Fire Ants on Lawns and Turf
David H. Oi and Philip G. Koehler
Imported fire ants are aggressive, reddish brown to black ants that are 1/8 to 1/4 in long. They construct nests which are often visible as dome-shaped mounds of soil, sometimes as large as 3 feet across and 1 1/2 feet in height . In sandy soils, mounds are flatter and less visible. Fire ants usually build mounds in sunny, open areas such as lawns, pastures, cultivated fields, and meadows, but they are not restricted to these areas. Mounds or nests may be located in rotting logs, around trees and stumps, under pavement and buildings, and occasionally indoors. When their nests are disturbed, numerous fire ants will quickly run out of the mound and attack any intruder. These ants are notorious for their painful, burning sting that results in a pustule and intense itching, which may persist for 10 days. Infections may occur if pustules are broken. Some people have allergic reactions to fire ant stings that range from rashes and swelling to paralysis, or anaphylactic shock. In rare instances, severe allergic reactions cause death.
In addition to stinging humans, imported fire ants can sting pets, livestock, and wildlife. Crop losses are also reported due to fire ants feeding on seedlings and even citrus trees. Harvesting machinery used on farms can be damaged by hitting hard fire ant mounds often found in clay soils. Electrical equipment and utility housings may serve as fire ant nest sites, sometimes resulting in short circuits.
The term imported fire ants generally refers to two species of ants: the black imported fire ant, Solenopsis richteri Forel, and the red imported fire ant, Solenopsis invicta Buren. The black imported fire ant was accidently introduced from South America into Mobile, Alabama, around 1918, and now infests a small area in Alabama and northern Mississippi. The red imported fire ant entered the United States probably in the 1930s. It was most likely introduced with cargo or in the soil used as ballast in ships from South America that were unloaded in the Mobile area. In the 1940s and early 1950s the red imported fire ant spread to Florida and other southern states in nursery stock and sod. Fire ants currently infest over 260 million acres in Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, and Puerto Rico ( Figure 1 ). They have the potential to establish in other areas where average minimum temperatures are above 10°F and rainfall is greater than 10 inches a year, or in irrigated areas. Localized infestations have also been reported in Arizona and Virginia.
To limit the spread of imported fire ants, a federal quarantine restricts the movement of soil, potted plants, plants with soil attached, grass sod, hay, and used soil-moving equipment to uninfested areas of the United States. These items must be certified that they are free from infestation. While there are no restrictions on the movement of regulated articles within Florida and other quarantine areas, any shipments outside the quarantine areas require inspection and certification from the Florida Department of Agriculture and Consumer Services, Division of Plant Industry (FDACS-DPI). Information on specific laws and regulations may be obtained from any local office of the FDACS-DPI or the U.S. Department of Agriculture Animal and Plant Health Inspection Service/Plant Protection and Quarantine, (USDA-APHIS/PPQ).
BIOLOGY
Red imported fire ants live in colonies that contain cream-colored to white immature ants, often called brood. The brood is comprised of the eggs, larvae, and pupae. Also within the colonies are adult ants of different types, or castes. The castes include winged males, winged females (which are unmated queens), workers of varying size, 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 do not 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 (or even farther in the wind), but most land within a mile.
New colonies do not 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 to 7 years.
Fire ants are omnivorous feeders, feeding on carbohydrates (e.g. honeydew, plant exudates, sugars, syrups), proteins (e.g. insects, meats), and lipids (e.g. grease, lard, oils from seeds). Their food preferences change depending on the nutritional requirements of the colony. In the spring and summer, when food is abundant, 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 during both the day and the night, 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. Large solids may be cut into smaller pieces so they can be carried back to the colony.
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 may 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 University of Florida, Institute of Food and Agricultural Sciences copyright this document.