This tarantula hawk was taking nectar on milkweed (Asclepias subulata) near a wash in the Mojave Desert. It's a female, since its antennae are curled---rather than straight, as in males. (Better seen in second image of series.)

After studying the wing-venation comments appearing here, I think this is a species of Pepsis rather than Hemipepsis. This is because (if I deciphered the wing venation correctly) the 1st recurrent vein appears to meet the 2nd submarginal cell in its basal third. (See third image in this series.) Confirmation, correction, comments appreciated.