Q: There were hundreds of these bugs on my Nellie Stevens holly, stripping the plant leaves. Then they up and left!
A: It’s a gray blister beetle, Epicauta caviceps.
It doesn’t usually eat leaves but tender new growth on a holly might be attractive. I don’t think they are worth spraying with insecticide because they appear and disappear so fast.
Avoid handling them…..or you’ll quickly find out why they are called “blister beetle”! Blisters from their “juice” will form on thin skin.