On Asclepias verticillata in a sand prairie and old field. I posted photographs of a similar bee from the same site on the same day previously, which were ID'd by Dr. Ascher as Protandrena cockerelli: https://www.inaturalist.org/observations/88683177. Reviewing my photos I found these and feel they are better shots - note the ivory-yellow knees on the first two photos. These might be the same bee posted previously though that seems unlikely. This series, at least, is of a single bee. My guess is that this is a female P. cockerelli.