Thanks to Wikipedia for the following information on Ants.
"Modern society considers the ant a pest and due to the adaptive nature of ant colonies, eliminating them is nearly impossible. Pest control with regard to ants is more a matter of controlling local populations than eliminating an entire colony. Attempts to control ant populations of any kind are temporary solutions.
Typical ants that are classified as pests include pavement ants (otherwise known as the sugar ant), Pharaoh ants, carpenter ants, Argentine ants, and the red imported fire ant. Control of species populations are usually done with bait insecticides, which are either in the form of small granules, or as a sticky liquid that is gathered by the ants as food and then brought back to the nest where the poison is inadvertently spread to other members of the brood — a system that can severely reduce the numbers in a colony if used properly.
Boric acid and borax are often used as insecticides that are relatively safe for humans. With the recent insurgence of the red imported fire ant, a tactic called broadcast baiting has been employed, by which the substance (usually a granule bait designed specifically for fire ants) is spread across a large area, such as a lawn, in order to control populations. Nests may be destroyed by tracing the ants' trails back to the nest, then pouring boiling water into it to kill the queen. This works in about 60% of the mounds and needs about 14 litres (3 gallons) per mound.
Ants that tend other insects can indirectly cause pest infestations. Many homopteran insects that are considered as horticultural pests are controlled by the use of grease rings on the trunks of the trees. These rings cut off the routes for ants and make the pest species vulnerable to parasites and predators."