Edited to add, it's actually a tarantula hawk. They kill and eat tarantulas!
FYI:
Tarantula hawk stings are considered to be the most painful of any North American insect. Christopher Starr wrote an article entitled, "A Pain Scale for Bee, Wasp and Ant Stings." On a scale of one to four, Pepsis formosa was one of only two insects to rate a four. This compares with a one for a Solenopsis xyloni (desert fire ant), two for a Apis mellifera (honey bee) and three for a Dasymutilla klugii (velvet ant).
Knowing that I proabably would not have got so close!
