Carpenter ants and moisture ants are the genuine structural insect problem in western Washington, and the most important thing about both is that neither one caused the condition it is living in.
Carpenter ants excavate galleries in wood that moisture has softened. Moisture ants go further and colonise wood that is already in advanced decay. Neither species eats wood or takes any nutrition from it. What they do is occupy a defect that already existed.
Which means the finding is diagnostic. An operator who treats the ants and says nothing about where the water is coming from has solved the visible half of a two-part problem, and left the half that determines whether you are calling again next spring.
Carpenter ants, moisture ants, termites
Three wood-associated insects turn up in Washington structures and they are routinely confused with each other, including by people selling treatment. The debris each leaves behind is the fastest way to separate them.
| What you found | What it means | What the wood is like |
|---|---|---|
| Clean fibrous shavings with insect parts | Carpenter ant frass, pushed out of a slit-shaped gallery opening. Often piled below trim, a window header or a joist. | Sound but damp-softened. Galleries are smooth and sanded-looking inside, following the grain. |
| Soil-like material packed into decayed wood | Moisture ants. They build carton nests using excavated wood and soil, in timber that is already rotting. | Actively decaying, often crumbling under a screwdriver. Usually wet to the touch. |
| Mud tubes on foundation or wood surfaces | Western subterranean termites. Not ants at all — they travel in covered tubes to keep contact with soil moisture. | Galleries packed with soil and faecal material, running with and across the grain. |
If it is the third row, the termite inspection page covers what a complete wood-destroying organism inspection involves and what the licence requirements are in this state.
Where they nest in Washington housing
The locations repeat, because the water sources repeat. Everything on this list is somewhere that a Washington building routinely gets wet and routinely stays wet.
The sill plate and rim joist above a damp crawlspace, particularly where the vapour barrier is incomplete. Framing around a leaking shower pan, bath or toilet. Window and door headers where flashing has failed and the trim has been repainted over the evidence. The wall cavity behind an outside tap. Roof sheathing beneath a failed valley or a chimney flashing. Porch posts and deck ledger boards. And the base of a door frame or siding run in direct ground contact.
Outdoors, the parent colony is frequently within a hundred yards — a stump, a woodpile stacked against the house, a landscape timber, or a tree with heart rot. Satellite nests inside the building are connected to it by foraging trails, which is why treating only what is indoors can leave the source untouched.
How the job should run
Locating the nest is the work. Everything else is straightforward once it is found. Technicians listen for the rustling a large colony makes in a quiet wall, probe suspect timber with a moisture meter and a screwdriver, follow foraging trails at dusk when carpenter ants are most active, and look for the frass piles that mark a gallery opening. A quote produced without any of that is a quote for spraying a perimeter.
Non-repellent materials, not contact sprays. The point is that foragers pick the material up without detecting it and carry it into the nest. Repellent products do the opposite — they push a colony deeper into the structure or cause it to bud into satellite nests, which is how a single-nest problem becomes a three-nest problem.
Direct treatment of the gallery where it is accessible. Dust applied into a located void treats the nest itself rather than the traffic around it, and it is the difference between one visit and four.
A written note of the moisture source. This is the deliverable that matters most and the one most often missing. Ask for it explicitly: where the water is entering, which timber is affected, and what the moisture readings were.
The repair is the actual fix
Treatment clears the colony. It does not dry the wood, and while the wood stays wet the site remains attractive to the next colony and to wood-rotting fungus, which is the organism doing most of the structural damage in Washington in the first place.
The repair list is unglamorous and largely about water management: clearing and re-pitching gutters and downspouts so they discharge away from the foundation, re-flashing the failure, laying a complete crawlspace vapour barrier and confirming the vents are clear, taking siding and trim out of ground contact, moving the woodpile off the wall, and cutting back vegetation that keeps a wall permanently damp.
Structural timber that has lost integrity needs replacing rather than treating. That is a carpentry quote, not a pest control one, and the two should be priced separately so you can see what you are buying.
What it costs in Washington
Treatment runs $250 to $600 in 2026, driven mostly by whether the nest gets located. Located and directly treated is often one visit. Working from trails alone means repeat visits and a longer non-repellent programme.
The wood repair and the water fix sit outside that entirely, from a few hundred dollars for a section of trim to five figures where a rim joist or sill plate has failed. The statewide pest control page carries the full cost table across all eight services, and ant control covers the nuisance species that do not damage buildings at all.