Montana Field Guide

For definitions and diagrams of bumble bee morphology please see the Montana State Entomology Collection's Bumble Bee Morphology page. A medium-tongued small species; queens 15-17 mm in length, workers 10-14 mm. Hair long and uneven; head length medium, the cheek as long as wide; mid-leg basitarsus with the back far corner rounded, hind-leg tibia outer surface flat and shiny (with a fringe of pale orange-tipped hairs), pollen basket present; hair on face, sides and top of head yellow with many black hairs intermixed; thorax yellow usually with many black hairs intermixed (appearing grayish or cloudy); T1 yellow, T2 largely or completely yellow, T3 often with some orange or completely black; tail (T4-5) usually with very pale orange hair, T5 orange. Males 11-14 mm in length. Eyes similar in size and shape to any female bumble bee; antennae moderately thick and long, flagellum 3.3X longer than scape; hair color pattern similar to queens and workers, although rarely T3-7 all black (Koch et al. 2012, Williams et al. 2014).

An early emerging species, queens reported from across the range February to September, workers April to September, males May to September (Williams et al. 2014). In Washington, queens reported February to August, workers March to August, males April to August (Koch et al. 2012). In California, queens reported early April tp late October, workers early April to late September, males early May to late September (Thorp et al. 1983).

Please see the Montana State Entomology Collection's Key to Female Bumble Bees in Montana. Females told from other Montana Bombus by a combination of the outer surface of the hind-leg tibia concave and shiny (not hairy), pollen basket present; T2 with yellow and/or black hair; cheek as long as, or slightly longer than, wide; T2-6 with some yellow and/or orange; scutum with yellow and black hairs intermixed, giving a cloudy appearance; scutellum also cloudy or with yellow hairs only; scutellum and scutum in front of wing bases similar in color or with the scutum darker than scutelum.

Open grassy sites (but usually not expansive prairie) at various elevations, chaparral and shrub steppe, and particularly aspen parkland and montane meadows to alpine and Arctic tundra (Hobbs 1967, Macior 1974, Richards 1978, Bauer 1983, Mayer et al. 2000, Wilson et al. 2010, Miller-Struttmann and Galen 2014, Williams et al. 2014). Also cropland of commercial Vaccinium (Ratti et al. 2008).

Feeds on a wide variety of plants, including Achillea, Apocynum, Arnica, Astragalus, Berberis, Calypso, Ceanothus, Cirsium, Collinsia, Crataegus, Delphinium, Dodecatheon, Epilobium, Erigeron, Eriogonum, Geranium, Heracleum, Linaria, Lonicera, Lupinus, Mentha, Mertensia, Onosmodium, Origanum, Pedicularis, Penstemon, Phacelia, Phyllodoce, Prunus, Rhamnus, Rhododendron, Ribes, Rosa, Rubus, Rudbeckia, Salvia, Senecio, Sisyrinchium, Solidago, Stachys, Symphoricarpos, Symphyotrichum, Taraxacum, Trifolium, Vaccinium and Vicia (Macior 1974, Ackerman 1981, Bauer 1983, Thorp et al. 1983, Mayer et al. 2000, Ratti et al. 2008, Wilson et al. 2010, Koch et al. 2012, Williams et al. 2014, Miller-Struttmann and Galen 2014). Also a frequent visitor to commecial Vaccinium (high-bush blueberry and cranberry) crops in southern British Columbia, where their activities contribute significantly to increased berry mass of both crops (Ratti et al. 2008).

Nests are built below ground, on the ground surface and above ground (Wiliams et al. 2014). Of 51 nests in southern Alberta aspen parkland, only one nest was built in open grassland on the surface, while 19 were built in artificial nests near the meadow-woodland ecotone (all but four of these were above ground), and the remaining 31 nests were built in woods (all but four nests located on or above ground). In southern Alberta, B. mixtus was the only bumble bee species that tended to avoid nesting underground (see also Hobbs 1967). Nest establishment occurred during mid May to mid June, with a peak in late May (see also Hobbs 1967). Only rarely were nest entrances camouflaged (Richards 1978). Numbers of larvae and pupae in first broods is 7-10. Queens sometimes invade hives occupied by the same species (Hobbs 1967. Males patrol circuits in search of queens; parasitism by cuckoo bumble bees not described but is likely (see comments in Hobbs 1967).