Over the last few years I have lost two large walnut trees, so I'm down to two younger ones in the garden, which I don't want to lose. Few months ago I decided to use some of the wood (from a branch) to make a gear shift handle and when I worked in on the lathe and sanded it I noticed a hole up the center and several small worm holes going across or up the grain.

I can't figure out what they are or more importantly, how to protect the remaining two trees.