Feeding on Desert Milkweed,
Asclepias subulata. Females of this species were very large, about 40 mm body length, and are identified by the curly antennae. In the male these are more straight. There were several individuals buzzing around the milkweed, and I could not keep individual insects straight, therefore these are all posted as unlinked images. Another individual in close proximity:
Comments indicate this is likely
Pepsis thisbe or
Pepsis formosa. Perhaps the newly-posted larger photos will help.