Fire Ants

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Red imported fire ant mound in red Georgia clay-soil.
Red imported fire ant workers.
Red imported fire ant worker in process of stinging.
Skin reaction after fire ant stings.
  • Scientific Name: There are many species of Soleopsis ants, but four economically important species are found in the United States: the native fire ant, Solenopsis geminata (Fabricius); the red imported fire ant, S. invicta Buren, the black imported fire ant, S. richteri Forell; and the southern fire ant, S. xyloni McCook. [Phylum Arthropoda: Class Insecta: Order Hymenoptera: Family Formicidae]

  • Common Names: fire ants

  • Climatic Zone: tropical and subtropical

  • Geographic Distribution: The red imported fire ant, the most important species, is a native of Brazil but was introduced into Alabama between 1933 and 1945. This pest now inhabits the area south of a line running from mid North Carolina across to the Oklahoma City, Oklahoma area and down to Corpus Christi, Texas. The black imported fire ant is a native of Argentina and Uruguay which was imported into the Mobile, Alabama area as early as 1918. This ant is still localized in the area were Mississippi and Alabama join. The native fire ant was originally located across the southern states but has been displaced by the imported species. Colonies are still scattered from South Carolina across Texas. The southern fire ant is a native to Arkansas, Oklahoma and Texas.

  • Damaging Stage: adults

  • Hosts: Mounds may be built in any turf situation.

  • Damage Symptoms: Fire ants do not attack turf directly but cause problems when they build earthen mounds for warming their eggs, larvae and pupae. They have a notorious sting which may cause a burning and itching sensation at minimum and serious welts or allergic shock at its maximum. Unsuspecting people and pets may be severely injured when the mounds are accidentally disturbed. In pastures, fire ants have been known to kill newborn animals that were dropped onto fire ant mounds.

Fire ants are beneficial in some aspects because they are major predators of other insects including mole crickets, cutworms and armyworms and white grub eggs.

The rounded cone-shaped mounds may be several inches to 2 ft in diameter and an inch to 8 inches tall. When the mound is disturbed, hundreds of ants emerge from the broken surface.

  • Description of Stages:

Fire ants are social insects with a complete metamorphosis. Though they have egg, larval, pupal and adult stages, the adults belong to several castes - wingless workers of several sizes, winged queens and winged males. Depending on the species and locality, fire ants may be yellow to reddish brown to black in color. An expert should be consulted if species determination is necessary. The following descriptions are based on the red imported fire ant.

Eggs: The translucent white eggs are laid in clusters which are picked up and stored by workers in the upper part of the mounds, They are slightly bean-shaped and about 1/32 inch (0.75 mm) long.

Larvae: The larvae are grub-like with no legs and a small head capsule. The larvae are usually white in color. Worker larvae may grow to 1/4 inch (6 mm) long while the reproductives may be twice as long.

Pupae: Fire ant pupae do not form a silk cocoon like some other ant species. The pupa looks like the adult but has the antennae, legs, and wing pads (if it is a reproductive) held close to the body. The pupa is first white but becomes darker before the adult emerges.

Adults: The workers can be several sizes and range from 1/16 to 1/4 inch (1.6 - 6 mm ) long and have the typical elbowed antennae of ants and a thin waist. The constriction at the waist, the petiole, has two nodes; the antennae are 10-segmented with the last two segments forming a distinct club. The reproductives are winged and the queens may be twice the size of the largest workers.

  • Life Cycle and Habits: Established colonies produce new queens and winged males during warm spring and summer months. These winged reproductives swarm periodically, usually five to nine times a year, often after a rain. Mated queens attempt to establish a new colony by digging a small hole in the soil and closing up the entrance. Inside this chamber, the queen lays 15 to 20 eggs in two to three days. More eggs are added over the next week by which time the first eggs hatch. The queen picks up the young larvae and sorts them into groups. The larvae are feed a liquid regurgitated by the queen.

After 20 to 25 days, the larvae pupate and tiny workers emerge four to seven days later. These first workers are about 1/5 the size of the smallest workers found in an older colony. These workers break open the nesting chamber and begin foraging for insect food and start to enlarge the nest. The queen, now fed by workers, begins to lay more eggs which are cared for by the workers. If food and water is adequate, the colony steadily grows over the next few months. If a colony is established in June, it may contain 6,000 to 7,000 individuals by the following December.

As the soil temperatures drop, the colony growth slows. By the following June, a one year old colony may have 10,000 to 15,000 workers and is producing new winged forms. Colonies 2 to 3 years old may have 20,000 to 200,000 workers. Established mounds will have a central pile of granular soil with openings and often smaller mounds around the perimeter. The mound is established as a solar collector to store and incubate the larvae and pupae. Fully mature colonies may have mounds 16 to 24 inches (40 - 60 cm) in diameter and 10 to 12 inches (25 - 30 cm) high.

Colonies may move the mounds in search of food, when regularly disturbed by mowing or when pesticides are applied. In clay soils, the mounds may become very hard and have been known to damage plows or mowing equipment. The mounds are generally not as tall in sandy soils.

  • Control Approaches:

Fire ants are important agricultural as well as urban pests so considerable effort has been expended in developing control techniques. Unfortunately, this group of pests have great powers of reproduction and the ability to move quickly when their nests are disturbed by chemical controls or mechanical disruption.

Traditional control of fire ants usually relys both curative and preventive approaches. The first step is to treat and eliminate colonies, then step two is to prevent new invasions.

Biological Controls - Pathogens and Parasites - Various fungal and bacterial diseases have been discovered and/or tested for fire ant control, but none produced consistent ant reduction or elimination. Insect parasitic nematodes have generally produced similar results. Most recently, thre is a small fly parasite that lays its eggs in foraging ants. The fly maggot kills the ant by eating its internal organs, then the maggot pupates within the head capsule causing it to fall off. This fly has been established in several areas and seems to be reducing fire ant colonies.

Chemical Controls - Mound Treatments - There are several insecticides that are applied as a powder or drench directly to the fire ant mounds. These insecticides are either picked up by the ants and distributed deeper in the colony, or the insecticide fumes may drift deeper into the colony. These types of insecticides rarely kill mounds but force the ants to move their colonies.

Chemical Controls - Ant Baits - There are several baits that contain growth regulators (these may cause the larvae to develop improperly, sterilize the queen(s), or affect other vital systems) or slow acting, acute toxicants (this allows the toxicant to be distributed throughout the colony before it takes its effect. Baits are generally broadcast where the ant workers are foraging. The workers pick up the baits, take them into the colony and feed their larvae the bait. Baits usually take several weeks to achieve maximum control. Baits should not be dumped onto mounds as this may disturb the colony and cause it to be moved before sufficient bait is picked up to destroy the colony.

Chemical Controls - Area Treatments - Certain insecticides have long residual action and large area applications can significantly reduce the fire ant populations. When these are used first, remaining ant mounds can be treated with baits or mound treatment products.