Bombus griseocollis. This is one of the species that seems to be holding its own in terms of numbers.  This worker has a corbicula on its tibia full of a mix of pollen and nectar, it is laying on a piece of black felt. I like the the contrast in textures
<a href="//www.flickr.com/people/54563451@N08" class="extiw" title="flickruser:54563451@N08">USGS Bee Inventory and Monitoring Lab</a>
CC BY 2.0
<a href="//commons.wikimedia.org/wiki/Flickr" class="mw-redirect" title="Flickr">Flickr</a>: <a rel="nofollow" class="external text" href="https://www.flickr.com/photos/54563451@N08/10058874876">Bombus griseocolis, F, Side, MD, PG County_2013-07-10-16.34.05 ZS PMax</a>