Bumble Bees of Montana
Bombus (Psithyrus) insularis (Smith, 1861) Indiscriminate Cuckoo Bumble Bee
Bombus insularis is a social parasite known to breed in colonies of B. appositus, B. fervidus, B. flavifrons, B. nevadensis, and B. ternarius. It has also been recorded inside colonies of B. rufocinctus, B. occidentalis, and B. terricola (Williams et al. 2014). It is known from the Rocky Mountains north to Alaska and scattered across the tundra/taiga and boreal regions of Canada to the maritime regions of eastern Canada and the U.S. (Williams et al. 2014). It is moderately common in Montana, collected mostly in the western half of the state.
Recorded Montana Distribution
Like all species in the subgenus Psithyrus, B. insularis lacks pollen baskets on the hind tibiae. Characters used for identification include yellow hairs on the face near the antennal bases and yellow hairs on the top of the head. The small ridges on abdominal S6 are relatively large and rounded in cross-section.
This species is mostly easily confused with B. flavidus and B. suckleyi, the other two species within the subgenus Psithyrus known from Montana. Bombus flavidus has black hairs at the base of the antennae, T6 is usually tightly curled below the abdomen, and the ridges on S6 are so small they are barely noticeable. Bombus suckleyi has no yellow hairs on the face or the top of the head and has ridges on S6 that are much more obvious.