Carpenter ants are large and dark and excavate wood to nest; moisture ants are small, light yellowish-brown and nest in damp rotting wood or soil. WSU PestSense describes both, and in a wet region such as Puget Sound the common thread is moisture. Fixing leaks and ventilation matters as much as treating the colony.
Key takeaways
- Size and color are the first clue: carpenter ants are large and dark, while moisture ants are only a few millimetres long and light yellowish-brown, according to Washington State University.
- Neither species eats wood. Carpenter ants excavate it to nest, and moisture ants colonize wood that is already damp and decaying, which is why moisture sits at the center of both problems.
- WSU says the parent carpenter ant colony is usually outdoors and many nests inside a house are satellite nests with no queen, so finding and treating only the indoor nest often leaves the source intact.
- Winged carpenter ants swarm from April to early June according to WSU, and winged ants indoors in spring can mean a nest in the building; they are easily confused with winged termites.
- The most useful steps are non-chemical: fix leaks, ventilate crawl spaces, keep wood off soil, remove stumps and rotting logs near the house and keep branches off the eaves. WSU recommends a professional for an active moisture ant colony.
What is the difference between a carpenter ant and a moisture ant?
Carpenter ants are large and dark and excavate wood to make galleries. Moisture ants are small, with workers a few millimetres long and light yellowish-brown, and they nest in damp rotting wood or soil while avoiding dry, sound wood. Carpenter ants favor wet or decayed wood. A technician can confirm either from one specimen.
Size is the quickest clue and the easiest to get wrong, because ants vary within a species. Put a coin beside a captured ant, or take a photo with something for scale. A technician can identify the species from a single specimen.
Behavior helps too. Carpenter ants forage at night along regular trails, often well away from the nest. Moisture ants tend to stay close to their damp nesting wood. If the ants you see are tiny and pale and come from a bathroom wall, think moisture ants. If they are large and dark and walking across a kitchen floor at night, think carpenter ants.
If you cannot decide, record the details: where you saw the ants, the time, the approximate size next to a coin, and whether any had wings. Those four facts let a technician make a likely identification over the phone and bring the right tools. Photos taken with a phone, close up with a coin for scale, are better than a description.
| Feature | Carpenter ants | Moisture ants |
|---|---|---|
| Size and color | Large, dark | Workers a few millimetres long, light yellowish-brown |
| Where they nest | Wet or decayed wood; the parent colony is usually outdoors | Damp rotting wood or soil; they avoid dry sound wood |
| Do they eat wood? | No, they excavate it | No, they use already damp, decaying wood |
| What their presence suggests | Decayed wood and possibly an outdoor colony | A moisture problem such as leaks or wood touching soil |
| Swarm timing | April to early June (WSU) | Winged forms can appear; ask for identification |
| Main prevention | Remove stumps and logs, keep branches off the eaves, fix damp wood | Fix leaks, ventilate crawl spaces, keep wood off soil |
Why does damp wood draw both?
Wet wood is soft, easy to hollow out and often rotting, which suits both species. Moisture ants need damp wood or soil to survive, and carpenter ants prefer wet or decayed timber for their galleries. In western Washington's long rainy season, shaded wood can stay wet for months, so the same leak or poor drainage can draw either ant.
The rainy season runs roughly from October to March. Roof edges, north-facing walls, decks and crawl spaces take the brunt, and small failures in paint, flashing or gutters let water behind the surface where it cannot be seen. By the time an ant shows up, the wood may have been wet for years.
This is why a visit that only treats the insects often disappoints. See wet wood and moisture pest prevention for how the moisture side is approached.
The risk follows moisture and decay more than location. A Seattle house with a leaking gutter and a stump nearby has more to offer than a dry house with none. No local count or neighborhood ranking is available, so treat the risk as likely wherever wood stays wet, and not as a statistic.
Older housing is a factor, as about half of Seattle's homes were built before 1960, based on older City of Seattle figures. Older houses tend to have more crawl spaces, additions and aged window sills. Mature conifers and shade make a difference on the Eastside and the lake shores.
A check before the wet season and again in spring costs little and may catch a soft sill early.
Are the colonies in the house or outside?
Often both. WSU PestSense says the parent carpenter ant colony usually lives outdoors, in a stump, log, tree or woodpile, and that many nests inside a home are satellite nests without a queen. A satellite depends on the parent, so a treatment that stops at the indoor nest may leave the source intact.
A technician traces trails to find where foragers come from. Following ants at dusk is a standard method, and a trail that leads over a fence rail to a stump twenty feet from the house is a good clue to the parent.
Moisture ants are different. Their nests may sit in a damp sill or in soil against a foundation, and the colony can be wholly inside the problem area. Either way, the visit looks outside as well as in.
When do carpenter ants swarm, and what does a swarm mean?
WSU PestSense gives April to early June for carpenter ant swarming. A swarm is winged males and females leaving the nest to mate. Seeing winged ants indoors in spring may mean there is a mature nest in the building; a few outdoors suggest a colony nearby. Swarmers are easily confused with winged termites.
Ants have a pinched waist, elbowed antennae and forewings longer than hind wings. Termites have a thick waist, straight beaded antennae and wings of equal length. WSU describes dampwood termites as swarming in late summer or fall, so a winged insect in April is more likely an ant than a termite, though a specimen settles it.
Keep a few in a jar or on tape, and note the date and room. Our guide on termites in western Washington covers the termite side.
- Pinched waist, elbowed antennae: ant
- Thick waist, straight antennae: termite
- Spring swarm indoors: possible nest in the building
- Fall swarm after rain: possible dampwood termites
What can I do before calling a technician?
Fix visible leaks, clear gutters, ventilate crawl spaces, keep firewood and lumber off the ground and away from the house, remove stumps and rotting logs and keep branches off the eaves. WSU lists these as non-chemical prevention steps. They reduce what attracts ants but do not remove a colony already established.
Do not spray. Contact sprays kill the ants you see but may disrupt trails a technician wants to follow and rarely reach the nest. If you can, note when and where ants appear, and save a specimen.
Check the crawl space with a flashlight in dry weather. Look for wet insulation, a torn vapor barrier and damp joists. These are the places where a moisture ant colony is likely to be.
What does a technician look for on a visit?
Trails, nest sites and the water behind them. The technician follows ants to their source, probes soft wood at sills, posts and window frames, checks the crawl space and attic, and looks at gutters and grading for leaks. The aim is to name the species, find the nest and list what keeps the wood wet.
Expect questions: when did you first see them, are they winged, have you had leaks or repairs, and are there stumps or logs on the lot? The answers often point straight at the cause, and a good technician explains what each finding means.
You should also hear what is optional. Nest treatment, bait, outdoor work and moisture repairs are separate items, and you can accept some and not others. The price is confirmed before work begins.
How are damp crawl spaces and sills usually put right?
By stopping the water, then drying and, where needed, replacing the wood. That can mean clearing gutters, extending downspouts, fixing a leak, improving crawl space ventilation or a vapor barrier, lifting wood off the soil and replacing rotted trim. Pest control treats the ants; drainage, plumbing and carpentry belong to other trades.
The order matters. Stop the source first, let the area dry, then repair. Sealing damp wood behind new trim hides the problem and keeps the colony comfortable.
For work that counts as construction, check the contractor's registration with the state before hiring, and ask for the scope in writing.
- Clear gutters and extend downspouts
- Fix plumbing and roof-edge leaks
- Ventilate and dry the crawl space
- Lift wood off soil and remove old form boards
- Replace rotted sills and trim
When does it need a professional?
WSU recommends a professional for an active moisture ant colony, and for carpenter ants whenever the nest cannot be found or ants keep returning. A technician identifies the species, finds the nest, treats it and lists the moisture repairs. Structural damage needs a contractor, and a sagging floor or soft sill should be checked promptly.
For the two services on this site, see carpenter ant control and moisture ant control. A same-day visit is offered only when a technician is free, and the price is confirmed before work starts.
If wood has been hollowed or rotted, a contractor handles repair. Washington requires many construction contractors to be registered with the Department of Labor and Industries, and you can check any contractor on the state's verification page before hiring.
Frequently asked questions
Do carpenter ants eat the wood in my house?
No. They excavate it to make nesting galleries and push the shavings out. They feed on other insects, honeydew and household food. The damage comes from the tunnels.
Are moisture ants dangerous?
They do little damage themselves. They matter because they signal wood that is wet enough to rot, and that wet wood can invite larger pests and decay.
How do I tell winged ants from termites?
Look at the waist, antennae and wings. Ants have a pinched waist, elbowed antennae and forewings longer than hind wings; termites have a thick waist, straight antennae and wings of equal length.
Can I just spray the ants I see?
It is better not to. Sprays rarely reach the nest and can scatter the colony or break the trail a technician follows. Bait and nest treatment, guided by identification, work better.
Will fixing the leak make the ants go away?
Often moisture ants decline as the wood dries, but no outcome is promised. For carpenter ants, treatment of the nest is usually needed as well, along with removal of outdoor nesting sites.
Sources
- Washington State University PestSense – Carpenter ants fact sheet
- Washington State University PestSense – Moisture ants
- Washington State University PestSense – Termites fact sheet
- Washington State Department of Labor & Industries – Verify a contractor or tradesperson
This guide is general information for King County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.