Montana Field Guide

For definitions and diagrams of bumble bee morphology please see the Montana State Entomology Collection's Bumble Bee Morphology page. Medium sized and short-tongued: queens 20-21 mm, workers 9-15 mm. Head short, cheek slightly shorter than broad; oceli on a line between the back (top) edges of the compound eyes (not below the line), hindleg tibia flat on outer surface and lacking long hair, but with long fringe on sides forming a pollen basket. Hair moderately short and even; upper surface of the thorax with at least a large central black spot (often a black band) between the wings. Abdominal T1 and T6 always black, T2 usually black anteriorly, but if predominantly yellow then head and thorax also predominantly yellow. If T2 and T3 entirely black then T4 and T5 completely or extensively white or yellow-orange. Even darkest individuals with white on tail have some grayish hairs on face and dorsal surface of head. Males 12-16 mm, hair color pattern similar to queens and workers, antenna short, flagellum just over 2x longer than the scape (Williams et al. 2014).

Queens reported March to September, workers May through September, males May to November (Koch et al. 2012, Williams et al. 2014). In California, queens reported early February to late November, workers and males early April to early November (Thorp et al. 1983).

Please see the Montana State Entomology Collection's Key to Female Bumble Bees in Montana. Queens and workers differ from other Montana Bombus by having a cheek slightly shorter than wide, and white or yellow hairs on abdominal T5 (and usually T4); T1 and T6 are always black, T2 usually black (at least on the posterior half), T3 may be black or yellow. B. terricola always has yellow hairs on T2 and T3 (Koch et al. 2012).

Open grassy areas, prairie, urban parks and gardens, sagebrush steppe, mountain meadows to alpine tundra (Macior 1974, Bauer 1983, Bowers 1985, Tommasi et al. 2004, Cook et al. 2011, Hatten et al. 2013, Williams et al. 2014). Also found in commerical highbush blueberry and cranberry fields in southern British Columbia (Ratti et al. 2008). Nests in southern Alberta constructed in woods, open meadows, and most often in the woods-meadows ecotone (Richards 1978).

A generalist forager. A short tongue requires it to often rob nectar from flowers it visits (Pyke et al. 2012). Flowers visited include Aconitum, Allium, Arnica, Astragalus, Balsamorhiza, Brassica, Calypso, Castilleja, Ceanothus, Centaurea, Chionophila, Chrysothamnus, Cirsium, Clematis, Corydalis, Delphinium, Dianthus, Dodecatheon, Epilobium, Erigeron , Eriogonum, Erysimum, Frasera, Geranium, Grindelia, Haplopappus, Heracleum, Ipomopsis, Iris, Lathyrus, Ligusticum, Linaria, Lotus, Lupinus, Malus, Medicago, Melilotus, Mentha, Mertensia, Nama, Onosmodium, Origanum, Orthocarpus, Oxytropis, Pedicularis, Penstemon, Phacelia, Polygonum, Potentilla, Prunus, Raphanus, Rhododendron, Ribes, Rosa, Rubus, Salix, Salvia, Sedum, Senecio, Sisyrinchium, Solidago, Symphoricarpos, Tanacetum, Taraxacum, Thermopsis, Trifolium, Vaccinium, Vicia and Zea (Hobbs 1968, Beattie et al. 1973, Macior 1974, Ackerman 1981, Bauer 1983, Thorp et al. 1983, Mayer et al. 2000, Rao and Stephen 2007, Ratti et al. 2008, Wilson et al. 2010, Colla and Ratti 2010, Koch and Strange 2012, Koch et al. 2012, Pyke et al. 2012, Williams et al. 2014, Miller-Struttmann and Galen 2014). May be involved in pollinating the fairy slipper orchid (Calypso bulbosa) in our region (Ackerman 1981). Visits commercial highbush blueberry and cranberry (Vaccinium) fields in British Columbia (Ratti et al. 2008, Colla and Ratti 2010), and prefers to visit flowers of alfalfa over sweetclover where both are available (Hobbs 1968).

Nests underground. In southern Alberta, nest initiation spanned mid-May to mid-June (Richards 1978). First broods average 8-9 eggs, larvae, and pupae, second and third broods about half that size (Hobbs 1968). One California colony in July contained five young queens and more than 80 workers (Plath 1934). Males patrol regular circuits in search of queens (Williams et al. 2014). Nests and broods destroyed by parasitic bees, including Bombus suckleyi. One Alberta study reported B. suckleyi depredated 80% of 15 B. occidentalis nests (Hobbs 1968).

On March 16, 2016, the U.S. Fish and Wildlife Service published a notice in the Federal Register indicating that, "Based on our review of the petition and sources cited in the petition, we find that the petition presents substantial scientific or commercial information indicating that listing the western bumble bee (Bombus occidentalis) may be warranted". Completion of status review is expected in 2023. Additional information on the species' management can be found on the U.S. Fish and Wildlife Service's Species Account