River Birch insect
I have a river birch in my front yard which I removed a large limb from about 6 years ago. This year the exposed cut has become peppered with bore holes. I cant find the culprit in the act so I'm looking for possible leads.
Oakland County Michigan insect issues
There are many potential culprits for the damage shown in your picture. The tiny holes could be the work of powderpost beetles. They will only attack dead wood, so by themselves they don't pose a threat to the tree. It is troubling that the wound has not healed over and I can see signs of fungal activity there that could affect the long term health of the tree.
What steps should I take to protect this tree from both the insects and the rot/fungus as well?
Anything you can do to keep the exposed wood as dry as possible will help. Pruning of the surrounding vegetation to reduce shading and increase air flow around the site will help. Removing the flag pole mount would also be a good idea. There is probably moisture trapped between the wood there that is creating conditions for improved fungal growth.
We do not recommend treating the exposed wood with paint or any of the commercially available products that are marketed to promote "healing of pruning cuts". Those coatings usually trap moisture underneath and lead to more problems than they solve.
If the tree seems healthy otherwise, then there is probably nothing more that you need to do at this time. You can see in the photo that the tree is forming callous tissue around the edge of the wound. Hopefully, this will eventually expand to cover the exposed wood completely. However, it is a large exposed area and the original pruning cut may have been a little too close to the main trunk, so the tree may never be able to heal over completely.
Thank you for your excellent advice Nate. My tree and I thank you!
Keep up the great work,