Imported fire ant (Solenopsis invicta and S. richteri) workers are polymorphic, which means that their workers can vary in size from 1/16 to almost 1/4 inch long. Mounds can sometimes reach 1 foot in height and 18 inches in diameter. When disturbed, the ants boil out from their mounds, climb up anything touching their mound, and sting. The sting produces a burning sensation and the next day a white pustule may form at the site.
Native fire ants are less common in imported fire ant-infested areas. Outside of California the tropical fire ant (S. geminata) is the most common species of native fire ants. In California the southern fire ant (S. xyloni) is most common. Telling apart the various fire ant species is not easy and should be left to local ant experts. The native species do not make the dome-shaped mounds described above. However, where grass is mowed frequently, the dome shaped mounds of imported fire ants can be easily destroyed, leaving only a scar visible in the grass where the mound was located. The ants will still be present just below the surface.
Common native ants sometimes confused with fire ants are the harvester ants (sometimes called red ants), which are larger than fire ants. Furthermore, all their workers are the same size. Their mounds are visible on the surface as large bare areas on the ground, devoid of grass, frequently surrounded by pebbles, and with a single entrance hole to the colony. These ants can also sting, but they are usually not very aggressive unless provoked.
See also FAQ #811: "How can I tell if I have imported fire ants?"
FAQ #1090: "How big are imported fire ants?"
FAQ #1088: "I have ants in my house. Could they be imported fire ants?"
