Fire Ant Biology and Identification

Imported Fire Ants February 25, 2015 Print Friendly and PDF

Biology and Identification

Larval stages of imported fire ant. Photo by Bart Drees.

Properly identifying ant species is the first step in determining the need and approach for control (see Identifying Fire Ants). Accurate identification can be especially important in the southwestern states where native fire ant species are common and red imported fire ants are rare. Although native fire ants are common urban pests, if they are controlled unnecessarily, especially in very dry climates, imported fire ants are more likely to invade these areas.

Where imported fire ants are common, most homeowners recognize them by the mounds they build or the stings they inflict. However, there are also other characteristics to look for. Their aggressive nature compared to other ant species is one such trait. If a mound is disturbed, usually hundreds of fire ant workers will swarm out and run up vertical surfaces to sting.

If you are unsure of the ant species you have, contact your county Extension office for help identifying them.

Imported fire ants (red imported fire ant, Solenopsis invicta Buren, and black imported fire ant, Solenopsis richteri (Forel) and their sexually reproducing hybrid) are social insects. Unlike many insect pests, they are very organized. Colonies consist of the brood and several types (castes) of adults. The whitish objects often found at the top of the mounds are actually the ant’s developmental stages or brood—the eggs, larvae and pupae.

Types of Adults

  • winged males (distinguished from females by their smaller heads and black bodies except in black or hybrid imported fire ants that may have darker females);
  • red-brown winged females;
  • one or more queens (wingless, mated females); and
  • workers
Imported fire ants in comparison to other ants (photo by S. B. Vinson)

Worker ants are wingless, sterile females. They vary in size within a colony from 1⁄16 to 3⁄16 inch long. They protect the queen by moving her from danger, defend the nest from intruders, and feed the queen only food that the workers or larvae have eaten first. They also forage for food and care for the developing brood.

Winged forms, or reproductives, live in the mound until their mating flight, which usually occurs in the late morning and afternoon soon after a rainy period. Mating flights are most common in spring and fall. Males die soon after mating, while the fertilized queen lands and walks around to find a suitable nesting site, sheds her wings, and begins digging a chamber in which to start a new colony. Sometimes, several queens can be found within a single nesting site.

The newly mated queen is 3⁄8 inch long, red-brown, and initially lays about a dozen eggs. When they hatch 7 to 10 days later, larvae are fed by the queen. These larvae will develop into small worker ants that will feed the queen and her subsequent offspring. Later on, a queen fed by worker ants can lay from 800 to 1,000 eggs per day. Larvae develop in 6 to 10 days and then pupate. Adults emerge 9 to 15 days later. The average colony contains 100,000 to 500,000 workers and up to several hundred winged forms and queens. Queen ants can live 7 years or more, while worker ants generally live about 5 weeks in summer and longer in cooler months. Larger workers generally live longer than smaller workers.

In addition to sexually reproductive hybrid imported fire ants, there are two kinds of red imported fire ant colonies -- the single-queen and multiple-queen forms. Workers in single-queen colonies are territorial. Workers from multiple-queen colonies move freely from one mound to another. This lack of territorial behavior by the multiple-queen form causes a dramatic increase in the number of mounds per acre. Areas infested with single-queen colonies contain 40 to 150 mounds per acre (rarely more than 7 million ants per acre), whereas areas with multiple-queen colonies may harbor 200 or more mounds and 40 million ants per acre.

Red imported fire ants build mounds in almost any type of soil, but prefer open, sunny areas such as pastures, parks, lawns, meadows and cultivated fields. Mounds can reach 18 to 24 inches in height, depending on the type of soil. Often mounds are located in rotting logs and around stumps and trees. Colonies also can occur in or under buildings.

Colonies frequently migrate from one site to another. A queen needs only half a dozen workers to start a new colony, and they can build a new mound dozens of feet away from their previous location almost overnight. Fresh water flooding causes colonies to leave their mounds and float until they can reach land to establish a new mound. Colonies also can migrate indoors.

Related Slide Presentation [PDF]

Imported Fire Ants, Competitor Ants and Impact of Bait Products

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.