Heil Ranch Open Space, Boulder County, Colorado, USA October 10, 2015 Size: ~ 20-22 mm
This largish bumble bee (not much smaller than B. nevadensis) was nectaring at Rabbitbrush at the Picture Rock Trailhead at Heil Ranch Open Space near Lyons, elevation ~5600'. I have another head/lateral view and an image of the abdomen which seems to have 7 tergites. Any help with its ID is much appreciated.