Carpenter ants are the largest ants most homeowners see inside, and they tend to show up in spring when the colony sends workers out to forage. Finding one or two in the kitchen in April is not necessarily an alarm. Finding a dozen in January, or a trail moving toward a wall, is a different matter. The question is whether they are foraging visitors from an outdoor colony or residents inside your structure.
Identification
The black carpenter ant, Camponotus pennsylvanicus, is the species most often found in Midwest and Mid-Atlantic homes. Workers are black to dark brownish-black with a covering of yellowish hairs on the abdomen. They are notably large: workers range from about 3/16 to 1/2 inch long, and the size varies within the same colony because the species is polymorphic, meaning a single colony produces workers of different sizes. Queens can reach nearly one inch long.
Two features distinguish carpenter ants from other large ants. Viewed from the side, the thorax has a smoothly rounded, arched profile with no bumps or notches. The waist has a single node. Those two points separate carpenter ants from most species you are likely to see.
The other identification question is whether what you are looking at is an ant at all. Subterranean termites swarm in spring and get confused with carpenter ant swarmers. Carpenter ant swarmers are larger, black, with a pinched waist, bent antennae, and front wings noticeably larger than the hind wings. Termite swarmers are smaller, pale to dark brown, with a thick uniform waist, straight beaded antennae, and four wings of equal size. Finding discarded wings after a swarm settles it: equal-length wings mean termites.
Behavior and Habitat
Carpenter ants excavate wood to create a nest. They do not eat wood. They chew it out, carry the debris away, and use the resulting galleries to raise their brood. The parent colony, where the queen lives and eggs are laid, almost always starts in moist or decayed wood. Outdoors that means dead trees, stumps, logs, and woodpiles. Indoors it means wood that has been softened by a leak: window frames with failed caulk, roof overhangs that hold moisture, wood around a plumbing penetration, sill plates in a damp basement.
Once a parent colony is established, it can extend satellite nests into drier, sound wood and into wall voids, insulation, and hollow doors. Satellite nests hold workers, pupae, and older larvae but not eggs, since eggs would dry out in lower humidity. Workers travel regularly between the parent and satellite locations. This matters for treatment: what you find inside is often a satellite nest, not the queen.
Carpenter ants are mostly nocturnal. Workers forage heavily after dark, following pheromone trails to food sources. They eat a wide range of things: other insects, honeydew from aphids and scale insects, and whatever is available in a kitchen. Activity slows in winter, but a colony with a satellite nest inside a heated wall will remain active through the cold months.
Colonies grow slowly. In the first year, a newly mated queen raises a small number of workers on stored fat reserves. It takes three to six years to reach maturity and produce swarmers. A mature colony holds roughly 2,000 to 3,000 workers.
Signs of an Infestation
The clearest sign is workers moving in a clear trail along a baseboard, pipe, or structural member, especially late at night. A single ant in the kitchen is not evidence of much. A column of large black ants traveling toward a wall is.
Frass is the other key indicator. Carpenter ants push debris out of their galleries through small kick-out holes. This frass is coarse and irregular, resembling pencil shavings. It contains bits of chewed wood fiber, soil, insulation, and insect parts, the body parts of ants that died in the nest. That mixture of materials is distinctive. Drywood termite frass looks like tiny uniform pellets, which is a different thing entirely. Subterranean termites pack their waste into mud and do not produce a visible dry frass pile at all.
A rustling or crinkling sound from inside a wall, especially when you tap on it, can mean an active gallery. Swarmers emerging indoors in spring, particularly from a window frame or a wall void, confirm that a colony is already inside the structure.
Health and Property Risks
Carpenter ants do not sting under normal circumstances and are not a health concern. The risk is structural.
A young colony in its first few years does limited damage. A colony that has been inside a wall for five or more years can hollow out a significant amount of wood. Structural members like sill plates, floor joists, and window headers that have been compromised by years of excavation may show deflection or soft spots. The damage is slow compared to termites, but it accumulates. The earlier you find and treat the colony, the less repair work follows.
Carpenter ants are also a signal that moisture is present. The parent colony almost always starts in damp wood, so finding an infestation often means finding a leak or a ventilation problem worth fixing regardless.
Treatment Options
Homeowners can try gel or granular bait for a light, early infestation. Place bait on the ant trails after dark, where workers are actively moving. Give it ten days to two weeks before expecting results. Baiting works best when foragers carry it back to the colony, so avoid spraying the trail with any repellent product while the bait is out. Repellent sprays break up the trail and scatter foragers without reaching the colony.
What does not work is perimeter spraying alone. Killing the workers you can see does not affect the queen and does not end the infestation. The workers are replaced.
Professional treatment for carpenter ants focuses on finding and treating the nest directly. A technician will inspect around the building for parent colony candidates: dead wood in contact with the structure, logs and stumps near the foundation, moist wood in crawl spaces or attics. Dust formulations injected into wall voids where satellite nests are suspected are a standard approach because dust sticks to the ants and moves through the colony on contact. Non-repellent liquid residuals applied to trails and entry points allow foragers to carry residue back without avoiding the product. Where the parent colony is outdoors and accessible, direct injection or drench treatments into the nest eliminate the queen.
Treatment is often a two-visit job: an initial treatment followed by a follow-up four to six weeks later to confirm the colony is no longer active. Finding and fixing the moisture source that made the nest possible is a required part of a lasting solution.
Prevention
Moisture control comes first. Repair any leak that has allowed wood to stay wet: roof overhangs, window flashing, plumbing penetrations, basement sill plates. Fix gutters that drain toward the house. Ventilate crawl spaces so they stay dry. A dry house is a much harder target for carpenter ants.
Keep wood away from contact with soil. Woodpiles stacked against the foundation are a common parent colony site. Move them away from the house and elevate them off the ground. Stumps and dead tree sections within 50 feet of the house are nesting candidates worth removing or treating.
Seal the structure. Caulk gaps around windows, doors, pipe penetrations, and utility lines. Carpenter ants are large and do not squeeze through tiny cracks the way small species do, but they will use any gap that fits a worker. Trim branches that contact the roof, since these serve as entry bridges.
What It Costs
A professional carpenter ant treatment for a single-family home typically runs $250 to $500 for an initial visit that includes inspection, nest treatment, and product application. More complex jobs, where the parent colony is difficult to locate or the satellite nest is deep in the structure, can reach $800. Most companies will schedule a follow-up visit, and some include it in the original quote. Recurring pest plans that cover carpenter ants as part of broader ant and pest coverage run $40 to $70 per quarter.
Structural repair costs if damage has gone undetected for years are separate. Replacing a compromised sill plate or a section of floor framing runs $500 to $2,500 or more depending on access and extent. This is the real reason to treat early: the pest control bill is small compared to the repair bill.
When to Call a Professional
If you are seeing workers in trails inside the house, especially in winter, a colony is already inside the walls and needs a professional. The same goes for any infestation where you find frass in a wall void or under a structural member. Bait alone rarely resolves an established indoor satellite nest because the nest site itself needs to be treated.
Call a professional if you have tried baiting for two to three weeks without a clear reduction in activity. Also call if the building is older, has a history of moisture problems, or has known soft spots in the wood, because in those situations a thorough inspection is worth as much as the treatment itself. Carpenter ants are manageable, but finding the parent colony is the job, and that takes experience.