So a year ago I discovered an borer in my apple tree:

http://forums.gardenweb.com/discussions/3387485/m=23/found-this-larva-in-my-apple-tree-trunk-could-it-be-the-problem

I thought I would share what I did as it seems to have worked!

First, I aggressively dug out the borers. I used a letter opener and peeled off the outer bark anywhere near the holes. I swear I very nearly girdled the trunk, but I figured I was close to pulling the tree out anyways... I found and killed 4 fattened borers. Very satisfying.

Next, I painted the trunk with interior latex paint. I don't believe I diluted it.

Next, I dug down a few inches around the trunk, took some window screen, and wrapped the trunk. The bottom of the screen was buried next to the trunk, and the top of the screen was zip-tied around the trunk. I folded the screen over several times where I zip-tied it, so it was firmly held in place but there was "give" and room for the trunk to grow. My theory was that this would prevent any remaining borers from emerging to complete their life-cycle, and would hopefully inhibit the adult beetle from gaining satisfactory access in laying its eggs.

Today I removed the screen to look for any frass and new borer damage and saw none! I re-painted the trunk and will wrap it again with layered window screen when the paint dries.

I've just discovered borers in my peach and nectarine trees as well, so I plan to try this technique on them. Interestingly, both peach trees are growing very well, and only the nectarine tree seems to be adversely affected by the borers.

Not a viable solution for vast commercial orchards, but I'm excited that it has worked for me on my tiny Chicago lot!