Most residents of the southeastern United States are very familiar with fire ants. These reddish brown ants are well known for their aggressiveness and stings that produce a burning sensation. The term “fire ant” actually applies to a group of New World ant species in the genus Solenopsis. Many people refer to them as “red ants,” although this term is also used to refer to the larger red harvester ants found in desert climates. In Spanish, the fire ant is sometimes called hormiga colorada or hormiga de fuego, and in Portuguese it is formiga de fogo. In North America, there are four native fire ant species, two introduced species and two hybrid forms. The two imported species in the United States are the red imported fire ant, Solenopsis invicta, and the black imported fire ant, S. rich-teri. S. invicta has spread throughout the southeastern part of the country, whereas S. richteri is restricted to northeastern Mississippi, northern Alabama, and northwestern Georgia. Seventeen fire ant species are currently described from South America.
ORIGIN AND SPREAD
There seems little doubt that the most important fire ant pest, S. invicta, traveled from South America to Mobile, Alabama, in ship ballast between 1930 and 1940. It spread in all directions from there, limited only by cold winters or desert drought conditions. S. richteri may have arrived earlier (perhaps in 1919), only to be largely displaced by S. invicta. The latter is currently established in 12 states (Florida, Georgia, Alabama, Mississippi, Louisiana, Texas, North Carolina, South Carolina, Arkansas, Tennessee, Oklahoma, and California) and Puerto Rico.
It was predicted that imported fire ants could not survive a winter when the minimum temperature was below — 12°C. However, infestations now occurring in southern Oklahoma and Tennessee have led to a revised estimate of — 18°C. Another limiting factor is rainfall. The deserts of west Texas have proven a barrier to the fire ant’s progression westward. It is likely that annual rainfall of less than 25 cm precludes S. invicta from becoming established unless there is also irrigation.
IDENTIFICATION AND BEHAVIOR
All fire ants in the genus Solenopsis can be separated from all other ants by having two segments in their narrow waists and antennae with 10 segments (including a 2-segment club at the tip). Their workers are polymorphic, ranging in size from small to large (about 2-5 mm in length; queens are about 7mm long).
Venom and Stinging Behavior
Fire ant venom consists mainly of piperidine alkaloids, which are highly unusual as animal by-products and are found only in the ant genera Solenopsis and Monomorium. The venom also contains a small amount of protein (<1%) that can cause anaphylactic shock in susceptible individuals. Fire ant alkaloids have hemolytic, phytotoxic, antibiotic, and insecticidal activity. Recent medical trials have shown that it is a potent angiogenesis inhibitor that could possibly be used to restricit the flow of blood to tumors. Among fire ant species there are both quantitative and qualitative differences in the mix of alkaloids in their venom.
To be able to sting, fire ants must first gain leverage with their mandibles by biting; they then curve around the abdomen to insert the stinger. The alkaloids in the injected venom produce a burning sensation, followed by the appearance of a red spot on the skin. The burning sensation is short-lived, followed by itching. In most people, a white pustule will develop at the site within a few hours. These pustules are sometimes called ” s terile pustules” because they are not produced by infectious bacteria. The pustules can last from days to weeks and can become infected if they are scratched. Fire ants can sting repeatedly; therefore, stinging ants should be brushed off rapidly.
Mounds and Foraging Behavior
When undisturbed, the typically dome-shaped mounds of S. invicta can reach heights of 30 cm or more above the ground. These mounds allow the workers to respond to local conditions by moving up and down with their brood and queens according to temperature and humidity. Exit holes are usually not apparent on the mounds themselves, but foraging trails extend outward from the mound just below the surface. During floods, fire ants move to the upper parts of the mound. If the water gets any higher, the ants grasp each other to form floating rafts that carry the brood and queens downstream. During droughts, fire ants can extend their tunnels down 6 m or more in search of moisture.
Fire ant workers can feed only on liquids: they have filters in their digestive tract that prevent the ingestion of solids. Only the fourth instar can digest solids directly, and it is the only path for processing of solid food particles in the colony. Workers deposit insect parts and other solids on the larva’s “food basket.” After feeding on these solid foods, the larva secretes liquids that are licked up by workers and distributed around the colony.
Mating Behavior
Like most other ants, fire ants have mating flights. In the southeastern United States, flights are most frequent in the spring following rain and subsequent sunshine. After the rain, workers fill the queens with food to prepare them for the flight. Dissection of queens at this time shows a large drop of yellow oil in their crops. Then, between mid-morning and mid-afternoon, the workers open large exit holes in the mounds to allow quick exit of the males and females. Workers become very agitated and start chasing the reproductives, which then climb vertical objects nearby from which they fly. Mating occurs in the sky. The males drop to the ground and die shortly thereafter. The queens also land, quickly shed their wings, and search for a place to dig a tunnel. The queen will close the tunnel and start to lay eggs, producing her first workers in about 1 month. Queens typically live 6 or more years. Because they mate only once, they must store live sperm for the rest of their lives. For this purpose, they have a transparent sac in their abdomens called the spermatheca that is filled with over a million sperm after mating. When the queen lays an egg, she can open a valve on the spermatheca, allowing the escape of sperm to fertilize her eggs. These diploid eggs give rise to females, either workers or new queens. If she does not release sperm, the egg she lays is haploid and becomes a male, as is typical for all haplodiploid social Hymenoptera.
Number of Queens
There are two social forms of S. invicta in the United States. The monogyne form has only one queen per colony, whereas the poly-gyne form has many queens. On the average, polygyne colonies also have smaller workers: there is a negative correlation between the number of queens and the average worker size. Worker genotypes at a single gene, Gp-9, determine the social form of the colony. This gene encodes a pheromone-binding protein which may influence how workers identify queens.
IMPACT ON PEOPLE AND
THE ENVIRONMENT
Imported fire ants arrived in the United States without their native parasites and predators. In addition, few native ant species are able to compete with them. When fire ants encounter other ant species, they vibrate their gasters (abdomens) and protruded stingers, spraying their venom and chasing away most other ants. In disturbed habitats, they can quickly become the predominant ant species and a significant pest. Fire ants will attack newly hatched birds both on the ground and in trees. Fawns and calves can be stung in their eyes and blinded if they are dropped on fire ant mounds. Soft plant tissue, such as okra, can be destroyed. Ants in mounds at the bases of trees can eventually girdle and kill trees. The sick and elderly, pets and children in backyards, campers, and picnickers all can become victims of fire ant stings (Fig. 1). One oddity about fire ants is their evident attraction to electrical fields. They frequently enter electrical boxes such as outside air conditioners, traffic boxes, and lights, where they chew wires and short out the circuits.
POSSIBLE REMEDIES
There are many insecticides that control fire ants. Baits are advantageous because it is not necessary to find the mounds; ants carry the bait back to their nests. Most fire ant baits consist of corncob grits coated with soybean oil as an attractant. Typically, a toxicant or insect growth regulator is dissolved in the oil. However, none of these
FIGURE 1 A child’s arm showing many pustules after numerous fire ant stings.
solutions are permanent. Efforts are now under way to bring into the United States some of the fire ant parasites and predators from South America. Decapitating flies (Phoridae) are one promising predator now being released in Florida and elsewhere. These flies lay their eggs on fire ants. The grub that hatches invades the ant’s head, where it consumes its brain. Ultimately the ant’s head falls off and a new fly emerges. Although these flies parasitize only a small percentage of ants, they do interfere with the ant’s foraging behavior and may make fire ants less competitive with other ants. The flies are spreading from their introduction sites, but their impact on fire ant populations is not yet known. Other potential biocontrol agents include protozoal parasites (Thelohania solenopsae and even Varimorpha invictae) and parasitic ants from South America (Solenopsis daguerri). A newly discovered fire ant picorna-like virus may kill brood in the colony. Ultimately, a broad approach using chemicals and biological agents will best manage this invasive species.
