In Washington state, a wood destroying insect species that is often seen is the moisture ant. The only wood that is attractive to moisture ants is lumber that is very damp in the first place. And if it is rotted, well they like that pretty well too. The most common place to find them is under plumbing fixtures that have been seeping for years. Other locations moisture ants hanker for would be in crawl space corners or behind deficient deck flashings that have been leaking.
The video below is quite revealing and it provides the non-home inspector with a view of moisture ants in action: I poked at their carton nest which was in a rotted beam under a largely inaccessible crawl space area. As is, I saw enough to know there were extensive structural problems.
This species does not infest or damage in a small way, there are a multitude of ants -- undoubtedly counting up in the thousands. And as long as conditions stay moist and conducive to their liking, the ant population will increase and damage will continue to spread.
Resolving moisture ant problems does not require chemical treatment. The solution involves locating all damaged wood and replacing it -- as if it was rotten which it will be. Then all conducive conditions and seepage must be eliminated. After that, the house should be good to go unless future problems develop due to uncontrolled moisture, leaks, etc.