What is in this fallen oak branch?

This is an insect called a twig pruner. The larvae is making tunnels in the branch and then bores away the wood in a circle causing the branch to fall. This is not a major problem and you do not have to spray. Just pick up the fallen branches and throw them in the trash. That will keep the larvae from becoming adults to continue the cycle. 

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Related Resources:
Fallen oak shoots may be work of the twig pruner
Twig Girdler and Twig Pruner