Broad-banded Hornet Fly - Spilomyia alcimus (or texana, see comments)
This fly was visiting Black Elderbery (Sambucus canadensis) flowers, popping in and out of very harsh light. It has broader yellow bands on its abdomen than the similar S. longicornis, which flies late summer to fall.
References
Update. See discussion and additional detail images added, trying to get a view of that pesky propleuron.
The Fells