Signs of Carpenter Ants in Your House: How to Spot an Infestation Early

TL;DR

• The main signs are frass, a dry rustling in the walls, many large ants in one spot, winged ants indoors and slit-like openings in damp wood.
• First, check it's a carpenter ant: large (6 to 25 mm), dark, with a slim waist and elbowed antennae. Termites have straight antennae and a thick waist.
• Frass is the clearest sign: small heaps of coarse, stringy sawdust mixed with bits of dead insects below baseboards, window sills or door frames.
• A few wandering ants may not mean a colony, but many ants in one spot, such as under the kitchen sink, is a warning sign (Health Canada).
• Large ants active in your house in winter, or many winged ants gathered inside your windows, point to a nest inside the house.
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

A small pile of what looks like sawdust under a window sill is easy to vacuum up and forget, yet it is one of the main signs of carpenter ants in a house. The ants do not eat wood: they chew out galleries and throw the shavings out of the nest, so fresh piles show up near the spot where they are digging.

A single clue proves little on its own, and Health Canada notes that a few wandering ants may not mean a colony is present. A dry rustling in a wall at night, many ants gathered under the kitchen sink or winged ants at a window in spring carry far more weight. Each of these signs only counts, though, if the insect behind it really is a carpenter ant and not a termite or a smaller species.

What Do Carpenter Ants Look Like?

Carpenter ants are the largest ants you are likely to find in a Canadian home. They are dark, with a slim waist and bent antennae, and they are often seen alone or in small numbers at night. Confirming that you are really dealing with carpenter ants is the first step, because the signs below only mean something if the ants match.

Carpenter Ant Size

Carpenter ant size ranges from 6 to 25 mm (about 1/4 inch to 1 inch), according to Health Canada. The workers you see on the floor are usually 1/4 to 1/2 inch (about 6 to 13 mm), according to Penn State Extension, and Alberta Health Services describes them as large black ants of 6 to 10 mm. If the ants in your kitchen are tiny, they are probably another species: see our guide to small ants in the house.

Carpenter Ant Colour

Carpenter ant colour comes in two common types in Canada, according to Health Canada. The black carpenter ant is dark brownish-black all over. The red carpenter ant has a dark brownish-black body with a reddish-brown upper body (the thorax, the middle section). Pest control companies report that the black carpenter ant is the more common one across the country, while the red one is found mainly in eastern Canada.

Carpenter Ant Waist

A carpenter ant waist is very slim, with a single small bump (a node) between the upper body and the abdomen. Seen from the side, the back of the thorax forms one smooth, even curve, which UC IPM uses as a key identification feature.

Carpenter Ant Antennae

Carpenter ant antennae are bent in the middle, like an elbow, and made of several sections (Health Canada). All ants share this elbowed shape, which makes it the easiest way to rule out termites.

Carpenter Ants Versus Termites

Carpenter ants and termites are easy to tell apart once you know what to look for. According to Health Canada, termites have straight antennae and a thick waist, while carpenter ants have elbowed antennae and a very narrow waist. When both have wings, Mississippi State University Extension notes that termite front and hind wings are the same length, while an ant's front wings are clearly longer than its hind wings. Termites are found only in certain areas of Canada, so in most Canadian homes, large dark insects tunnelling in wood are carpenter ants.

What Are the Signs of Carpenter Ants in Your House?

The signs of carpenter ants in your house are frass, a dry rustling in the walls, many large ants in one spot, winged ants indoors and slit-like openings in damp wood. Here is what each sign looks like and what it tells you.

Frass Piles

Frass piles are the clearest sign of carpenter ants. Small heaps of sawdust-like material appear below baseboards, window sills or door frames, pushed out of the nest as the ants dig. Health Canada lists these piles of sawdust-like borings as a key sign. Frass is covered in detail in the next section.

Rustling Sounds in Walls

Rustling sounds in walls come from an active colony moving inside a wall void or a hollow door. Health Canada describes it as a dry rustling noise, best heard at night when the house is quiet. Press a wine glass or a stethoscope against the wall to hear it better. If you hear it, our guide to ants in walls explains what is going on behind the drywall.

Large Ants After Dark

Large ants after dark are typical of carpenter ants, which are most active after sunset (Health Canada). If you see big dark ants walking along a baseboard or a counter at night, watch where they go. According to Health Canada, a trail may connect your house to a parent colony outside, often through a window, a gap in the foundation or a utility cable.

Many Ants in One Spot

Many ants in one spot is a stronger sign than a few wandering ants. Health Canada notes that a few individual ants walking around may not mean there is a colony in your home, but a high number of ants in one area, such as under the kitchen sink, is a warning sign. A single carpenter ant in spring may simply be a scout that came in from outside.

Winged Ants Indoors

Winged ants indoors are one of the most reliable signs of carpenter ants in a house. Winged males and females leave established colonies on warm days in spring and early summer. Penn State Extension notes that if many winged ants gather at your windows on the inside, they probably came from a nest inside the house. Alberta Health Services adds that sawdust-like shavings combined with many winged carpenter ants indoors usually means there is a nest inside. Our guide to flying ants covers swarms in general.

Carpenter Ants in Winter

Carpenter ants in winter are a strong sign of an indoor nest. Outdoor colonies are inactive during the cold months, but according to Penn State Extension, they stay active if the nest is in a heated part of a building. If you see large ants in your house in January or February, the colony is very likely somewhere inside your walls, floors or ceilings.

Slit-Like Openings in Wood

Slit-like openings in wood are the small doors carpenter ants cut to push frass out of their tunnels. Health Canada lists them as a sign, usually in woodwork such as trim, window sills and wooden steps. Look for them just above any frass pile.

Hollow-Sounding Wood

Hollow-sounding wood can point to tunnels inside. Pest control companies suggest tapping suspicious trim, sills and door frames: wood that sounds hollow may have been dug out. This is a useful check, but it is not proof on its own.

What Does Carpenter Ant Frass Look Like?

Carpenter ant frass looks like coarse, stringy sawdust mixed with bits of dead insects. It is not food waste: carpenter ants do not eat wood, they carve it out and throw it out of the nest, according to Health Canada.

What Frass Contains

Carpenter ant frass contains coarse, stringy wood particles, dead insect parts and other debris, according to Penn State Extension. The insect parts make sense: carpenter ants eat other insects as well as sweet foods, according to Health Canada. If you look at a pinch of it under a bright light or a magnifying glass, the mix of wood fibres and shiny bits of ant bodies is a giveaway.

Where Frass Piles Up

Frass piles up just below the opening the ants use to push it out, so the nest is usually right above or behind the pile. Penn State Extension lists cracks, the backs of mouldings and the spaces under porches as common spots. Indoors, check under baseboards, window sills, door frames, and inside closets and basements near damp wood.

Frass Versus Termite Droppings

Carpenter ant frass is fibrous, like wood shavings, while drywood termite droppings are hard 6-sided pellets, according to UC IPM. Carpenter ant galleries also contain no soil or droppings, because the ants clean them out (Penn State Extension). If what you find is gritty pellets or mud, call a professional to identify it.

What Does Carpenter Ant Damage Look Like?

Carpenter ant damage looks like smooth, clean tunnels carved inside wood, often with very little visible on the surface. You usually notice the frass, the openings or the ants long before you see the tunnels themselves.

Smooth Galleries

Smooth galleries are the signature of carpenter ant damage. According to Penn State Extension, the tunnels are smooth-sided and contain no soil particles or droppings. The surface of a damaged board can look normal, since the ants work from the inside.

Damp Wood First

Damp wood is where carpenter ant damage almost always starts. Carpenter ants usually avoid sound, dry wood with a moisture content under 15 percent (Penn State Extension), and UC IPM notes that nests often begin where there is water damage or wood decay. Window sills, wood around leaky pipes, roof leaks, bathrooms and basement framing are common starting points. Alberta Health Services warns that finding carpenter ants at home may point to a moisture problem.

How Serious the Damage Is

Carpenter ant damage is usually slow and limited, but it should not be ignored. Health Canada says carpenter ants are well known for their ability to damage wooden structures. UC IPM adds that from a nest in damaged wood, they can expand into sound wood and weaken it. Penn State Extension, on the other hand, notes that it is rare for a nest that expands into a building's timbers to cause structural damage. Either way, the leak or rot that attracted the ants also needs fixing.

Carpenter Ant Galleries Versus Termite Galleries

Carpenter ant galleries are clean and smooth, while termite galleries are packed with soil and droppings, according to the descriptions from Penn State Extension and UC IPM. Clean tunnels and loose frass outside the wood point to carpenter ants. Mud and pellets inside the wood point to termites.

What Should You Do After Finding Signs of Carpenter Ants?

Finding signs of carpenter ants means it is time to confirm the species, locate the nest and fix the moisture that drew them in. The nest matters more than the ants you see, so take these steps first.

Check at Night

Checking at night gives you the best chance to see carpenter ants at work, since they are most active after sunset (Health Canada). Penn State Extension suggests checking with a flashlight from May through July, between 10:00 p.m. and 2:00 a.m. Look in the basement, the attic, the garage and around the outside walls.

Follow the Trail to the Nest

Following the trail of foraging ants is the most reliable way to find the nest. Indoor nests are usually satellite nests of a larger parent colony outside (UC IPM), and Penn State Extension notes the parent colony is often within 100 yards (about 90 metres) of the house, in a tree, a stump or a woodpile. Our step-by-step guide on how to get rid of carpenter ants explains how to track the trail and treat both nests.

Fix Moisture Problems

Fixing moisture problems removes what drew carpenter ants into your house. Repair leaks, replace soft or rotting wood and keep firewood out of the house, as Health Canada recommends. Alberta Health Services advises keeping indoor humidity at or below 50%. If you also see ants outside, our guide to getting rid of ants outside can help.

When to Call a Professional

Calling a professional makes sense when you cannot find the nest, when the signs are inside walls or ceilings, or when you see winged ants indoors. Health Canada recommends a professional exterminator in difficult situations, and Alberta Health Services suggests a certified pest control professional for severe infestations. Our guide to carpenter ant pest control explains what a professional does, and you can get a quote from a recommended local exterminator through Pest Agent's referral form. If you treat yourself, compare options in our best ant killer picks and only use products with a PCP number on the label, which shows Health Canada approval, and check that the label allows indoor use. For more on other species, visit our ant guides.