By far the most common wood destroying insect in Western Washington is the Anobiid Beetle(sometimes called the Western Death Watch Beetle) Hemicoelus Gibbicollis. Often misinformed people call this the "powder post beetle". True powder post beetles(Lictyds) infect hardwoods and appear in Washington primarily in furniture. The Anobiid infects softwoods; important because it infects structural lumber. They are common in crawl spaces and thrive in a moisture level of 13 to 18%. The larvae spend as much as five years eating wood before exiting to breed and continue the cycle. Control includes removal and replacement of any severely damaged wood (There could be serious structural damage) and eliminating conducive conditions; primarily by ensuring adequate ventilation of the crawl space, removing cellulose debris and making sure the vapor barrier is properly installed. Chemical control is possible, but it takes treatment over a period of years to effectively control them.
Carpenter Ants(Camponotus modoc is the most common) generally nest in tree stumps outside the house. They enter the house foraging for food and to establish satellite nests. They do not eat wood, but excavate tunnels. The initial nests in houses is started in decaying wood, but once it is established, they expand their network of tunnels into sound wood. They can do considerable damage to a structure. If the colony is located they can be controlled by chemical treatment. Again, non chemical control includes reducing conducive conditions: proper ventilation in attics and crawl spaces, trimming vegetation back away from the house, removal of any wood affected by decay fungi, keeping soil away from wood trim and siding. Closing any openings to the outside (entry of gas pipes, etc.) also will help keep them out.
There are two species of termites known to be in Western Washington. The first, Pacific Dampwood Termite is mostly found in crawl spaces. The wood must be wet to support them. A common source of infestation is formwood left on footings. Out door firewood storage (especially against the house) is another common source of infestation. Best control is to remove any conditions conducive to their support; remove footing forms and cellulose debris in crawl space, ensure proper ventilation of the crawl space, do not store fire wood or other wood products against or near the house, repair any leaks in plumbing. Subterranean Termites, the other type, fortunately is relatively rare in Western Washington. It nests in the ground and must be in contact with moisture at all times. They build mud tubes for protection and moisture when foraging away from the nest. Chemical control is the most effective way to get rid of them. This can be quite involved.
Moisture Ants are usually associated with rotting wood. They are not considered a primary structural pest. Removal of rotted wood and controlling conducive conditions are generally adequate to control them.
Wood Decay Fungi, often mistakenly called "dry rot", is a very common WDO in Western Washington. Water must be present (generally a moisture level at 20% or above), optimum temperature for growth is 60 to 90 degrees f, and a food source (the wood). The primary control for wood decay fungi is remove and replace any damaged wood and control conducive conditions (eliminate any sources of moisture).
There are other WDOs in Washington, but they are not considered to be a primary problem because of biological conditions. The WSDA has identified another insect, the Velvety Tree Ant, that they are classifying as a WDO, but information isn't widely disseminated yet.