Red Imported Fire Ant Solenopsis invicta Buren Hymenoptera Formicidae

Natural History

  1. Red imported fire ant was accidentally introduced to the United States at Mobile, Alabama between 1933 and 1941. It gradually spread over most of the southeast, occupying much of Georgia, Alabama, Mississippi, and Louisiana by 1957, and then adjacent states from North Carolina to Texas by
  2. Isolated infestations occur elsewhere periodically, including in California, and this species seems destined to occupy all of the southern half of the United States except for very arid areas. Red imported fire ant also has successfully invaded Puerto Rico. Activity decreases during cold weather, and this species seemed ill-adapted for cold-weather conditions, which was limiting northward spread. The northern limits now are southern Tennessee and southern Oklahoma. The origin of Solenopsis invicta is South America, where it is most abundant in southwestern Brazil and in Paraguay.

Red imported fire ant is not the only fire ant in North America, but it is the only species that commonly affects vegetable crops. Two native species, Solenopsis geminata (Fabricius) or tropical fire ant, and S. xyloni McCook or southern fire ant, also occur in the southeastern states, with S. xyloni also extending to California. Another immigrant species, S. richteri Forel or black imported fire ant, was also introduced from South America, but likely from areas south of the homeland of red imported fire ant. Black imported fire ant is quite similar to red imported fire ant, though darker in color, and so initially the two species were confused. Black imported fire ant has a very restricted distribution, and is limited to northern Alabama and Mississippi, and western Tennessee. Red imported fire ant tends to replace most other species of ants when it invades a new area, including the aforementioned Solenopsis spp. (See color figure 179.)

Host Plants. These ants are omnivorous, and feed on a mixture of plant and animal matter. They are effective predators of insects, spiders, earthworms, and other small invertebrates. Plant feeding is limited, and often occurs when ants are deprived of other food. Nevertheless, fire ants are known to feed on such vegetables as bean, cabbage, corn, cucumber, eggplant, okra, potato, and sweet potato, and on other crops such as young citrus trees, peanut, sorghum, soybean, and sunflower. Okra fruit is particularly at risk. Sweet plant exudates and honeydew from homopterous insects are readily consumed. (See color figure 22.)

Natural Enemies. Many natural enemies of red imported fire ant are known, but few seem to be effective under either South American or North American conditions. Competition from other ants is believed to be one of the most important factors limiting abundance and dispersal in their native land, but North American ants seem to be ineffective competitors of fire ants, at least within disturbed habitats. Colony survival, particularly of incipient colonies, is also affected by competition among fire ant nests. Minim workers from new colonies often raid brood from nearby nests, boosting size of receiving colonies and allowing new colonies to achieve maturity sooner. Although raided colonies disappear, workers and queens from the losing colonies may be amalgamated into the receiving colonies (Tschinkel, 1992).

Among the promising pathogens of fire ant are the microsporidian Thelohania solenopsae and the fungus Beauveria bassiana. Neither has demonstrated consistent effectiveness in North America, though there is some evidence that Thelohania may be important in South America.

A South American parasitoid, Pseudacteon sp. (Diptera: Phoridae), which decapitate ants during the final stages of development has been introduced to the southern states, and related species are being introduced. An unusual South American parasitic ant, Solenopsis daguerri (Santschi) (Hymenoptera: Formici-dae), invades fire ant colonies and takes over control of the fire ant colony, possibly through food diversion. This and other parasites are being investigated for potential to achieve fire ant suppression. Natural enemies have been discussed by Jouvenaz (1983, 1986), Banks et al. (1985), and Wojcik (1986a).

Life Cycle and Description. Ants are colonial insects, with the number of ants per colony depending on colony age and availability of food. Activities of the colony members are highly structured, with some members (castes) specialized for reproduction, tending of the young, and obtaining food. The seasonal reproductive cycle usually begins in March, and eggs from mature colonies give rise to both worker and reproductive castes. Workers predominate, with the maximum proportion of sexual forms (about 10%) produced in June. A life cycle can be completed in about 30 days. Longevity of colonies is uncertain, though individual queens can live 6-7 years.

  1. Colonies may be initiated by a single mated queen or several queens. Mating flights may occur at any time, but peak abundance is during May-August. Flights are preceded by rain, normally within 1-2 days, and at least 80% relative humidity. After the mating flight the queen usually breaks off her wings and begins excavation of a burrow within four hours of her mating flight. Initially the burrow consists of a vertical tunnel 6-12 cm deep into soil, and small cells. The queen lays 15-20 eggs within 2-3 days and produces 20-125 eggs by the time the first larvae hatch. The newly deposited egg is whitish, oval in shape, and measures about 0.2 mm wide and 0.3 mm long. As the egg approaches maturity it acquires the larval form. Only about half of the eggs are fertile, the remaining serve as food for the young larvae. Mean duration of the egg is 8.4, 5.2, 5.0, and 5.8 days when cultured at 25°, 30°, 32°, and 35°C; optimal develop ment of eggs and other stages occurred at about 32°C (O'Neal and Markin, 1975b). Once the colony is firmly established, the queen can produce up to 2000 eggs per day.
  2. There are four instars, and all stages are whitish, C-shaped, and lack distinct appendages. As described by Petralia and Vinson (1979), first instars are hairless, and measure 0.27-0.42 mm long. Head capsule width is 0.14-0.16 mm. Second instars have only a few simple hairs, measure 0.42-0.57 mm long, and have a head capsule width of 0.16-0.19 mm. Third instars have moderately numerous short hairs of various types, measure 0.59-0.91 mm long, and have a head width of 0.20-0.25 mm. Fourth instars are similar to third instars in general appearance, though the body hairs are slightly longer. The body length of fourth instars is 0.8-1.8 mm, and the head width is 0.260.32 mm. Head capsule widths increase slightly during both the third and fourth instars. Duration of instars 1-4 was 1,1,2, and 3 days, respectively, when cultured at 32°C (O'Neal and Markin, 1975b). Porter (1988) reported that duration of the larval stage decreased from 28 to 11 days as temperature increased from 24° to 35° C. The castes are quite similar in appearance during the larval stage; the principle difference is size. Mean size is greatest among larvae destined to be sexual forms, followed by major, minor and minim workers. Development time is proportional to size, with the larger reproductives requiring nearly twice as much time as the minim workers, and the other castes intermediate.
  3. Pupal development is often reported to require about 6-8 days, though Porter (1988) indicated that duration of the pupal stage decreased from 28 days when cultured at 21 °C to less than 7 days at 35°C. The pupa greatly resembles the adult in form, though it is whitish. Size varies considerably, depending on the caste of the adult form. The pupae, along with the eggs and larvae, comprise the ant brood, and are moved about within the colony by workers according to environmental conditions.
  4. A total of 20-30 days is normally required between egg deposition and emergence of the first mature worker ants, called minim workers. Within 90 days of colony founding, the colony may consist of 200 minims and a few minor workers. Porter and Tschinkel (1986) described the importance of minims in colony founding and success. Within five months there may be 1000 minor and a few major workers. Within seven months the colony may consist of 6000-14,000 workers, and may contain about 3% major workers. The number of ants per colony is estimated at 11,000, 30,000, and 60,000 workers at 1, 1.5 and 3 years. After three years the colony is considered to be mature, and workers number about 230,000 per colony. Abundance typically decreases each winter, however, and reproduction ceases at northern latitudes and decreases in more southern locations. Colonies may begin to produce reproductives at 5-7 months, but normally it is older colonies that produce most reproductives.

Adult worker ants vary considerably in size, ranging from about 1.6-6 mm long. They lack wings, and vary in color from light reddish-brown to dark-brown. The gaster is sometimes marked with an orange spot. Assignment of workers to castes is based on arbitrary head width categories, not on distinct morphological or behavioral differences. Adult reproductives bear two pairs of wings, with the front wings longer than the hind wings. The females resemble the workers in color, but the males are black except for pale antennae.

The physical structure of the nest changes markedly over time. As noted earlier, initially it is a small burrow with chambers. As workers are produced, however, the tunnels are enlarged and additional chambers are produced. Within 90 days of colony founding, a soil surface mound is produced, typically 5-7 cm in height and 3-7 cm in diameter. At this time there are 5-15 vertical tunnels extending a meter or more into the soil down to the water table. Mounds in areas with sandy soil tend to be relatively flat, whereas in clay soil they can be 0.5-1.0 m in height. Large colonies may construct several mounds. They may continue to use the same mound for several years, or may construct new ones. The mound enables the colony to optimize environmental conditions for the brood. During cool weather the brood is moved to the sunny side of the mound near the surface, while during hot weather the brood is found deep within the underground tunnels.

Food for the colony is collected by foraging workers, which often forage for considerable distances from the colony, with the distance dependent on colony food requirements. Maximum foraging occurs between the soil temperature (5 cm depth) of 21-35°C, but some foraging can occur between 10-37°C. Foraging is aided by the construction of tunnels leading back to the nest; these often extend 15-25 m from the mound. Foraging ants search randomly for food, but once food is located, ants returning to the tunnels deposit trail pheromones that serve to recruit other ants. Soon there is a stream of ants leading to the food. Food obtained by the foraging workers is quickly distributed throughout the colony. Food typically passes from foragers to nurses, and then to larvae and queens. Colony organization and coordi nation is maintained by secretion of pheromones. Several pheromones associated with fire ants have been identified, including the brood, trail, queen tending, and nestmate recognition pheromones.

There are two types of colonies: single-queen (monogyne), and multiple-queen (polygyne). The monogyne colonies contain about 100,000-240,000 workers, fight with other colonies, and tend to be situated farther apart, with mound densities of 100-350 per hectare. In contrast, polygyne colonies contain 20-60 queens and 100,000-500,000 workers, do not fight with other polygyne colonies, and have densities of 500-2000 mounds per hectare. Polygyne colonies tend to erect lower mounds than monogyne colonies, and certain smaller workers which are lighter in color. Polygyne colonies are becoming more common in the southern United States.

The fire ants were described by Buren (1972). Hung et al. (1977) provided keys to the common species of fire ants. Creighton (1950) presented a comprehensive treatment of North American ants, though red imported fire ant was not distinguished in this treatment. Wheeler and Wheeler (1990) provided a good key to North American genera. Biology of fire ant was reviewed by Lofgren et al. (1975), Vinson and Greenberg (1986), and Vinson (1997). Vander Meer (1988) discusses caste structure. A laboratory rearing method was described by Williams et al. (1980). Bibliographies were published by Banks et al. (1978), Wojcik and Lofgren (1982), and Wojcik (1986b). Zoogeography was summarized by Buren et al. (1974).


Red imported fire ant thrives in disturbed habitats such as cropland and pastureland. Thus, agricultural environments are particularly likely to be infested, but similar sites such as roadsides, irrigation ditches, and athletic fields also support high densities. Red imported fire ant does not survive well in climax plant communities, particularly if dense shade is provided by trees and shrubs.

An interesting feature of these ants is that they are unable to ingest solid food. They place solid food near

Adult worker red imported fire ant.

the mouth of fourth instar larvae, which then secret digestive enzymes. The liquefied food is then passed around the colony. Nevertheless, fire ants readily collect solid materials, including plant tissue. Young seedlings, in particular, can be killed by fire ants. For example, Adams (1983) reported destruction of over 50% of eggplant seedlings in Florida. Damage was in the form of stem girdling and destruction of the growing point of the young plants. Seedling injury is the most frequent form of damage, but ants also feed below-ground on seeds, roots and tubers; they remove bark from young citrus trees; and cause abortion of okra flowers by feeding at the base. Plant injury occurs most commonly when ants are deprived of food, as when land is first cultivated. Fire ants are not normally considered to be primary plant pests except for okra, where damage can be severe and frequent. However, they affect crop production indirectly because their mounds interfere with equipment operations, and laborers are reluctant to harvest heavily infested crops due to the toxic sting of fire ants.

Fire ants are well-known for their venomous sting which induces a burning sensation in victims—in fact, this is the basis of their common name. Although the individual sting is much less toxic than that of bees and many other stinging insects, the abundance of the ants and the tendency of victims to receive multiple stings represents a real threat to human health. Humans are not the only animals at risk. Reptiles, ground-nesting birds, young mammals, and even fish have been reported to be killed by fire ants. Animals reproducing during the warmest periods of the year are at great risk because ant foraging is reduced during cool weather. Rates of reproduction of several forms of wildlife reportedly increase following area-wide suppression of fire ants. This is also of interest to livestock producers, because newborn livestock are at risk; blinding of young calves is the most common form of livestock injury. The impact of fire ants on plant, animal and human health was reviewed by Adams (1986), Lofgren (1986), Jemal and Hugh-Jones (1993), and Allen et al. (1994).

Damage is somewhat offset by the beneficial predatory behavior of red imported fire ant. Predation of fire ants on sugarcane borer, Diatraea saccharalis (Fabricius) (Lepidoptera: Pyralidae), boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae), and lone star tick, Amblyomma americanum (Linneaus) (Acari: Ixodidae), is particularly well-documented. However, predation occurs on many species of insects, particularly caterpillar larvae on plants and fly larvae breeding in animal manure, and red imported fire ant is usually considered to be a beneficial insect within the context of cotton and sugarcane production sys tems. Reagan (1986) reviewed the beneficial aspects of fire ants.


  1. Fire ants and their mounds are easily detected, and visual examination of fields is adequate for most purposes. Density is usually expressed as number of mounds per unit of land area, though colony size is an important variable.
  2. Insecticides are effective for ant suppression, but reinvasion of treated areas from untreated areas can occur, sometimes giving the impression that insecticides were not effective. Ants are often treated by application of toxin-treated bait. The attractive component of the bait is normally soybean oil, and the granular carrier can be any of a relatively inert substances, often corncob grit. The toxin is usually a slow-acting insecticide or an insect growth regulator. Slow induction of mortality is advantageous, because it allows the toxin to be transported through the colony to the brood and perhaps even to the queens. Unfortunately, few bait-based insecticides can be legally applied to vegetable crops owing to failure by manufacturers to register their products for this use. Treatment of field perimeters is allowable, however, and foraging ants may collect bait deposited in perimeter areas and return it to colonies located within crop fields.

An alternative method of suppression is application of granular, dust, or liquid insecticide to mounds, or to tilled soil, but mortality is often less complete than with bait formulations. Ants from mounds treated with insecticide may relocate following treatment, necessitating one or more reapplications of insecticide before the colony is eliminated.

Cultural Practices. There are few effective non-chemical approaches to managing fire ants. Suppression of other insects is helpful because it deprives ants of their principal food supply. Individual mounds can be treated with boiling water, but at least 10 liters of water are necessary to penetrate most mounds, and its effectiveness is limited. Boiling water also kills nearby plants. Physical destruction of mounds by digging or tilling, and shoveling two mounds together in hopes of stimulating a fatal fight, are of little value. Large individual plants can be protected from foraging ants by placing a wide barrier of adhesive around the base of the plant. This approach is often best preceded by wrapping the trunk or stem with tape or foil to prevent the adhesive from disfiguring the plant or damaging the plant tissue.

Biological Control. The diseases and parasites discussed under "natural enemies" are being inocu-

lated for permanent establishment and are not gener- Heterorhabditidae) and the straw itch mite, Pyemotes ally available from commercial sources, but other tritici (Lagrexe-Fossat and Montane) have been sug-organisms have been suggested for biological suppres- gested for biological suppression. However, perfor-sion if applied regularly. In particular, entomopatho- mance of these biotic agents have been disappointing genic nematodes (Nematoda: Steinernematidae and under field conditions.

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