Question
How do I get rid of pests that are boring into the trunk of my mature trees? The bark has peeled off in several areas especially in the krotches. The pests have bored into the trees quite deep and it appears that some of the 5 inch branches may actually break away from the tree. Will Malathion work?

Answer
Malathion will not penetrate the bark and is not the right insecticide to use. This is a borer type insect and the best insecticide would be either Merit or Onyx-both of these will sink into the bark and kill the insect under the bark. Wet the area with the spray solution to the point of runoff. Sounds like you may also have some decay fungi in the trunk. This is not a real bad problem. Decay fungi enter the wood of the trunk where there has been a wound. and over time may hollow out the woody cells. They do not infect the living cells just under the bark. A tree can live a life time as a hollow tree. IF the decay fungi grows up the trunk and enters a limb it could cause the limb to break.

Here is some information on a hollow tree.
The heartwood consists of the growth rings in the center of the trunk. They are aged sapwood and thus form part of the Xylem. They tend to have a darker color than the sapwood, because they no longer transport any water and are therefore much drier, and they tend to be clogged up with gums, resins and other substances, depending on tree species. Because they are no longer taking part in the trees vital processes, they can be said to be 'dead wood'.
A hollow tree is a tree where the heartwood, which has less resistance to rot than the living parts, has gradually decayed away. Surprisingly, this makes no difference to the vigor of the tree and may even enhance it ability to stand up to severe gales, due to the immense strength of a tubular structure. So it is important to realize that a tree with decaying heartwood is NOT a decaying tree! It is a natural process in many older trees and makes them all the more valuable as a habitat for many different forms of wildlife.  

I would fertilize the trees with 10-10-10 fertilizer at the rate of 1 lb of fertilizer per inch of trunk diameter scattered around the tree and watered in good. Apply the fertilize just before a rain storm and you will not need to water. This will increase the overall health of the tree form decay and also protect it from borers.