Ants are the most abundant social insects on the planet. There are all kinds of ants roaming the earth, including more than 450 species in North America alone. Some may be considered a nuisance, especially when they invade your home, favorite picnic spot or sting you. Generally speaking, however, ants are beneficial insects, and the ones commonly found in landscapes don't actually eat plants. They also aerate the soil as they tunnel to create their nests.
Some ants can cause damage to plants indirectly, however.
These types of ants actually herd and protect aphids from their natural predators, such as ladybugs. There are several non-chemical ways of controlling ants. Nurseries and other stores sell a sticky material that you can brush or spray on plant stems or tree trunks to prevent ants from going up the plant, and one application lasts several weeks. In the home, an ant bait that includes boric acid that you sprinkle along their trails is your best bet. The imported fire ant has invaded many parts of the United States, and it's a very serious, and in some cases, very dangerous critter with a brutally-painful sting .