Ants are the most common household pest in the United States, and many species that originated elsewhere are now well established. Most ants can be beneficial to soil and, with the exception of fire ants, they aren't necessarily a problem if they stay outside. The tiny foragers present a nuisance inside the house, though, where you must avoid toxic chemicals to control them. You may have success with a combination of natural control methods instead.
Block the Pheromone Trail
Ants communicate by natural scents called pheromones; when a forager finds food, other ants pick up the trail by detecting the pheromones with their antennae. Spraying your kitchen counters, floor, baseboards and walls with white vinegar is an effective way to disrupt this pheromone trail. Vinegar causes mass confusion among ants, because, besides masking the natural odors they use to navigate, it's just generally repulsive to them. Essential oils, especially peppermint, clove, lemon and tea tree oil, have similar effects, and they smell much better to humans. Drop a few drops of one or a combination of oils on a cotton swab, and wipe along the baseboards or anywhere else you see ants entering.
Offer a Tasty and Lethal Treat
Boric acid is as natural an ingredient as you can find, being derived directly from borax, a naturally occurring mineral. Using it as a bait is an effective way to control ants, especially when used in low concentrations that allow the foragers to stay alive long enough to feed the bait to the rest of the colony. When a baiting program is successful, the entire colony dies. You must first identify the ants you're trying to control so you can make a bait that will attract them -- some ants like sugar, some like protein, and a few enjoy either. Place bait stations where the ants can find them and wait for the ants to disappear -- it usually takes about two weeks.
Note: Boric acid is a skin irritant and is toxic when ingested or inhaled. Do not use boric acid or any product containing boric acid where children or pets may contact it.
Scratch 'Em and Dry 'Em
Food-grade diatomaceous earth looks like flour, and it's safe enough to eat in limited quantities, but it's actually composed of microscopic phytoplankton fossils. It works by lacerating ant exoskeletons and drying out the insects' vital fluids. Spread it around the inside and outside of the house, especially along baseboards and other places where you see ants. The idea is to force the ants to walk through it -- those that do will die. Destroy a colony by pouring DE around or directly on a nest or anthill. Diatomaceous earth isn't always effective -- in humid climates, there may be enough moisture in the air to counteract its desiccating effects.
As with boric acid, never use diatomaceous earth where children or pets may contact it.
Get the Nest
Many ants prefer to nest outdoors, and you can usually locate the nest by following an active trail. If he nest is in the ground or an old woodpile, you can usually kill it by dousing it with boiling water. Be sure to wear protective clothing if you do this to a fire ant nest. If the ants are nesting in your walls, follow the trail to the point where the ants are emerging and drill a series of evenly spaced, 1/4-inch holes in the wall for 2 or 3 feet on either side of this entry point. Blow boric acid or diatomaceous earth dust into the holes, using a plastic ketchup bottle. This should kill the nest.