Ants can be beneficial to your garden, so an outdoor colony isn't always a problem. It's a different matter if the ants are coming indoors, especially if they are fire ants, which pack a painful sting. If your goal is just to keep them out of the house, perimeter treatment may work, but baiting is best if you want to kill the colony. You can do both effectively with homemade control methods.
If you have determined that the ants invading your counters are coming from outside -- and not from the walls -- you may be able to keep them out by dusting around the foundation with diatomaceous earth. This powder, which is formed from the fossilized remains of prehistoric organisms, is low-toxicity and will not harm plants, but it should not be used where children or pets can be exposed to it. DE is lethal to ants that walk through it. It creates microscopic incisions in their exoskeletons and dries out their body fluids. It works best if you form a continuous line around the house that the ants have to cross, and make sure you replenish it regularly.
Attacking the Colony
The best strategy to control ants that interfere with your outdoor activities or are simply troublesome -- such as fire ants -- is to destroy the colony. If you can locate it by following an ant trail, it may be shallow enough to destroy by simply dousing the colony boiling water. Avoid pouring boiling water on any desirable plants. This method isn't always effective, because it may leave stragglers and, in the case of fire ants, may expose you to bites and stings from escaping ants. You can spray or dust the nest with a product that contains carbaryl or pyrethroid, two effective insecticidal chemicals, but safer alternatives --diatomaceous earth and boric acid -- work just as well. Spread either powder where the insects are forced to walk through it.
Boric Acid for Outdoor Ant Control
Like DE, boric acid is a low-toxicity, naturally occurring compound, but it can be toxic if ingested in large enough quantities. Do not use boric acid where children or pets may be exposed. The mechanism whereby it kills ants isn't completely understood, but it may desiccate the insects in much the same way that DE does. In addition, it probably interferes with their neurological systems. Large quantities kill ants quickly, so dusting it directly on a nest is a good way to kill the nest. It also kills insects that walk through it, so spreading it on the ground around the nest or around your house can be an effective control strategy.
Boric Acid Bait
Baiting ants with boric acid is a good way to kill a nest you can't access. The bait should include a low concentration of boric acid -- from 1 to 5 percent by weight -- which kills slowly enough to allow the worker ants to feed the bait to the colony and kill the queen. The bait should also include sweet or oily attractants, depending on the species of ant. To make an effective all-purpose bait that attracts both sweet- and grease-loving species, include ingredients such as honey and peanut butter. Distribute bait stations in several places in your yard, and replenish them regularly. If the bait works, the ants should disappear in one to two weeks.