Common Name: Fire Ants
Scientific Name: Solenopsis spp.
These ants get their common name from their ability to inflict especially painful bites and stings. The two most important species are the southern fire ant (Solenopsis xyloni McCook) and the red imported fire ant (Solenopsis invicta Buren). The southern fire ant is a native species and ranges from California to southern South Carolina to northern Florida. The red imported fire ant is from central Brazil and is found in the southeastern United States, from Virginia through Texas.
Workers polymorphic, about 1/16-1/4″ (1.6-6 mm) long; queens average 1/4″ (6.6 mm) long. 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 alcohol-collected specimens; readily inflict painful stings. Characteristics of the mandible and petiole (1st node of pedicel) will separate these 2 species. The mandible of S. xyloni has 3 distinct teeth on its inner/biting surface whereas, those of S. invicta have 4 teeth and the petiole of S. xyloni has a ventral tooth near the node’s attachment to the thorax whereas, S. invicta lacks such a tooth.
(1) Fire ant (Solenopsis geminate) with ridge on lower front margin of mesothorax having 1 or more teeth, 1st node in profile with rear margin almost straight. (2) Little black ant (Monomorium minimum) with antenna 12-segmented, club 3-segmented. (3) Acrobat (Crematogaster spp.), big-headed (Pheidole megacephala), harvester (Pogonomyrmex spp.), and pavement (Tetramorium caespitum) ants have spines on upper surface of thorax; in addition acrobat ants with heart-shaped abdomen and pedicel attached to upper surface of abdomen, big-headed ants with soldier with head very large and 3-segmented antennal club, harvester ants with underside of head with a brush of long bristles (coarse hairs/setae called psammophores), and pavement ants with head and thorax covered with distinct ridges. (4) Other small dark ants have 1-segmented pedicel.
For the red imported fire ant, single-queen mounds usually number 30-100/acre (0.4ha) with typically 80,000 but up to 250,000 individuals per colony. Multiqueened colonies may number 200-700/acre (0.4ha) but contain fewer individuals per colony, and there is less fighting between the colonies. Typical mounds are rounded, being up to 18″ (48 cm) high and 24+” (61+ cm) in diameter, each with several tunnels just under the soil surface extending out several feet. A queen in a large colony is capable of producing her own weight in eggs each day or about 1,500 or more. Developmental time (egg to adult) for workers ranges from 22-38 days. A mature colony can produce as many as 4,500 swarmers during the year, with 6-8 mating flights occurring between spring and fall. Mating flights usually begin about 10 am, 1-2 days following a rain if it is warm (about 75 degrees F / 24 degrees C), sunny, and not very windy. Minor workers live 30-60 days, intermediates (medial) 60-90 days, and majors 90-180 days or longer. Queens live 2-6 years. Males die shortly after mating. Typical mounds of the southern fire ant are flattened and irregular, covering 2-4 sq ft (0.17-0.37 sq m). Swarms occur from May through October in the afternoon to early evening of warm days. Developmental time can require as few as 44 days. Its biology has not been thoroughly studied.
Fire ants are typically ground-nesting ants. However, the southern fire ant will sometimes nest in the wood or masonry of buildings, especially in areas near the soil or warmth such as fireplace hearths. When the southern fire ant nests outside near a house, it is usually in the vicinity of the kitchen. Outside nests are usually situated under stones or other covering objects, or in the soil at the base of a tree or shrub, or in clumps of grass. The red imported fire ant typically nests outside. Each colony has its own territory, and there is usually no movement between colonies. However, they will sometimes nest in areas of exposed soil within buildings such as bath traps. They also have the habit of building outside nests adjacent to foundation walls. They are commonly introduced into new areas via potted or balled shrubs and trees. Fire ants are attracted to electrical junction boxes of traffic signals, air conditioners, etc. When they mass around the electrical contact points, they cause the equipment to malfunction. They will also nest in gas and water meter boxes and then follow the pipes into the building. Fire ants prefer food with a high protein content but will feed on almost anything, plant or animal. The southern fire ant has been known to remove insulation from phone and electrical wires, and to gnaw on clothing, especially if soiled. They usually feed on seeds, insects, young tree bark, honeydew and other sweets, preferring oily meats and nuts. Red imported fire ants are particularly destructive to vegetation. Workers forage in established trails.
Fire ant control is difficult. It usually requires repeated applications of liquid or granular residuals to eliminate the colony. Particularly effective with a single application are residual aerosols applied under high pressure (160 psi) with a long injection probe. Although baits are slower acting, they are effective. Baits containing only a stomach poison require several applications each season to control newly emerging workers when the queen(s) is/was not killed, and new colonies. Baits containing only an insect growth regulator can provide year-long control with 1 or 2 applications in the residential situation when followed in 7-10 days with a liquid residual application to kill the active foragers. Newer baits containing avermectin,, which acts as both an insect growth regulator and slow-acting stomach poison, give good control without liquid application.