Carpenter ant

How to recognise it

These are the features that identify a carpenter ant. Each one is described in words so the identification works without the diagram.

1. The thorax seen from the side is evenly rounded in a single smooth arch, with no spines or dips. This is the most reliable carpenter ant feature and it separates the genus from nearly every other large ant.
2. There is one node — a single raised bump — on the narrow waist between thorax and abdomen. Count it from the side against a light background.
3. The antennae are elbowed, bending sharply partway along their length, and the body is clearly divided into a narrow, pinched waist.
4. Workers vary continuously in size within one colony, from about 6 to 13 mm, so finding both large and small ants together is normal rather than evidence of two species.
5. Colour is usually solid black, though several species are black with a dull red or reddish-brown thorax.

What it leaves behind

• Coarse sawdust-like shavings, often mixed with insect body parts, pushed out of a crack or a small round hole. The material is fibrous, not pelletized.
• A dry rustling or faint crackling inside a wall or window frame, most audible at night.
• Winged swarmers indoors in spring, which indicates a mature colony in or very near the structure.

Why an infestation persists

Carpenter ants do not eat wood. They excavate galleries in it to nest, which is why they concentrate in wood that is already wet or decayed — around leaking window frames, poorly flashed chimneys, sill plates over damp crawl spaces, and porch columns. A mature colony often keeps satellite nests indoors while the parent colony stays outside in a stump or a woodpile, which is why removing one nest frequently does not end the activity. The moisture that made the wood habitable is the actual problem, and correcting it is what prevents recurrence.

Where it occurs

Throughout the continental United States, with the largest species most commonly reported in the Northeast, Upper Midwest, and Pacific Northwest.

Where to look for it

• Window and door frames on the weather side of the building, particularly where the sealant has failed and the timber below feels soft to a probe.
• The sill plate where the frame meets the foundation, which is the most common place for a satellite nest in a house with any history of damp.
• Around chimney flashing and roof valleys, where slow leaks wet structural timber for years before anything shows indoors.
• Porch columns, deck posts, and step stringers, especially any timber in direct contact with soil or standing water.
• Hollow doors, wall voids behind bathrooms, and the void beneath a bath or shower tray.
• Outside, in stumps, woodpiles, and standing dead limbs within about 30 metres, since the parent colony is frequently outdoors while satellites are in the structure.

When you see it, and why

Indoor activity peaks from late winter into spring, and that timing is diagnostic rather than incidental. Workers seen indoors in February or March, when it is far too cold for foraging outside, indicate a nest already inside the heated envelope of the building rather than one outdoors sending workers in. Swarmers appear indoors in spring from colonies that are several years old. Summer activity is ambiguous, because workers forage outdoors and enter opportunistically. By late autumn activity drops sharply as the colony becomes dormant, which is why an untreated problem often appears to resolve on its own in November and returns in March.

What getting the identification wrong costs you

The expensive error runs in both directions. Treated as an odorous house ant — a sugar-seeking nuisance — the moisture damage that made the timber habitable goes uninvestigated, and the structural problem continues quietly for years. Treated as a termite, a homeowner may buy a termite bond and a soil treatment that address a colony living in the ground, when the actual colony is in a wet window frame two metres up. The distinction is free to make: carpenter ants have a sharply pinched waist and elbowed antennae, termites have neither.

What a professional actually does

The valuable part of the visit is the inspection, not the treatment. A competent inspector uses a moisture meter and a probe to find where the timber is wet and how far the softening extends, traces activity to locate nests rather than treating where ants were merely seen, and identifies the water source that made the wood habitable. Expect a written finding that names the moisture source and the affected members. A quote given without that inspection is a guess, and a treatment that does not address the water will be repeated.

What it costs

Inspection and treatment for carpenter ants typically sits well below termite work, because the colony is accessible and the scope is smaller. The variable that actually moves the figure is the carpentry: replacing a decayed sill plate or a rotted porch column is a construction cost, not a pest control one, and it is frequently the larger half of the bill. Ask for the two to be quoted separately, because they are different trades and the timber repair is often work you can competitively tender.