Approximately 700 species of ants inhabit the United States; the National Pest Management Association calls them America's number-one nuisance pest. The three species you're most likely to encounter in your house are carpenter, odorous and pavement ants. Handling a colony in the walls can be tricky, because it's often difficult to pinpoint its location, and you don't always have access. The most effective control strategy is baiting, but it isn't the only one.
Ant-Proofing Your Home
Before you attempt to eliminate an ant colony in your walls, investigate how they got there so you can prevent a reinfestation. Look around the house for tree or bush limbs that touch the walls or roof, and cut them back. Seal cracks in wood siding with caulk, and fix cracks in concrete foundations with hydraulic cement. Ants are drawn to water, so look for sources of leaks that could produce moisture in the walls and fix them. These leaks may be caused by overflowing gutters or poorly sealed window or door trim. Fix the gutters and caulk the trim, as needed.
Locating the Nest
You may already be able to see the hole or crack the ants are emerging through, but if not, there's a simple way to find it. Attract the ants with two different types of bait, since not all ants like sweet things. Spread jelly and peanut butter on several pieces of masking tape and spread them around. Once you see ants starting to collect, follow their trail back to the wall. You may see the ants passing through a gap between an electrical plate or vent grille and the wall. Seal the hole with caulk or joint compound, but don't kill any ants. Keep the bait out, and wait until they find a new hole. This helps you pinpoint the location more accurately.
Dusting the Colony
The best way to stop an ant infestation is to destroy the colony and kill the queens, and one way to do this is to spray boric acid -- available at drug stores -- inside the walls. It kills the insects by interfering with their neurological systems. If you have located the nest, drill a series of small holes in the wall around it and spray in the powder, using a plastic ketchup container. You can also kill the workers -- thereby starving the colony -- by dusting diatomaceous earth along baseboards, around electrical fixtures and ventilation ducts, and any where else you see ant activity.
Boric Acid Baits
Another way to use boric acid or borax is to make bait that kills worker ants slowly enough to allow them to feed it to the colony. A water solution containing 1 percent boric acid is usually best, although you may have to experiment by strengthening it to as much as 5 percent. Mix the solution with jelly, honey or peanut butter, depending on whether the ants like sweet or greasy foods, and set up several bait stations. Remove the stations that get no traffic and replenish the bait in the ones that do. Baiting takes a few weeks to work; you'll notice a gradual reduction in the number of ants until they disappear completely.