgrass prairie patches in the Midwest (Hines and Hendrix 2005, Colla and Dumesh 2010). Throughout wooded areas of foothills and river valleys of southern Alberta, but not on the prairies (Hobbs 1965a); the observations from Alberta are questionable and probably represent misidentified B. nevadensis for reasons stated above (see Range section).

Feeds on a variety of flowers; nine species reported in southern Ontario. Species include Astragalus, Carduus, Cirsium, Crataegus, Dalea, Delphinium, Dipsacus, Dodecatheon, Echinacea, Hypericum, Iris, Lonicera, Monarda, Penstemon, Ranunculus, Rubus, Solanum, Spiraea, Taraxacum, Thermopsis, Trifolium and Vicia (Macior 1968, Macior 1974, Colla and Dumesh 2010, Colla et al. 2011, Williams et al. 2014, Miller-Struttmann and Galen 2014, Tripoldi and Szalanski 2015).

Nests are small, sometimes underground, more often (perhaps) on the ground surface in abandoned mice nests; one occupied artificial nest in Illinois was 45 cm underground, a second nest was in an abandoned mouse nest in a hollow cement block in the foundation of a small cabin (Frison 1917, 1918; Plath 1934; Williams et al, 2014). Nests in Illinois were established from mid-May to mid-June. A nest on 24 June contained 9 eggs, 3 larvae, 5 pupae, 1 new worker, and the queen; as many as 8 workers were present on 7 July. This queen produced a total of 45 eggs before her death on 28 July; eggs were laid one per cell, not in small batches. A second nest contained 10 workers, 3 new queens, and 3 males on 6 September. Males perch on grass stalks and fence posts, and chase moving objects in search of queens (Frison 1917, 1918). Nest parasites poorly known.