Finding myself unnable to adequately answer client questions about carpenter ants and thier effect on trees, I did some web research. Its not hard to find pest controll people who will set out to save a tree by eradicating ants and other insects, but the entomological sites all report that they do not remove wood unless it has already started to decay. This still leaves the issue of whatever caused the initial decay, but either way its not the ants. I have not read anywhere that carpenters are actually beneficial to trees, but wouldnt this make sense? Seems like it would be easier for the tree to compartmentalize if all rotting tissue were gone and no longer spreading.

The entomologists are correct (but of course, I learned it from them).

Carpenter ants, and most termites, live in wood already beginning to decay. The ants delve through the wood and carry it out. Termites actually eat the wood. I heard reports that Formosan termites will go through anything with cellulose.

As to being beneficial, I believe and have heard from one or two entomologists that the nests may tend to moderate the humidity in the wood and may slow the decay. That would depend on many other factors, however, like how close the nest is to the advance line of the decay, etc.

Since the wood is already decayed, the ant nests do little to affect structural strength of the tree. The only adverse affect of the nest is its proximity to wood structures and the increased possibility that winged females will find a suitable site for a new colony in someone’s house.