Fire ants, which were introduced to the United States from South America over 70 years ago, have now spread throughout the southern United States. These aggressive ants can be 1/8″ to 1/4″ long, with reddish-brown to black coloring. In general, they build their large, mounded nests in sunny, open areas (often visible along sidewalks and driveways). However, nests may also be constructed around trees and stumps or even under buildings.