Likely Eucera hurdi Longhorn Bee Male - Melissodes -
Stevensville, Ravalli County, Montana, USA September 1, 2019 Size: 1.3 cm?
This is a good match for Eucera hurdi as there are four Eucera listed for Montana and Eucera hurdi is the only one that looks like this bee. It was nectaring on Rabbitbrush. Thank you!