Montana Field Guide

For definitions and diagrams of bumble bee morphology please see the Montana State Entomology Collection's Bumble Bee Morphology page. [Remarks pertain to the North America form B. bohemicus ashtoni, recently reduced to subspecies status of the Eurasian form] A medium-sized bumble bee: queens (no workers) 17-19 mm in length. Hair medium length; hind leg tibia with outer surface convex and densely hairy, lacking pollen basket. Hair on face and upper side of head (occiput) black (sometimes with yellow hairs towards back of head); sides of thorax predominantly black; T3-T4 yellowish to white at sides but T4 usually white at least in middle of back. Male 11-17 mm in length; eyes similar in size and shape to eyes of any female Bombus; antennae of medium length, flagellum 3X length of the scape; sides of thorax extensively black; T2 entirely black, T4 yellow without extensive black, T7 black (Colla et al. 2011, Williams et al. 2014).

[Remarks pertain to the North America form B. bohemicus ashtoni] Across range, queens reported April to September, males June to October (Colla et al. 2011, Williams et al. 2014). In southern Ontario, queens reported April to August, males June to October; earliest record 21 April (Colla and Dumesh 2010).

Please see the Montana State Entomology Collection's Key to Female Bumble Bees in Montana. [Remarks pertain to the North America form B. bohemicus ashtoni] Queens differ from other Montana Bombus by a combination of the outer surface of the hind leg tibia convex and hairy, pollen basket absent; occiput and face predominantly with black hairs, black hairs around base of antennae; hairs on side of thorax predominantly black; T4 yellowish white to white (at least medially); large obvious ridges on edge of S6, but barely extending laterally beyond T6.

[Remarks pertain to the North America form B. bohemicus ashtoni] Found close to wooded areas, higher elevation woodlands in northeastern mountains, boreal woodlands (Colla and Dumesh 2010, Williams et al. 2014).

[Remarks pertain to the North America form B. bohemicus ashtoni] Does not gather pollen. Reported feeding at a number of plants, including Allium, Aralia, Cirsium, Eupatorium, Hieracium, Melilotus, Penstemon, Rhododendron, Rubus, Rudbeckia, Solidago, Symphyotrichum, Symphytum, Taraxacum, Trifolium and Vaccinium (Plath 1934, Colla and Dumesh 2010, Colla et al. 2011, Williams et al. 2014).

[Remarks pertain to the North America form B. bohemicus ashtoni] A social parasite. Does not build its own nest but queen invades and claims host nests of other Bombus species, getting the host workers to raise her young. Thus, does not produce a worker caste. The primary host in the northeast is B. affinis, but also parasitizes B. terricola, and is suspected of parasitizing B. occidentalis and B. cryptarum (Plath 1934, Williams et al. 2014). In the Boston area, parasitized 30.6% of 36 B. affinis colonies, but only 14.3% of 14 B. terricola colonies (Plath 1934). Host nests found by odor recognition. Invading queen B. bohemicus ashtoni do not kill host queens (as other cuckoo bumble bees do), but instead prevent new host queens and males from developing by eating the host queen's eggs and ejecting their larvae. This strategy employed because the invading parasitic queen cannot suppress ovarian development in queen-less host workers, but may "maul" host workers into submission, reducing the likelihood of successful egg production. Parasite queens guard their eggs to prevent them from being discovered and eaten by host workers, but as host colony size increases, successful defense of parasite eggs decreases (Fisher 1983a and b, 1984, 1987).