Ants in Walls: How to Spot the Signs and Get Rid of Them for Good

Can Ants Live in Your Walls?

Ants in walls are common in Canadian homes: several species nest inside wall voids, insulation and damp wood, out of sight. Health Canada notes that indoor ant nests can be located in wall voids, and that the species that invade Canadian homes include carpenter, little black, odorous house, thief and pharaoh ants. Knowing which one you have matters, because the right treatment depends on the species.

Why Ants Nest Inside Walls

Ants nest inside walls because the space gives them food, moisture and protection that are better than what they find outdoors. A wall void is dark and sheltered, it is close to kitchen and bathroom food and water, and it often holds pipes, insulation or wood that stays damp. Carpenter ants in particular favour moist, decaying wood, so Health Canada points out that their presence can be a sign of a moisture problem or rotting wood in the house.

Carpenter Ants

Carpenter ants are the largest ants you will find in a Canadian home, about 6 to 25 mm long according to Health Canada. The two most common species in Canada are the black carpenter ant, dark brownish-black all over, and the red carpenter ant, which has a reddish-brown upper body. Carpenter ants do not eat wood. They dig galleries in it and push out sawdust-like shavings. Indoors, they may nest in wall voids, attic spaces, hollow doors and styrofoam insulation.

A carpenter ant nest in a wall is often a satellite colony of a parent nest outside, in a stump, a tree or a woodpile. To go further, see the guides on the signs of carpenter ants in the house and how to get rid of carpenter ants.

Odorous House Ants

Odorous house ants are small, dark brown to black ants, usually about 1/8 inch (about 3 mm) long, that often smell like rotten coconut or blue cheese when you crush them. According to the University of Wisconsin extension, they nest in wall voids and attics, in damp wood or insulation, and near plumbing fixtures. Their colonies can form networks of connected nests with many queens, so treating one nest may not solve the problem. The odorous house ants guide covers them in detail.

Pavement Ants

Pavement ants are dark brown to black ants, typically about 2.5 to 4 mm long, that usually nest under slabs and sidewalks and often come inside through cracks in foundation walls and floors (Penn State Extension). You may find small piles of sand or soil where they dig near a crack at the bottom of a wall.

Pharaoh Ants

Pharaoh ants are tiny yellow to light brown ants that nest in wall and cabinet voids, moving between rooms along wires and pipes. They need special care: Alberta Health warns that spraying them can make the colony split into new nests. See the full guide to pharaoh ants if your ants are tiny and pale.

Carpenter Ants Versus Termites

Carpenter ants differ from termites by their body shape: carpenter ants have bent antennae and a very slim waist, while termites have straight antennae and a thick waist (Health Canada). Termites occur only in certain parts of Canada, so most wood-boring insects found in Canadian walls are carpenter ants. The debris is different too: UC IPM describes carpenter ant sawdust as fibrous, unlike the six-sided pellets left by drywood termites.

Can Ants Damage Your Walls?

Carpenter ants are the only common wall ants that damage the structure. Health Canada says they tunnel into building trim, wooden steps and window sills. Odorous house ants, pavement ants and pharaoh ants are not known to tunnel into wood. They are a nuisance and a hygiene problem when they reach your food, but they are not known to weaken the wall.

What Are the Signs of an Ant Infestation in Walls?

An ant infestation in walls shows up as sawdust or soil piles, rustling sounds, winged ants indoors, or trails that keep disappearing into the same gap. One ant wandering across the floor does not necessarily mean a nest: Health Canada advises looking for a concentration of ants rather than individuals.

Sawdust Piles

Sawdust piles below baseboards, window frames or door trim are the classic sign of carpenter ants nesting in the wall. Health Canada describes them as piles of sawdust-like borings pushed out of the galleries, often below small slit-like openings in the woodwork.

Rustling Sounds at Night

Rustling sounds inside a wall can come from an active carpenter ant colony. Health Canada says the dry rustling noise is easiest to hear at night, when the house is quiet, and that holding a wine glass or a stethoscope against the wall can help you hear it.

Winged Ants Indoors

Winged ants coming out of a wall, a window frame or a baseboard inside your home suggest a nest inside the house, not just outside. Health Canada notes that carpenter ant swarms usually leave the house in spring. Penn State Extension adds that pavement ants can swarm inside heated buildings at any time of year.

Ant Trails Along Baseboards

Ant trails along baseboards, the carpet edge along walls and cabinet edges are often the most visible sign (Health Canada). If the trail always leads back to the same crack, outlet or gap in the trim, the nest is probably inside that wall.

Soil Piles at Foundation Cracks

Small piles of sand or soil next to a crack in a concrete floor or at the top of a foundation wall often point to pavement ants digging a nest underneath (Penn State Extension).

Many Ants in One Spot

A high concentration of ants in one place, such as under the kitchen sink, is usually a sign of a nest nearby (Health Canada). Moist spots near pipes, dishwashers and bathroom walls are the first places to check.

How to Get Rid of Ants in Walls

Getting rid of ants in walls means reaching the colony inside the void with slow-acting bait or a nest treatment, not spraying the ants you see. Here is how to do it, step by step.

Identify the Ant First

Identifying the ant comes first, because each species reacts differently. Large black or red and black ants with sawdust nearby usually point to carpenter ants. Small dark ants with a rotten coconut smell are odorous house ants. Tiny pale ants may be pharaoh ants, which must never be sprayed, though UC IPM notes they look similar to thief ants. If you are not sure, a magnifying glass or a pest control professional can confirm the species.

Find the Nest

Finding the nest makes every method work better. Carpenter ants are most active after sunset, so inspect with a flashlight in the evening (UC IPM, Health Canada). UC IPM suggests setting out a small amount of sweet, nontoxic food and following the workers back to where they disappear. If the trail leads outside, look for a parent colony in stumps, woodpiles or dead trees: Health Canada says that for a satellite colony, the parent colony should be found and destroyed too.

Slow-Acting Baits

Slow-acting baits are the best way to reach a nest you cannot see. Worker ants carry the bait back into the wall and share it with the rest of the colony, including the queens. Baits matter even more for carpenter ants, because UC IPM notes that only a small share of the colony forages at any time. Health Canada recommends to:

• Place bait stations right on the ant trails, out of reach of children and pets
• Use plenty of stations, and two different baits at once, which Health Canada says will give better results
• Keep baits available for at least two weeks, as Health Canada advises, and replace them as they empty
• Avoid spraying ants while you are baiting, because sprays stop the bait from working

Check the label: some liquid borax baits are not meant for carpenter ants, while others list them. Baits containing boric acid are generally low in toxicity to other animals, according to Health Canada. Compare options in the guide to the best ant killers.

Why Spraying Trails Backfires

Spraying ant trails inside the house kills the workers you see but not the colony in the wall. Health Canada warns that spraying indoor trails may split the colony into several new colonies, and advises switching to bait if ant numbers go up. With odorous house ants, the University of Wisconsin extension notes that activity can even increase for a while after baiting, as more workers are recruited to the bait. That is a sign the bait is working, so keep it in place instead of reaching for a spray.

Dust Treatments for Wall Voids

Dust treatments reach a nest directly inside the wall. Health Canada notes that dusts can be puffed into wall voids, and NC State Extension says effective control often requires treating the nest directly. In practice, a professional usually puffs an insecticidal dust through small holes drilled in the drywall or trim near the nest, then patches them. UC IPM lists borate dusts and desiccant dusts among the products used. Avoid breathing in any dust, and only use a product whose label allows indoor use in wall voids.

Diatomaceous Earth

Diatomaceous earth is a non-chemical dust that dehydrates ants. Health Canada says it can be placed in cracks and crevices, stays active while dry, and is non-toxic to humans and pets, though you should avoid inhaling it. It kills ants that walk through it, so it works best in the gaps where ants enter and leave the wall, not as a replacement for bait.

Essential Oil Sprays

Essential oil sprays such as peppermint or citrus may repel ants from a surface for a while, but they are unlikely to reach the colony inside the wall; UC IPM says baits are more effective than sprays. A repellent can also push ants to new spots, which is the same problem Health Canada describes with sprays on indoor trails. Use them, if at all, away from your bait stations.

Choosing a Registered Product

Choosing a registered product keeps the treatment legal and safe. Health Canada advises looking for a Pest Control Products (PCP) number on the label, which shows the product is registered in Canada, and checking that the label allows indoor use before using it inside. Follow every label direction and warning, and keep children and pets away from treated areas. Health Canada also suggests leaving dead ants where they are, because other ants may carry them back to the nest. For a broader method that covers ants anywhere in the house, read how to get rid of ants.

When Should You Call a Professional for Ants in Walls?

Calling a professional for ants in walls makes sense when you see carpenter ant sawdust, when you cannot find or reach the nest, or when weeks of baiting have not reduced the trails. Health Canada recommends a professional exterminator in difficult situations, and Alberta Health says severe infestations may need a certified pest control professional and several visits over days or weeks.

What a Professional Treatment Involves

A professional treatment usually starts with an inspection of the home, inside and out, to identify the species and locate the nest, including the wall voids. The technician then treats the nest and the trails with baits, dusts injected into the wall voids or targeted sprays, and often a perimeter treatment outside. Most companies plan follow-up visits to confirm the colony is gone, and many give sanitation and structural advice, such as fixing a leak or replacing damp wood. Tell the technician if you have children or pets (Alberta Health).

Questions to Ask Before You Hire

Questions to ask before you hire help you compare companies:

• Is the technician licensed in your province?
• Which species do they think it is, and where is the nest?
• Which products will they use, and do they have a PCP number?
• Will they drill into the wall, and who patches the holes?
• How many visits are included, and is re-treatment covered if the ants come back?

Prices vary with the species, the size of the infestation and how hard the nest is to reach. The guide to ant extermination costs in Canada explains what affects the price, and you can get a quote from a recommended local exterminator through Pest Agent's referral form.

How to Prevent Ants in Walls

Preventing ants in walls comes down to dry wood, sealed gaps and no food left out. These steps also help a treatment last.

Moisture Control

Moisture control removes what draws carpenter ants and odorous house ants into walls. Fix leaky pipes and clogged gutters, improve ventilation in attics and crawl spaces (UC IPM), and keep indoor humidity at or below 50%, as Alberta Health recommends.

Sealing Cracks

Sealing cracks cuts off the ways ants get into your walls. Health Canada recommends repairing and sealing visible cracks in the foundation and exterior walls, and caulking along baseboards, cracks and crevices indoors. Pay attention to the spots where power and phone cables, pipes and heating ducts enter the house. If you are baiting, it usually helps to wait until the colony is gone to seal the gaps around the nest, so the ants keep taking the bait.

Food Storage

Food storage keeps foraging ants from settling near your kitchen. Store food in glass jars with rubber seals or plastic containers with tight lids, sweep up crumbs, and empty the kitchen garbage often (Health Canada). If you rent, the guide to ants in an apartment covers your next steps with the landlord.

Rotting Wood

Rotting wood is a favourite nesting material of carpenter ants. Health Canada advises replacing decaying or infested structural wood with sound material, clearing decaying wood near the house, and moving firewood away from the building instead of stacking it against a wall.

Trimming Branches

Trimming branches and shrubs away from the house removes the bridges ants use to reach your roof and walls (UC IPM). For more ant guides, visit the ant hub.