The big ants showing up in your kitchen or bathroom are almost certainly carpenter ants, the largest ant species commonly found in U.S. homes. Workers range from about 1/4 inch to 1/2 inch long, and queens can reach a full inch. Unlike termites, carpenter ants don’t eat wood, but they tunnel through it to build nests, and that can cause real structural damage over time. Getting rid of them takes a combination of finding the nest, choosing the right treatment, and eliminating the conditions that attracted them.
Make Sure They’re Carpenter Ants
Size alone isn’t enough to identify these ants, because a single colony produces workers of several different sizes. What sets carpenter ants apart is their body shape: viewed from the side, the middle section (thorax) is evenly rounded and arched, and they have a single small bump (node) at the waist. They also have a visible ring of fine hairs at the tip of the abdomen. Workers are typically black, or red and black, though some species are brown, tan, or yellow.
Field ants are the most common lookalike. They’re a similar size but have a noticeably uneven, dented thorax. This distinction matters because the treatment strategies are completely different. If you’re unsure, capture a few workers in a small container and compare them to reference photos from your local university extension service.
Find the Nest First
Killing the ants you can see won’t solve the problem. Carpenter ants maintain a parent colony, usually outdoors in a dead tree, stump, or woodpile, and often establish one or more satellite colonies inside your home. The satellites tend to settle in wood that’s already soft or damp: rotting windowsills, framing around sinks and tubs, and areas near roof leaks are all common sites.
To locate a nest indoors, watch where the ants travel after dark. Carpenter ants are most active at night, and their foraging trails often lead back to a wall, cabinet base, or door frame where they’re entering a void. Tap along baseboards and wood trim with a screwdriver handle. A hollow sound, or a rustling noise from disturbed ants, points to a gallery inside. Small piles of fine wood shavings (called frass) on the floor beneath a wall opening are another strong clue. These shavings look like sawdust mixed with insect body parts, and they accumulate right below the nest entrance.
Use Bait to Reach the Colony
Spraying the ants you see with a contact killer feels satisfying, but it mostly kills foragers without touching the queen or the rest of the colony. Bait works differently. Foragers carry the bait back to the nest, where it spreads through the colony as ants feed each other. This process isn’t fast. Expect 5 to 10 days before you notice fewer ants, and several weeks before the colony is fully eliminated.
Liquid sugar baits with a low concentration of boric acid (around 0.5% to 1.0%) tend to have the greatest colony-level impact. The low dose is intentional: it keeps foragers alive long enough to deliver the bait deep into the nest. Place refillable bait stations along the trails you’ve observed, near entry points, and close to any suspected nest sites. Don’t spray insecticide near the bait stations, because repellent chemicals will keep ants away from the bait you want them to find.
Resist the urge to clean up ant trails during this phase. The chemical trails foragers leave behind are what guide other workers to the bait. Let those trails do the work for you.
Treat Wall Voids Directly
If you’ve located a nest inside a wall, you can treat it more aggressively with an insecticidal dust. Boric acid, silica gel, and diatomaceous earth are the three most common active ingredients for this purpose. Each works by damaging the ants’ outer coating or digestive system after contact.
The application method is simple but requires some care. Use a squeeze-bulb duster (or a small plastic bottle with a narrow tip) filled no more than one-third full. Insert the tip snugly into the nest entrance hole or a small drilled opening in the wall, then puff the dust in by squeezing and releasing the bottle. You want a light coating inside the void, not a heavy pile. Too much dust and ants will avoid the area. Make sure the dust doesn’t drift into spaces where people or pets spend time, and store any leftover product in its original labeled container.
If you don’t have an obvious entry point, drill a small hole (about 1/8 inch) in the suspected wall section, ideally behind a baseboard or inside a cabinet where it won’t be visible. Inject the dust, then plug the hole with a small piece of tape or caulk.
Fix the Moisture Problem
Carpenter ants choose nesting sites based on moisture. Dry, solid wood is hard for them to excavate. Soft, water-damaged wood is easy. If you eliminate the ants but leave the moisture source in place, a new colony will eventually move in.
Check for leaky pipes under sinks and around tubs. Inspect the roof and attic for signs of water intrusion, especially near valleys, vents, and flashing. Look at windowsills and door frames from the outside for paint bubbles, soft spots, or visible rot. Any wood that’s been structurally compromised by moisture and ant tunneling should be replaced, not just treated.
Beyond repairs, a few exterior changes reduce the odds of reinfestation significantly. Trim tree branches and shrubs so nothing touches your house; carpenter ants use vegetation as a bridge from outdoor colonies to your siding. Move firewood, lumber, and stumps away from the foundation. Make sure gutters drain well away from the house, and fix any grading that channels rainwater toward the structure.
When DIY Isn’t Enough
A small satellite colony near a bathroom leak is a manageable DIY project. But carpenter ant infestations can be more complex than they appear on the surface. If you’re seeing large ants in multiple rooms, finding frass in more than one location, or noticing winged ants emerging indoors during spring (a sign the colony is mature and producing new queens), the colony has likely been established for years. A mature carpenter ant colony contains 10,000 or more workers, and the structural galleries can be extensive.
A pest management professional can use moisture meters and thermal imaging to locate nests inside walls without tearing them open, and they have access to non-repellent liquid treatments that ants walk through unknowingly and carry back to the colony. If you suspect the damage extends into structural framing, getting a professional assessment of both the pest problem and the wood damage is worth the cost. Ignoring a large, established colony risks the kind of hidden structural deterioration that gets expensive to repair.