Q: Look at this worm that we pulled out of a fallen oak we were chopping for firewood. As you can see, they tunnel into the oak, eating with their little pinchers and leave a trail of sawdust-type substance behind.

What in the world is this thing?

A: It’s a flatheaded borer, the larvae of a common beetle. The beetle lays her eggs on the bark of a stressed tree and if conditions are right the eggs hatch and the larvae bore into the tree.

The larvae chew through the living cambium under the bark. If several are present the trunk can be girdled, leading to quick death.

Flatheaded borers can continue eating inside the tree after it dies. That’s probably what happened in your case. The oak died, you cut it for firewood and then discovered the odd-looking creatures.

I don’t think you have cause for worry unless the rest of your trees are thinning and looking stressed.

see
http://warnell.forestry.uga.edu/service/library/for99-001/for99-001.pdf