This morning was cool and overcast and although the bumblebees showed up for work on the Helianthus, they were definitely on the slow side, in need of a coffee or two. This plant is always abuzz with bees, but when it's warm they move from flower to flower so rapidly it is difficult to photograph them with a glacial focus-speed digital camera.
But today they were too stoned by thermal shortfall not to pose. I think these are workers of Bombus griseocollis, which is widespread in North America. Here is a site with an interactive key to help with bumblebee id.