I think I have carpenter ants in my house. What should I do?

— A reader in Fairfax Station, Va.

A: Carpenter ants can cause serious structural damage to a house, so it’s smart to be concerned. These unusually large ants — often a quarter to half an inch long — typically nest in rotting stumps or other wood outdoors, but they also establish nests in wood within walls, basements, attics, porches or decks. Their damage isn’t usually as widespread as damage from termites, but it does compromise the strength of the wood where they nest.

First, understand why they move into a house. Carpenter ants don’t actually eat wood. They dine on a whole range of other foods that other ants like, too: insects, honeydew from plants, pet food and scraps of people food (both meat and sweets). If you find only occasional carpenter ants indoors, they are probably just searching for food. These might annoy you, but they don’t pose a structural problem. But if you find dozens on a regular basis, it’s worth investigating whether there is a nest.

How to tell if you have a carpenter ant nest

To create a nest, carpenter ants tunnel into wood. But they just discard the fibers; they don’t digest them. This fact can help you determine whether you’re dealing with carpenter ants or termites.

Termites do digest wood. Although they might leave some sawdust behind, their frass — a fancy word for droppings — comes out as uniformly shaped pellets, each with six ridges visible with a hand lens. Drywood termites leave piles of pellets near wood they are eating, but subterranean termites, more common in many parts of the country, recycle their frass to construct mud tunnels, which keep them moist as they travel between nests in the ground and the wood they are chewing.

If you find piles of sawdust that don’t contain little ridged pellets, you know that carpenter ants have created — or are trying to create — a nest. They aren’t just foraging for food.

Carpenter ant nesting habits

Scraping tunnels through wood is a lot of work, which is why carpenter ants aren’t attracted to dry, sound wood, although they do sometimes establish satellite nests in hollow doors and insulation. For the main nest, they go for damp wood, especially if it’s already starting to rot and become spongy. A plugged gutter that dumps water where it splashes up against siding, a roof that collects piles of leaves or needles, a leaking pipe in a basement — these are classic situations that can lead to carpenter ants making a nest.

If you don’t find sawdust-type fragments, it’s possible they are piling up inside a wall cavity, out of view. Sometimes it’s possible to locate the nest by putting an ear — or a stethoscope — to a wall or beam. The sound these insects make as they scrape the walls of their tunnels has been described as a faint rustling or scratching, or like cellophane being crushed. You can listen to YouTube or Facebook videos, although it’s hard to tell how much the sounds are amplified. (Search online for “carpenter ant sounds.")

Another sure sign of a problem: finding masses of winged carpenter ants swarming indoors around a window. In Virginia, this usually occurs in May through July, and only after a colony is mature enough to rear reproductive males and females. A nest usually takes three to six years to reach that stage, then it produces more winged ants each year.

Male and female termites and other kinds of ants also grow wings before mating, so be sure to identify the insects. Ants have narrow waists and wings that are about as long as their bodies; termites don’t have indented waists and have wings twice as long as their bodies. Carpenter ants have an evenly rounded thorax (the segment behind the head), while the thorax for other ants is bumpy.

Of course, if you wait for a nest to get big enough to produce winged ants before doing anything, you’ll have bigger problems to deal with — if not from the ants’ tunnels, then from the rot that attracted them and continued to expand while you didn’t deal with the leak or other underlying moisture problem.

How to get rid of the ants

If you can locate the nest, you can treat it with an insecticide that can legally be sold to consumers. Look for one that’s specifically labeled as effective against carpenter ants; it probably also lists other kinds of ants and insects. You’ll also need to fix whatever caused the moisture problem, which often also means replacing rotting wood. Opening up walls just to locate a nest might cause more damage than is warranted, though.

If you can’t find the nest, try using bait stations. Terro Liquid Ant Baits (six for $5.98 at Lowe’s) contain a sugary substance to attract the ants plus borax, which they can’t detect. They will carry it back to poison their nest mates. Avoid using kill-on-contact sprays in combination with bait stations. The instant-kill formulas won’t cause enough damage to wipe out the colony, but they will warn other ants to find a pathway that avoids the bait stations.

Consider calling in the pros

Using bait stations won’t point you to the moisture issue that attracted the ants, though. And it’s hard to know whether you’ve eliminated the nest. Because it can be tricky to find a nest, and because only licensed pest-control operators can legally purchase some of the pesticides that are most effective when the location is not known, it makes sense to consult with a professional. People who deal with pest problems every day know more signs to look for. They often have other equipment, such as moisture meters, that can help pinpoint problems.

Ask what product or products they plan to use and go online to read the safety data sheet and the label for each product. The safety sheet may list precautions most appropriate for people who handle large amounts in factories and shipping plants, but it also will give you an idea of the relative risk to people and pets. The label will list details of how the product is to be applied, whether it is suitable for use indoors as well as outside, and whether you need to keep people and pets away for a specific time.

Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.