Our best guess is an Indiscriminate Cuckoo Bumble Bee (Bombus insularis). This species is commonly encountered from May to August in Canada and forested areas of the western United States, but rarely found in the northeastern United States, where it is restricted to northern and mountainous regions. Both females and males visit flowers for nectar, including flowers from the aster family (Asteraceae). Female cuckoo bumble bees are “social parasites” of true bumblebees. They displace the resident queen in an established nest and trick the host workers into raising their offspring. For more information about this fascinating process click here.
Photo by Glenn Marangelo on 5/16/20 in Condon, MT