Carpenter Ants: How to Identify Them, What They Damage, and What to Do

What are carpenter ants?

Carpenter ants are large ants in the genus Camponotus that nest inside wood. Workers are 1/4 to 1/2 inch (6 to 13 mm) long and black, red and black, or brown. Two features separate them from other ants: one node at the waist and an evenly rounded back. Their bent, elbowed antennae separate them from termites.

The common one in the Midwest is the black carpenter ant, Camponotus pennsylvanicus, according to Illinois DPH. In the West, UC IPM names the western black carpenter ant, C. modoc, which is black with dark red legs, and C. vicinus, usually red and black. The genus is huge. Wikipedia counts over 1,500 species, but only a few nest in wood.

Their name comes from the way they work. They chew galleries into wood to make room for the colony, then push the shavings out. Hundreds of people a month search for “carpet ants”; as far as we can tell they mean carpenter ants, since no ant goes by that name.

How big are carpenter ants?

Most carpenter ant workers are 1/4 to 1/2 inch (6 to 13 mm) long. Queens are bigger, up to 3/4 inch (19 mm) according to UK Extension and up to 1 inch (25 mm) according to the University of Minnesota. A few small species are much smaller: Minnesota mentions one at only 3/16 inch (5 mm).

Our reading: workers are 1/4 to 1/2 inch. A single ant of 3/4 inch or more, often seen alone in spring, is most likely a queen.

Is it a carpenter ant? Big black ants and look-alikes

Not every big black ant is a carpenter ant. Check the side view first: one node and a smooth, rounded back mean carpenter ant. Other large ants, such as field ants, also have one node but an uneven, bumpy back. Then check size, about 1/4 to 1/2 inch for workers, and look for coarse sawdust nearby.

Ant features from the University of Minnesota and Illinois DPH. Termite features from Minnesota, Illinois DPH and UC IPM. Small house ant sizes from our ant control guide.

Tiny black ants marching in a line to the sugar are a different job, covered in our guide to getting rid of ants in the house. Big winged ants at a window in spring are explained in our flying ants guide. Still unsure? A sharp side-on photo through the free insect identifier will name it.

Carpenter ants or termites?

Look at the waist, the antennae and the wings. Carpenter ants have a pinched waist, elbowed antennae and hind wings shorter than the front ones. Termites have a thick waist, straight antennae and four wings of equal length, as the University of Minnesota explains. Termite workers are pale and almost never seen in the open.

The damage is different too. Illinois DPH says carpenter ant tunnels are smooth and clean and can run with or against the grain, while subterranean termite tunnels are lined with a mud-like material and follow the grain. UC IPM adds that carpenter ant sawdust is fibrous, unlike the tiny six-sided pellets of drywood termites.

Are carpenter ants dangerous? Do they bite?

Carpenter ants are not dangerous to people. They cannot sting, but UC IPM says their powerful jaws can give a painful bite, and they may spray formic acid into it, which burns. Workers are only 1/4 to 1/2 inch. UK Extension says they pose no human health threat and bite only when the nest is disturbed or when picked up.

If a bite breaks the skin, wash it with soap and water. The sting of the acid fades on its own. A pet that snaps at one may get a painful nip, but the ants carry no venom that harms it. For bites you cannot explain, our bug bite identification guide covers the insects that bite indoors.

Do carpenter ants eat wood? Damage and signs

No, carpenter ants do not eat wood. They dig it out to make nest galleries and drop the shavings outside. Their food is insects, honeydew from aphids and scale insects, and household sweets, meats and pet food, according to the University of Minnesota. Even a colony of up to 10,000 ants digs slowly, so serious damage usually takes years.

The digging starts where wood is wet or decaying. UC IPM explains that a large colony can then spread from the damaged area into sound wood and weaken the structure. They also nest in wall voids, hollow doors and insulation. On the other hand, the University of Minnesota says damage builds slowly, often over years. The longer a colony stays, the worse it gets.

Signs of carpenter ants in the house

• Coarse sawdust (frass) below small slits in wood. The University of Minnesota calls it coarse, and PestWorld notes it can hold insulation and insect body parts.
• 20 or more workers seen indoors at the same time, again and again. Minnesota lists this as a sign of a nest.
• Large numbers of winged ants indoors from late winter through spring.
• A dry rustling sound in a wall or frame, louder when you tap on it.
• Smooth, clean galleries with a sanded look when damaged wood is opened up.
• Ants around damp spots at night: sinks, tubs, dishwashers, rolled-up towels.

What it means when you see carpenter ants

What a sighting means depends on the season and the number. Carpenter ants seen indoors in winter almost always come from a nest inside the building, say Minnesota and Michigan State. In spring and summer, 1 or 2 may be foragers or queens from outside. Seeing 20 or more indoors again and again points to a nest inside.

Built from the University of Minnesota and Michigan State Extension. PestWorld says “If you see one ant, it likely signals you may have an infestation”; the extension sources show it depends on the season.

Why do I suddenly have carpenter ants?

Usually because a colony has found a way in, or found damp wood worth moving into. The main nest, with the queen, is often outdoors in a stump, a dead tree or a woodpile. It then sets up satellite nests, holding workers and older young, in nearby buildings. Illinois DPH says the outdoor nest may be up to 100 yards away.

• A leak or condensation around a roof, chimney, skylight, window, tub or dishwasher.
• Branches touching the house, the roof or utility wires, which work as bridges.
• Firewood stacked against the walls, in a garage or in the basement.
• Gaps where utility lines enter, plus cracks around doors and windows.
• Land clearing nearby. UC IPM says new homes can get them when work disturbs colonies in the area.

Carpenter ant life cycle, colonies and swarms

A carpenter ant colony grows slowly. UC IPM says it takes several years to mature and start producing winged ants. A mature colony can hold up to about 10,000 ants, according to Illinois DPH, and PestWorld puts large western black carpenter ant colonies at over 50,000.

UK Extension says many species have only 1 egg-laying queen, though UC IPM notes C. vicinus nests can hold as many as 40. After a mating flight, the new queen sheds her wings, seals herself into a small cavity and lays her first eggs. UK Extension says the first batch is 15 to 20 eggs. The larvae are white and grub-like. Once the first workers emerge, they take over the feeding and digging, and the queen only lays eggs.

From the University of Minnesota, Illinois DPH (active from March or April to September or October) and UK Extension. Exact months vary by region and species.

Swarm timing varies by region and species. UK Extension (Kentucky) puts mating flights in late May to early June. PestWorld gives May to August in the East and February to June in the West. Our guide to flying ants and what each swarm means has a month-by-month calendar for the common US species.

How to find a carpenter ant nest

To find a carpenter ant nest, watch after dark and follow the workers home. Start about 15 minutes after sunset, put out a lure such as tuna packed in water or sugar water, and track the ants that carry food away. Then confirm the spot with coarse sawdust, damp wood and a rustling sound.

Finding the nest matters more than any product. Illinois DPH calls it the critical step, and the University of Minnesota says the best control is to locate and destroy the nest and fix the moisture. Allow a few evenings for it.

How to get rid of carpenter ants

To get rid of carpenter ants, find the nest, fix the moisture that drew them, and remove or treat the nest wood. Replace soft wood, cut branches back, move firewood away and keep mulch to 2 to 3 inches. If exclusion and sanitation are successful, insecticide treatments are not required. Nests inside walls are a job for a licensed professional.

Start with what costs nothing. The steps below run from free fixes to products, and the University of Minnesota is clear that the nest and the moisture come first.

1. Fix the moisture (free to fix, the biggest step)

Carpenter ants follow water. Repair roof, chimney and plumbing leaks, clear clogged gutters, and make sure the ground slopes away from the walls. NPIC suggests better ventilation in attics, basements and crawl spaces, and insulating pipes that drip with condensation. Then replace wood that has gone soft.

2. Cut off their routes (cheap)

• Trim branches so they do not touch the house, the roof or the wires.
• Move firewood off the ground and several feet from the house, as UC IPM advises.
• Remove stumps and old roots near the house, a favourite place for parent nests.
• Keep mulch to 2 to 3 inches deep along the foundation, as UC IPM advises.
• Seal gaps where pipes, cables and wires enter, and around doors and windows.
• Store food in containers with tight lids, so foragers find nothing inside.

3. Remove or treat the nest

If the nest is in a stump, a woodpile or a dead tree outdoors, removing that wood is the cleanest fix. If it is inside the house, it has to be treated where it is. The University of Minnesota says professionals may drill holes of about 1/8 inch (3 mm) into a wall or frame and puff an insecticidal dust into the nest.

Spraying the ants you see does not reach the colony. Illinois DPH says spraying around the foundation or into an open attic usually gives little or no control. It also unnecessarily exposes people and other animals to the product.

4. Baits, if you cannot find the nest

Baits can work, but the evidence is mixed. The University of Minnesota and UC IPM say slow-acting baits can destroy a nest because the workers share the food. However, Michigan State says shop-bought bait stations are “relatively ineffective” against carpenter ants. Minnesota adds that carpenter ants have complex food preferences, so a bait may not hold their interest long enough.

To give a bait its best chance, Minnesota advises placing it only where ants are active, checking it within 24 hours, swapping any bait they ignore, and adding more when they eat it. Never spray near a bait. Expect weeks or months, not days. UC IPM adds that the label must list ants as a target pest.

5. Insecticides, last and only if needed

If the moisture is fixed and the nest wood is gone, you may never need an insecticide. For a nest that stays, UC IPM lists pyrethroid sprays, borate dusts and desiccant dusts applied directly into the nest. It notes that diatomaceous earth is sold in stores, stays effective while dry, and should not be breathed in. Use only a product registered as an insecticide, puffed into the nest or cracks as its label directs.

Some of the most effective tools are for professionals only. UC IPM and UK Extension both say fipronil sprays work very well against carpenter ants but must be applied by a licensed applicator.

What does not work, or is not true

The most common mistake is spraying the ants you can see. Only 10 to 15% of a colony forages at once, so the nest carries on, and the ants are back next spring. Several popular beliefs about carpenter ants are also wrong. Here is what the extension sources actually say.

• Myth 1

  “Carpenter ants eat wood, like termites.”

  Truth: They do not. Minnesota, UC IPM and NPIC all say they only dig nests in wood and feed on insects, honeydew and household food.

• Myth 2

  “One carpenter ant means your house is infested.”

  Truth: Minnesota says a few winged or wingless queens in spring do not prove a nest, and Michigan State says summer sightings may come from outside. Winter sightings and 20 or more workers are the real signs.

• Myth 3

  “They only live in rotten wood.”

  Truth: They start in damp or decayed wood, but UC IPM says a colony can spread into sound wood, and Minnesota finds satellite nests in drier spots such as insulation, hollow doors and wall voids.

• Myth 4

  “Spraying the trail will get rid of them.”

  Truth: Illinois DPH says spraying the foundation or an open attic usually gives little or no control. The nest has to be found and treated.

• Myth 5

  “Bait stations always work.”

  Truth: Michigan State calls them relatively ineffective against carpenter ants. Minnesota and UC IPM say baits can work, slowly, with careful placement and swapping.

• Myth 6

  “They eat the fungus in rotten wood.”

  Truth: No extension source we read lists fungus as food. Their diet is insects, honeydew, sweets and meats. Rotten wood is simply easier to dig.

• Myth 7

  “A homemade borax and sugar mix is the fix.”

  Truth: Borax is a pesticide. We do not give home recipes, because there is no tested dose. If you bait, use a product registered with EPA for ants, as labelled.

How to prevent carpenter ants

To prevent carpenter ants, keep the wood in and around your house dry and keep their bridges away. Fix leaks promptly, store firewood several feet from the walls, keep mulch to 2 to 3 inches, and trim branches off the roof. A check of damp spots each spring helps catch problems early.

• Check in spring around windows, the chimney, skylights and the bathroom for soft or stained wood.
• Ventilate attics, crawl spaces and basements, and fix condensation on cold pipes.
• Lift stored lumber off the ground so air can move around it, as Minnesota advises.
• Remove earth-to-wood contact at porches, decks and siding, as UC IPM advises.
• Clear clogged gutters, which UC IPM lists among the causes of wood decay.

Perfectly round holes about 1/2 inch across on the wood surface are not carpenter ants at all. Those belong to the carpenter bee, the big black bee by your deck, which needs a different fix. For other tiny indoor bugs, see our small black bugs guide.

When to call someone

Call a licensed pest professional when the nest is inside a wall, ceiling or beam, when you see carpenter ants indoors in winter, or when 2 rounds of careful baiting have failed. The University of Minnesota says that because control is complex, it is best to hire a professional.

• A pest professional for nests in walls, ceilings or floors, or ants you cannot trace.
• A roofer, plumber or carpenter for the leak and the soft wood. The ants will come back if the damp stays.
• A home inspector if you are buying a house where ants were found. Ask for the moisture source and the extent of soft wood.
• Your landlord if you rent. Leaks, rot and treatment inside walls are not a tenant’s job.

Not sure what you have? Take a clear photo from the side and run it through our bug identifier by picture before paying for a treatment.

Pest control disclaimer. Photo-based identification can help, but it is not always perfect. For dangerous spiders, large infestations, or anything you are unsure about, contact a licensed pest control professional.