Taking nectar on Rosinweed, Silphium species. See guide page for identification references. Looks like this is one member of the genus that is distinctive. This really looks like a small Carpenter Bee (Xylocopa). On this female, however, you can see the distinctive pollen-collecting hairs on the underside of the abdomen, showing it is a Megachilid.

Thanks to Lynette, Richard, and Eric for working out the identification of this bee--excellent detective work. I realized what my photos were based on their discussions--see other images in guide, for example:



Identification references also given in guide page for the species.