Integrated Pest Management Manual
Fire ants are so called because their venom, injected by a stinger like a wasp's, creates a burning sensation. They are also active and aggressive, swarming over anyone or anything that disturbs their nest, be it wild animals, domestic animals, pets or people. An encounter with a fire ant nest can leave a lasting memory of burning pain, followed by tiny, itching pustules.
Because of this, and occasional stories of animals or people killed by multiple stings, people fear fire ants. In some areas infested with certain species of fire ants, playgrounds, parks, and picnic areas lie abandoned, unused because of the presence of fire ants. In campsites of state and national parks in fire ant infested areas, it is often difficult to put up or take down a tent without being stung by angry fire ants.
Fire ants are pests in other ways besides their stinging. They damage crops such as soybeans, eggplant, corn, okra, strawberries, and potatoes by feeding directly on the plants or by protecting other insects that damage the crops. They chew the bark and growing tips of citrus trees and feed on the fruit. Fire ant mounds interfere with farming and mowing operations and turn recreational fields into disfigured moonscapes. Fire ants have caused sections of roads to collapse by removing soil from under the asphalt.
Increasingly, fire ants have been found nesting in wall voids, around plumbing, and under carpeting in structures. The ants have also been found invading outdoor electrical equipment, apparently attracted to the electrical fields. Infested sites include household electric meters, traffic signal control boxes, and even airport runway lights.
Fire ants are voracious predators and sometimes feed on pests such as boll weevils, sugarcane borer, ticks, and cockroaches. The imported fire ant is thought to have dramatically reduced the range of the lone star tick, a serious livestock pest.
BIOLOGY AND IDENTIFICATION OF FIRE ANTS
Pest Species of Fire Ants
There are many species of fire ants in the United States, but the most serious pests for National Park Service personnel are four in the genus Solenopsis: the red imported fire ant, the black imported fire ant, the southern fire ant, and the fire ant. Distinguishing between imported and native species of fire ant is difficult, even for experts. Identification usually requires 40 or more randomly collected worker ants for study. The following sections describe the four fire ants of major concern.
Red Imported Fire Ant
Introduced from South America, this species becomes the number one fire ant pest wherever it occurs. The red imported fire ant (Solenopsis invicta) is associated with disturbed habitats, mostly created by humans, and is abundant in old fields, pastures, lawns, roadsides and many other open sunny areas. It often inhabits fields used for agricultural purposes where its large above-ground mounds create problems in planting and harvesting crops. In areas where grass is periodically cut, mounds are flush with the ground and are hard to see. This species is rarely found in mature forests and other areas with heavy shade, unless part of the area has been disturbed by fire or storms.
The red imported fire ant builds mounds that are, on average, 10"-24" in diameter and 18" high. But larger mounds are not uncommon. They also may extend 6' underground. The primary function of mounds, beyond that of the simple ground nests of other ants, is microclimate regulation--controlling the temperature and humidity. The ants can maintain a temperature inside the mound much higher than that outside, allowing them to continue colony growth during cool weather.
The mounds are symmetrical piles of excavated soil, rich in organic materials, laced with interconnected galleries and chambers. The soil below ground also contains galleries and chambers. During foraging periods only a small percentage of ants may be inside the mound; the rest are out gathering food and exploring.
A newly established nest rapidly produces young, and winged reproductives are produced for most of the year (8-10 months), much longer than native species. Red imported fire ants quickly spread through a suitable habitat, and the species is now found throughout most of the southeastern United States and west into Texas.
Black Imported Fire Ant
The black imported fire ant, Solenopsis richteri, is very similar to the red imported fire ant. It is currently limited to a small area of northern Mississippi and Alabama. It may be displaced from established habitats by the red fire ant.
Scientists have long thought that the black and red fire ants were two distinct species. Recently it has been discovered that hybrids of these ants produce viable offspring, and some scientists now wonder whether they are simply two races of the same species, varying in color and perhaps behavior.
Southern Fire Ant
The southern fire ant, Solenopsis xyloni, is a native species that occurs from North Carolina south to northern Florida, along the Gulf Coast and west to California. Colonies may be observed as mounds or more commonly may be constructed under the cover of stones, boards, and other objects or at the base of plants. These ants also nest in wood or the masonry of houses, especially around heat sources such as fireplaces. Nests often consist of loose soil with many craters scattered over 2 to 4 square feet. In dry areas nests may be along streams, arroyos, and other shaded locations where soil moisture is high. Southern fire ants usually swarm in late spring or summer.
The fire ant, Solenopsis geminata, is a native species sometimes called the tropical fire ant. This ant ranges from South Carolina to Florida and west to Texas. Very similar to the southern fire ant, it usually nests in mounds constructed around clumps of vegetation, but may also nest under objects or in rotting wood.
The Ant Colony and Life Cycle
The life cycles of the four fire ant species discussed above are very similar.
Development of the individual: Like all ants, an individual fire ant begins life as an egg, which hatches into a legless, grub-like larva. The larva is very soft and whitish in color. It is also helpless and depends totally on worker ants for food and care. The larva is specialized for feeding and growing, and almost all growth occurs during this period. As in all insects, growth is accomplished by periodic molting, or shedding of the cuticle (skin). Having reached its final size, the larva becomes a pupa in which various adult structures, such as legs, and in some cases wings, become apparent for the first time. The pupal stage is the transitional stage between the larva and the adult that emerges during the final molt. In insects in general, the adult stage is specialized for reproduction and dispersal; with ants, some adult individuals are capable of reproduction (queens and kings) and the remainder are sterile workers.
The colony: The social unit of fire ants is the colony. Colonies, like individuals, pass through a characteristic life cycle.
Fire ants are very typical of ants in general. In addition to workers and a queen, mature colonies contain males and females capable of flight and reproduction. These individuals are generally called "reproductives." On a warm day, usually one or two days following a rain, the workers open holes in the nest through which the reproductives exit for a mating flight. Mating takes place 300' to 800' in the air. Mated females descend to the ground, break off their wings, and search for a place to dig the founding nest, a vertical tunnel 2" to 5" deep. They seal themselves off in this founding nest to lay eggs and to rear their first brood of workers. During this period they do not feed, instead utilizing reserves stored in their bodies. The first worker brood takes about a month to develop; these are the smallest individuals in the entire colony cycle. They open the nest, begin to forage for food, rear more workers, and care for the queens. Hereafter, the queen or queens essentially become egg-laying machines, each able to lay up to 1,500 eggs per day.
Multiple queen colonies are fairly common. A single colony may have 10 to 100 or more queens, each reproducing. Multiple queen colonies can mean up to 10 times more mounds per acre. The queens generally mate several times and may live for several years. Workers are less long-lived and usually will not survive an entire season.
The colony grows rapidly by the production of workers that gradually enlarge the original vertical tunnel into multiple passages and chambers. Colony maturity is attained when reproductives are once again produced. The reproductives leave to mate and form new colonies. A mature colony of red imported fire ants can produce as many as 4,500 reproductives during the year in 6-10 mating flights between spring and fall. Nearly 100,000 queens may be produced per acre in heavily infested land, but mortality rates, mostly from predators, can reach 99%.
Colony size: Colonies of red and black imported fire ants become territorial as they grow; they defend an area against all other fire ants. Therefore, fire ant colony populations often reach an upper limit depending on the territory size of mature colonies. A typical figure for pasture land seems to be about 20-50 mounds per acre in single queen nests and up to 250 mounds or more in multiple queen nests. Mature colonies of imported red fire ants consist of an average 80,000 workers, but colonies of up to 240,000 and more have been reported.
The oldest and most expendable 20% or so of the colony's workers leave the nest to search for food. They explore 50-100 feet from the nest with an efficient looping pattern. Although the worker ants can chew and cut with their mandibles, they can only swallow liquids. When they encounter liquid food in the field, they swallow it and carry it back to the nest. Solid food is cut to reasonable size and carried back to the nest.
Like other ants, fire ant workers share their food with their nest mates by regurgitating it so that it can be licked or sucked by other ants. In this way, most ants in the nest get fed equally. This food sharing is also why baits can be an effective control tactic against fire ants.
Fire Ant Stings
In infested areas, fire ant stings occur more frequently than bee, wasp, hornet, and yellowjacket stings. Stepping on a mound is almost unavoidable when walking in heavily infested areas. Furthermore, many mounds are not easily seen, with many lateral tunnels extending several feet away from the mound just beneath the soil surface. Ants defend these tunnels as part of their mound.
A person who stands on a mound or one of its tunnels, or who leans against a fencepost included in the defended area, can have hundreds of ants rush out to attack. Typically, the ants can be swarming on a person for 10 or more seconds before they grab the skin with their mandibles, double over their abdomens, and inject their stingers.
Although a single fire ant sting hurts less than a bee or wasp sting, the effect of multiple stings is impressive. Multiple stings are common, not only because hundreds of ants may have attacked, but because individual ants can administer several stings. Each sting usually results in the formation of a pustule within 6 to 24 hours. The majority of stings are uncomplicated, but secondary infections may occur if the pustule is broken, and scars may last for several months. Severe infections requiring skin grafting or amputation have been known to occur.
Some people experience a generalized allergic reaction to a fire ant sting. The reaction can include hives, swelling, nausea, vomiting, and shock. People exhibiting these symptoms after being stung by fire ants should get medical attention immediately. Death can occur in hypersensitive people. Individuals who are allergic to fire ant toxins may require desensitization therapy.
Fear of Fire Ants
An important indirect effect of the presence of fire ants is fear of being stung. Fear and anxiety about fire ants may limit the use of sites where fire ants are present by park visitors and personnel alike. In some parks, playgrounds, athletic fields, and campsites are not used because of fear of fire ants in the area.
MONITORING AND THRESHOLDS FOR FIRE ANTS
The first step is to identify the species of fire ants in the area (see Pest Species of Fire Ants above). Population monitoring for fire ant control generally consists of determining the number of active mounds in a particular unit area. Any mound where at least three ants are observed after mound disturbance should be considered active. Heavily infested fields can contain over 100 active mounds per acre.
Another method of estimating ant populations for comparison studies is by collecting ants attracted at baits in a test area. A small piece of hamburger and a small piece of agar containing 40% honey are each placed on a small piece of aluminum foil or in a small plastic cup. The two baits are placed on the ground at each bait station, 1'-3' apart, at each bait station. Bait stations are placed about 10 yards apart. The number of ants attracted to the baits per unit time is determined.
Threshold/Action Population Levels
The threshold population levels for fire ants will vary according to the species and the sites. In certain camping and recreational areas, for example, very few active mounds per acre would likely be tolerated, particularly of the imported species. In contrast, a few active mounds per acre probably would be acceptable in other types of sites; little-used hiking areas, for example. Every effort should be made to correlate fire ant populations observed through the use of monitoring techniques with complaints received from park visitors and personnel. In this way, a complaint threshold level can be established for each park site.
In areas where fire ants are not causing any problems, the best solution may be to do nothing. Some sites will only support a limited number of fire ants. These may be in the form of a few large colonies or many small ones. Established mounds defend territories, preventing the establishment of new colonies. Maintaining several large, and perhaps well-marked, colonies may be a sound way to stabilize fire ant populations in an area, as long as there is a low risk of people or pets stumbling into the nest.
Some researchers believe it may be best to selectively control fire ant colonies- -allowing native species to flourish as a way to prevent the introduction of the imported species, or leaving single queen imports alone to prevent the area from invasion by a multiple-queen "supercolony."
Mounds built by fire ants in fields often interfere with mowing and farming operations. Not only is equipment damaged by dried and hardened fire ant mounds, but operators may refuse to enter fields infested by ants. The number of mounds per acre that can be tolerated as regards equipment damage must be determined on a case- by-case basis.
NON-CHEMICAL CONTROL OF FIRE ANTS
Fire ants, particularly red and black imported fire ants, pose a serious dilemma in parks. On the one hand, there can be no doubt that the fire ant is a major pest, stinging visitors and park workers, disfiguring the landscape, even attacking native animals. In one private preserve, imported fire ants were killing hatchlings of the brown pelican, a threatened species. On the other hand, aggressive insecticide treatment of critical habitat can have negative impact on a sensitive environment.
Fire ant management consists of a series of questions and decisions: What species are in the area? How extensive is the infestation? How high is the risk that visitors or park personnel will be stung? How much damage are the ants doing? Is control action justified? What are the best strategies of control? Answering these questions requires inspection and monitoring to determine the nature and extent of the problem.
Boiling water has been added to individual mounds with varying degrees of success reported. Approximately 3 gallons of hot water poured into each mound will eliminate about 60% of the mounds treated. Surviving mounds will need to be treated again. Water has also been applied as steam, using a steam generator, usually on a cool day. Both techniques are cumbersome in the field, especially where large numbers of mounds are involved.
Area-wide flooding or prescribed burning of fire ant infested areas has proved ineffective, and may promote the establishment of new colonies.
Mounds can be dug up and moved or destroyed, but not without some risk that the fire ants will successfully attack the digger. Dragging, or knocking down, mounds may provide a limited level of control, but only if mounds are dragged just before the first hard freeze. Mounds are destroyed by pulling a steel I-beam drag, weighing about a ton, behind a tractor across the ant-infested area. Destroying mounds during the warm season will not reduce the number of active mounds; ants quickly rebuild their nests.
A number of mechanical mound pulverizers, ant electrocuters, even nest exploders, have been developed for fire ant control, but so far the effectiveness and practicality of these devices is open to question.
A number of biological enemies of the fire ants have been evaluated as biocontrol agents, including nematodes, bacteria, fungi, viruses, and microsporidia. Some show promise, but biological control is not yet a proven effective control tactic for fire ants.
So far, the most effective of these is a nematode, Neoaplectana carpocapsae. In trials, one application has inactivated about 80% of treated mounds in 90 days. The straw itch mite, Pyemotes tritici, has also been shown to inactivate fire ant mounds. Three to ten applications at about two week intervals gave 70% control. Practical use of this mite for fire ant control must await the development of more efficient methods of mass production and increased effectiveness. Another problem is that this mite is a pest of people and animals; it bites and causes a dermatitis.
Fire ants, like other ants, may be nesting near buildings and can enter and move through a structure through innumerable tiny cracks and openings. Caulking and otherwise sealing cracks and crevices being used by fire ants can often have great effect in suppressing the population inside. Many effective, easy-to-use silicon sealers and expandable caulk products have been recently developed, including some designed specifically for pest management.
The most effective measure for preventing injury to park visitors and personnel is education. Visitor activities should be directed away from highly infested areas. Park visitors should be informed about the habits of fire ants, how to recognize them, and how to avoid them. Visitors should be encouraged to use proper sanitation so that fire ants are not attracted to such sites as picnic areas. And if the worst happens, information should be available on what to do if a person is stung.
CHEMICAL CONTROL OF FIRE ANTS
Many different types of chemical products are available for fire ant control. There are three major ways to manage fire ants with chemicals: treating individual mounds, broadcast treatment of a large area, and spot treatment in and around structures. Remember to consult with your IPM coordinator for specific pesticide recommendations for your area.
Treating individual ant mounds is time consuming, but it is generally the most effective method of control. It takes from a few hours to a few weeks to "kill" the mound, depending on the product used. Individual mound treatment is usually most effective in the spring. The key is to locate and treat all the mounds in the area to be protected, not always a simple task. If many young mounds are missed, reinfestation of the area can take place in less than a year. The following discussions describe different ways to treat individual mounds.
Mound drench. Follow directions for dilution of the insecticide and gently wet the mound and surrounding area with insecticide. Then break open the top of the mound and pour the insecticide dilution directly into the galleries.
Mound drenches are most effective after rains when the ground is wet and the ants have moved up into the drier soil in the mound. During excessively dry weather, effectiveness of the treatment may be enhanced if you soak the soil around the mound with plain water before you treat.
A few granular insecticides are labeled for application to fire ant mounds. After application, the granules are watered into the mound.
Mound injection. A growing number of insecticide products are designed to be injected directly into fire ant mounds. They may be injected using a "termite rig" with a soil injector tip, a standard 1-3 gallon compressed air sprayer with a fire ant injector tip, or a special aerosol soil injector system. The mound is injected in a circular pattern, usually at 3 to 10 points. A new product combines insecticide treatment with high temperature vapors to increase penetration in the mound.
Baits. A few bait products are available that may be used for individual mound treatments. The baits take from several days to several weeks to eliminate a fire ant colony, but they can be very effective and are simple to use. Baits are available with the either a toxicant, a sterilant/toxicant, or a growth regulator. The baits are sprinkled around and sometimes on the mounds. During hot weather it is best to apply the bait late in the afternoon or early in the evening when the ants begin to forage. Baits must be kept dry.
Dusts. A few insecticide dusts are labeled for dusting individual fire ant mounds. The dusts are evenly distributed over the top of the mound. Dusts must be dry in order to be effective.
Fumigation. Large fire ant mounds can be eliminated through fumigation. Check with your IPM coordinator to see if these products are registered for use in your area. Only those who have been specifically trained in the use of fumigants should conduct such fumigations.
Several different types of products are labeled for application over wide areas to control fire ants. Granular insecticides are often applied with hand-operated fertilizer spreaders or agricultural application equipment. Sprays also are sometimes used. Because of the broad spectrum of such treatments and their effects on nontarget species, broadcast application of standard insecticides is not a good choice for park land.
A better choice is broadcast treatment with an insect growth regulator bait, which poses much less risks to nontarget species. For example, fenoxycarb bait has been shown to be very effective for suppression of fire ant populations when applied in one application over a wide area.
Spot Treatment with Insecticides
If fire ants are nesting in a structure (in a wall void, for example), the nest should be treated directly, usually by drilling and injecting with a residual insecticide. Treatment of ant trails or barrier treatment to keep fire ants from foraging in occupied areas are generally not acceptable choices for Park Service facilities.
Summary of Management Recommendations
Park visitors and personnel should be directed away from infested areas and encouraged to observe proper sanitation procedures so that fire ants are not attracted to recreational sites.
Mechanical and other nonchemical control measures should be considered first if control is deemed necessary. Remember that control may not be necessary in many cases. When it is necessary, chemical control, particularly the use of baits, may be attempted if other control measures have failed and the use of pesticides has been approved by the National Park Service.
(Sources of Additional Information:)
1. Conniff, R. 1990. You never know what the fire ant is going to do next. Smithsonian. July 1990.
2. Hamman, P. J., Drees, B. M., and S. B. Vinson. 1986. Fire ants and their control. Texas Agricultural Extension Service Publication B-1536.
3. Mallis, A. 1990. Handbook of Pest Control (7th ed.). Franzak and AMP; Foster Co. Cleveland, OH.
4. Silva, J. M. 1990. Turning up the heat on fire ants. Agricultural Research, July 1990. U.S. Department of Agriculture.
5. Vinson, S. B., and A. A. Sorensen. 1986. Imported Fire Ants: Life History and Impact. Texas Department of Agriculture. Department of Entomology, Texas A and AMP; M University. College Station, TX.