Fire Ant Biology

Worker ants are wingless, sterile females (Fig 1). They protect the queen by defending the nest from intruders, by feeding the queen only food that the workers have eaten first, and by moving the queen from danger. They also forage and care for the developing brood. The brood is made up of cream-colored eggs, larvae, and pupae of all the castes.

The winged forms, or reproductives (Fig. 2), live in the mound until their mating flight, which usually occurs in the afternoon soon after a rainy period. Mating flights are most common in spring and fall. Males die soon after mating, while the fertilized queen alights to find a suitable nesting site, sheds her wings, and begins digging a chamber 3-12 cm deep in which to start a new colony. Sometimes, several queens can be found within a single nesting site.

Figure 1
Picture of ant life stages from larva to adult worker. workers

Figure 2
Picture of ant life stages from larva to winged, reproductive adults. Picture taken by V. E. Bertagnolli, Auburn University Research Assistant, Auburn, AL.

A newly mated queen lays about a dozen eggs. When they hatch 7 to 10 days later, the larvae are fed by the queen. Later on, a queen fed by worker ants can lay up to 800 eggs per day. Larvae develop 6 to 10 days and then pupate. Adults emerge in 9 to 15 days. The average colony contains 100,000 to 500,000 workers and up to several hundred winged forms and queens. Worker ants exhibit a range of sizes.

Queen ants can live 7 years or more, while worker ants generally live about 5 weeks, although they can survive much longer.There are two kinds of red imported fire ants -- 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, which has resulted in 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).

In areas with multiple queen colonies, there may be 200 or more mounds and 40 million ants per acre.The red imported fire ant builds mounds (Fig. 3) in almost any type of soil, but prefers open, sunny areas such as pastures, parks, lawns, meadows and cultivated fields. Mounds can reach 35 cm in height and 30-50 cm in diameter 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. The queen needs only half a dozen workers to start a new colony.

They can develop a new mound several hundred feet away from their previous location almost overnight. Flooding causes colonies to leave their mounds and float until they can reach land to establish a new mound. Colonies also can migrate to indoor locations.Fire ants can be beneficial. They are omnivores. They feed primarily on insects and arthropod pests, which can reduce the need for insecticides in commercial agriculture. In urban areas fire ants feed on flea larvae, chinch bugs, cockroach eggs, ticks and other pests.

Figure 3
Fire ant mound.
Picture taken by V. E. Bertagnolli, Auburn University Research Assistant, Auburn, AL.

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Fire Ant Identification

Solenopsis invicta (Fig 1). Workers are polymorphic , about 1.6-6 mm (1/16-1/4") long; queens average 6.6 mm (1/4") long (fig 3). Head and thorax yellowish red and abdomen black; reproductives darker. Antenna 10-segmented, with 2 segmented club. Thorax lacks spines, profile unevenly rounded. Pedicel 2-segmented. Stinger extruded in most. The mandible has 4 teeth.

Figure 1
Common morphological characters used in identifying the red imported fire ant, Solenopsis invicta.

Figure 2
Fire ant queen shown with workers and brood. Courtesy of Sanford D. Porter, USDA, Gainesville, Florida.

Figure 3
Workers demonstrate polymorphism. Picture taken by Dr. Samford Porter, USDA-CMAVE, Gainsville, FL.

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Fire Ant History

Most ant stinging problems in North America are due to the two species of imported fire ants in the southern United States: Solenopsis invicta (Fig. 1) and S. richteri (Fig. 2).

The black imported fire ant, S. richteri, was introduced from South America into the United States at Mobile, Al, in the early 1900s, followed by the introduction of the red imported fire ant, S. invicta, at the same port in the 1930s in the soil used as ballast or dunnage in ships carrying agricultural goods.

Figure 1
Solenopsis invicta. Picture taken by V. E. Bertagnolli

Figure 2
Solenopsis richteri. Picture taken by Joe MacGown, Mississippi Entomological Museum, Starkville, MS.

Since that time, few other stinging insects have created more controversy, generated more research, or received more publicity than the red imported fire ant. The two ants now inhabit a major portion of 12 southern states, with S. invicta occupying 95% of the infested areas; S. richteri occurs only in parts of Mississippi, Alabama, and Tennessee.

At first the red imported fire ant was of little concern. However, by the 1950s, it became obvious that the ant was rapidly expanding its range accompanied with an increase in red imported fire ant density and a decrease in the density of other ants in the areas where it occurred.

William Buren described the red imported fire ant with the specific epithet name invicta meaning invincible because of its resilient nature and the belief that it would be difficult to manage. The ant has proven to be appropriately named.

Last Updated: March 28, 2013