I know long-horned bees can be difficult to identify even down to genus, say nothing of species, but I thought I'd give this one a try. There were several males along with this female getting pollen from the flowers of cream wild indigo (Baptisia bracteata)on a dry prairie remnant, but this female is the only one which finally showed her face. After comparing her features and those of the males to examples of long-horned bees on BugGuide, they looked very similar to Eucera hamata, though I am sure there are other similar species and a positive ID may not be possible.