There were many on an old oak tree in my yard this evening. I livein northern MI and I originally thought these were cicada killer wasps. After a little research, I found that not to be so. I have no idea what these are! Some appeared to be laying eggs in holes in the tree. They are quite large. The body is 3" or more and the tail is quite long (3"+). They seemed quite passive and was even able to touch them without them flying off or attacking.