Mystery Insect - answer: sawfly

Several people correctly identified the mystery insect, but for those who didn't read through everybody's comments, the answer is that the critter is a sawfly, probably Acantholyda erythrocephala:
http://bugguide.net/node/view/15779

My understanding is that sawflies are technically a type of wasp, even though their bodies are very differently shaped from what we normally think of when we think of wasps.