Carpenter ant control

A trail of small ants across the kitchen counter in May is an annoyance. Large black ants appearing indoors through the winter is a different message, because it means a nest is inside the building. Telling those two situations apart is the first job, and it changes everything that follows.

Identify before you treat. Carpenter ants are the large ones, and Health Canada gives their size as 6 to 25 mm. They are distinguished from termites by a very narrow waist and elbowed antennae, where termites have straight antennae and a thick waist (Health Canada, carpenter ants). Pavement ants and the other small species that trail along counters are nuisance ants and are a far simpler problem.

What carpenter ants are telling you. They do not eat wood. They excavate galleries in it to nest, and they prefer moist, decaying wood. Health Canada notes that their presence can signal a moisture problem or decaying wooden structures. That is the important sentence on this page: a carpenter ant infestation is usually a symptom of a leak, and treating the ants without finding the water means they come back.

Step one: find the nest. Look for piles of sawdust-like borings outside their galleries, slit-like openings in woodwork, and heavy ant traffic in one particular area. Health Canada even describes listening for a dry rustling noise at night with a glass or a stethoscope held against the wood. Common Ottawa locations are around a leaking bathroom, a chimney chase, a window sill with failed caulking, a roof leak, a deck ledger and rotted trim.

Step two: bait rather than spray. Non-repellent baits are the modern approach for both carpenter and nuisance ants. Workers carry the bait back and share it, which reaches the queen and the brood. Spraying visible ants with a contact product kills the ants you can see, tells the colony to avoid that area, and can cause a carpenter ant colony to bud into satellite nests, leaving you with several problems instead of one. Resist the urge to spray a trail.

Step three: treat the nest directly where it is found. Where a nest is located, a registered product can be applied into the void, which is generally faster than baiting alone. This is where a licensed technician earns their fee, because it requires correct product choice and placement.

Step four: fix the moisture, then the structure. Repair the leak. Replace decayed wood. Correct grading and downspouts that dump water at the foundation. Move firewood away from the house, since Health Canada advises removing firewood from buildings and correcting humidity problems. This is the step that determines whether the ants return.

Step five: exclusion and yard work. Seal gaps around windows, doors, pipe penetrations and utility entries. Cut back branches touching the roof and siding, which act as bridges. Remove tree stumps and decaying wood near the house, which are common outdoor carpenter ant nests supplying the satellite nest inside.

How long it takes. Nuisance ants generally respond to baiting within one to two weeks. Carpenter ants take longer, commonly two to six weeks, because the bait has to reach a dispersed colony and satellite nests may need to be found. Expect at least one follow-up visit. The moisture repair is a separate trade and its own timeline.

What it costs in Ottawa in 2026. Indicative ranges, matching this site's price guide. A carpenter ant programme, including follow-up visits, generally runs $300 to $800, more where the nest is hard to locate or the building is large. A general ant treatment for nuisance species such as pavement ants is usually $150 to $400. Both figures exclude the carpentry and the leak repair, which frequently cost more than the pest work and are the part that actually solves the problem.

What makes an Ottawa house different. Carpenter ants thrive on the combination this climate provides: heavy snow load sitting against roof edges and trim, ice damming that pushes meltwater back under shingles into fascia and wall cavities, and a spring melt that saturates everything at ground level. Houses with wood decks, mature trees close to the building and older wood window sills are the typical callers, which describes a great deal of Alta Vista, Manor Park, Westboro and the older Nepean stock. Season matters too: carpenter ants indoors between December and March are strong evidence of an indoor nest, because outdoor colonies are dormant then. Newer suburban houses in Barrhaven, Kanata and Riverside South see them less often, and when they do it usually traces to a failed exterior seal or a deck ledger.

Who should do the work, and the licence. Consumer ant baits are sold legally and are a reasonable first attempt for nuisance ants. For commercial application in your home, Ontario requires a pesticide exterminator licence in the structural class, and the business needs a pesticide operator licence, regulated by the Ministry of the Environment, Conservation and Parks under O. Reg. 63/09 (Ontario, pesticide licences and permits). Ask to see it.

What to ask before you hire. Which species is it, and how do you know. Where is the nest, and have you found it or are you guessing. Are you baiting or spraying, and why. Have you looked for the moisture source, and where is it. How many visits are included. What is the guarantee and how long. A contractor who inspects the bathroom, the roofline and the deck before quoting is doing the job properly.