Red Imported Fire Ants

The red imported fire ant (Solenopsis invicta), is an introduced species from South America. It is well known for its aggressive nature when disturbed and its painful sting, hence the name "fire" ant. Red imported fire ants (RIFA) can harm people and native wildlife, cause damage to buildings and electrical equipment, as well as interfere with harvesting and maintenance of pastures and crops.

Biology   

Red imported fire ants live in colonies that contain cream-colored to white immature ants, called brood. The brood is comprised of the eggs, larvae, and pupae. Within the colonies are adult ants of different types, or castes. The castes include winged males, winged females (unmated queens), workers, and one or more mated queens. The winged males and females fly from nests, usually in the spring and early summer, to mate in flight. Upon landing, mated females will shed their wings after finding a suitable nesting site. All the males die after mating. While thousands of winged males and females can be produced per year in large colonies, they don't sting, and fewer than 10% of the females will survive to produce a colony. Newly-mated queens can fly as far as 12 miles from the nest, but most land within a mile.

New colonies don't make conspicuous mounds for several months. Once a colony is established, a single queen can lay over 2,000 eggs per day. Depending on temperature, it can take 20 to 45 days for an egg to develop into an adult worker. Workers can live as long as 9 months at 75°F, but life spans usually are between 1 and 6 months under warmer outdoor conditions. Queens live an average of 6-7 years. Fire ants are omnivorous feeders, feeding on carbohydrates (honeydew, plant exudates, sugars, syrups), proteins (insects, meats), and lipids (grease, lard, oils from seeds). Their food preferences change depending on the nutritional requirements of the colony. In the spring and summer the colony produces new offspring and the protein needs of the colony increase. Adult ants require carbohydrates and/or lipids to sustain themselves throughout the year. Fire ants are only able to ingest liquids. Solid proteinaceous foods are liquified by placing them on a depression in front of the mouth of the oldest larvae (the fourth instar stage), which then regurgitate digestive enzymes onto the food. Once liquified, the fourth instar larvae suck up the protein and regurgitate it to the workers, which pass it on to the rest of the colony.

Workers will forage for food more than 100 feet from the nest. They can forage generally when air temperatures are between 70° and 90°F. When a large food source is found, fire ants recruit other workers to help take the food back to the colony. Liquids are ingested at the food source, and stored within the ants until they are regurgitated to other ants within the colony. Liquids from solid foods are extracted at the source or are carried back as solid particles.

There are two types of fire ant colonies:

• single-queen, or monogyne, colonies, and
• multiple-queen, or polygyne, colonies.

Single-queen colonies have only one egg-laying queen, and may contain as many as 100,000 to 240,000 workers. Multiple-queen colonies have many egg-laying queens (usually 20 to 60), with 100,000 to 500,000 workers. Single-queen colonies fight with other fire ant colonies. Because of this antagonistic behavior, colonies are farther apart, resulting in a maximum of 40 to 150 mounds per acre. Multiple-queen colonies generally do not fight with other multiple-queen colonies. Consequently, mounds are closer together, and can reach densities of 200 to 800 mounds per acre. Multiple-queen mounds may also be inconspicuous, often times being clusters of small, flattened excavations, in contrast to the distinct dome-shaped mounds of single-queen colonies. Workers from single-queen colonies vary in size, ranging in length from 1/8 to 1/4 in, and are usually reddish brown to black in color. Workers of multiple-queen colonies are generally smaller (1/8 to 3/16 in), have only a few large workers, and are lighter in color (orangish-brown) than single-queen colony workers.

The large colony sizes, and the presence of numerous queens makes multiple-queen colonies more difficult to eliminate than single-queen colonies. Since 1973, multiple-queen colonies have been found in eight of the 11 fire ant infested states, including Florida. Multiple-queen colonies produce fewer winged, or alate, queens that will start new colonies after a mating flight than single-queen colonies. However, multiple-queen colonies can establish new colonies by budding, where a portion of the queens and workers splits off from a colony.

The spread of fire ants into new areas depends on many factors, such as climate, surrounding fire ant populations, and the native predators and competitors in the areas. Areas with an abundance of natural enemies and competing ant species may hinder colony establishment because the enemies prey upon newly-mated queens and compete for resources. However, if an area is disturbed, for example, by clearing land for pastures or urban development, natural enemies or competitors may be adversely affected and fire ants may colonize the area more rapidly.

It can take as long as 11 years for single-queen fire ant colonies to become the dominant ant species in a new area which has been disturbed by urbanization, and has not been treated with insecticides to control ants. Multiple-queen colonies may become dominant in new areas at a slower rate because they spread more by budding than by establishing numerous new colonies scattered throughout an area after mating flights.

In areas where native ants and fire ant populations have been reduced or eliminated with insecticides, reinfestation by fire ants may be noticeable within a month after treatment. Fire ants reinfest these areas more rapidly and outcompete other ant species because of their tremendous reproductive capacity and faster colony development. If fire ant control is not maintained, the subsequent reinfestation of an area may result in even greater fire ant populations than existed before the application of insecticides.

The RIFA build mounds in a variety of soil types but seem to prefer open, sunny areas. Mounds can reach 18 inches in height. They can also establish colonies in rotting logs, around stumps and trees, and in or under buildings. The average colony contains up to 500,000 workers, several hundred winged ants and one or more queens. The ants can vary in size from around 1/8" to 1/4" in length. Worker ants are wingless, sterile females and care for the queen and brood, forage for food, and protect the colony from intruders. The winged ants are reproductives and live in the mound until their mating flight. Mating flights are most common in the spring and fall, soon after a rainy period. Males die soon after mating, while the fertilized queen looks for a suitable nesting site. There she will shed her wings and begin digging a chamber in which to start a new colony. The new queen lays about a dozen eggs, which hatch 7-10 days later and are fed by the queen. Larvae develop into pupae in 6-10 days. Adults emerge from pupae in 9-15 days. When the queen is cared for by the workers she can lay up to 800 eggs a day. The queen can live seven years or more while the workers usually live about five weeks.

Distribution

These ants were first introduced to the United States in 1930's Alabama. The red imported fire ant is now established over much of the Southeast and Alabama, Florida Louisiana, Arkansas, Georgia, Mississippi, Oklahoma, North Carolina, South Carolina, Tennessee, Texas, and Puerto Rico. Isolated colonies have been found as far west as California, and as far north as Kansas City, Missouri. RIFA were officially detected in Los Angeles and Orange Counties in November 1998. It is believed that they have been present since at least 1996. The ants are expected to colonize irrigated agricultural areas and lawns throughout California.

Health Risk

Fire ants are aggressive and will defensively attack anything that disturbs them. They can sting repeatedly. After firmly grasping the skin with its jaws, the fire ant arches its back as it inserts its stinger into the skin, injecting venom, then inflicts an average of seven-eight stings in a circular pattern. Symptoms of a sting include burning and itching, which usually subsides within 60 minutes, followed by a small blister at the site of each sting within a few hours. A white pustule forms at the sting sites in a day or two and infection can occur if the pustule is scratched or broken. Although the stings are not usually life threatening, they are easily infected and may leave permanent scars. On rare occasions, anaphylaxis can occur and may be life threatening. Signs of anaphylaxis include flushing, general hives, swelling of the face, eyes, or throat, chest pains, nausea, severe sweating, loss of breath or slurred speech and requires emergency medical assistance.

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