Pepsis species. Tarantula wasp. It's a female. You can tell by her curly antennae.
They like to nectar on milkweed. They catch tarantulas to feed to their young. We haven't noticed any tarantulas at the arboretum, but I can always hope!
On the same plant, a large Polistes wasp.
Then, on the same plant (!) but also moving among adjacent plants, was this wasp that I identified as Ammophilia. These wasps catch and paralyze caterpillars, put them in a burrow under the sand, and lay an egg on them. The baby wasp hatches with a ready food supply.
I have also been finding lots of caterpillars in that area of the arboretum, so maybe that's what the wasps are after.