Feeding on flowers of Mule's Ears (Wyethia mollis), a large flowered perennial composite (Asteraceae).

After carefully going through the key in Thorp et al(1), I'm fairly confident this is a male Bombus centralis. The key breaks were all fairly clear, distribution map included records in the area, and the color pattern (Fig. 116 on pg 73 of Thorp et al(1)) fit perfectly. In particular, the mostly yellow thorax with a black band between the wing bases, and the "front-to-back" sequence of abdominal segment colors (two terga yellow, two terga red, remainder black) seem to be distinctive of B. centralis.

Nevertheless, I've been known to make errors with bees...so confirmation or correction is much appreciated.