Fire Ant Research

Female phorid flies are attracted to fire ants swarming over a disturbed mound or foraging along a trail to food. They hover over ants looking for a preferred individual. (Each phorid species has a particular size range of fire ant workers which it prefers.) When the target is chosen, the phorid darts in, injects an egg into the ant's body, and gets away at high speed. The attack takes a fraction of a second and leaves the ant disoriented before she staggers off to join her sisters!

The injected egg develops in the ant's thorax until after about two days when the larva moves into the head and consumes the jaw muscles and other head tissues. After about 10 days, the ant dies and its head falls off when the larva pupates in the safety of the hard chitinous head capsule shell. Adult flies emerge from pupae about 30 to 45 days after the original attack. This process of killing individual ant workers is termed the direct effect of mortality Estimates of the number of infected ants in a single colony seldom exceed 3 or 4%, and is usually less than 1%, so direct mortality only has a low impact on colony growth.

Another thing that phorids "do" to ants is more significant from the standpoint of biocontrol. As phorids fly above ants looking for victims, the ants respond by hiding, pilling on top of one another, retreating into the nest, and posturing in various odd ways. This fly harassment disrupts the economy of provisioning the nest with food and protecting home and territory. Native ant species can then take advantage of the RIFA's distraction and reclaim lost territory. This more indirect and subtle effect has been identified as the mechanism by which phorids might reduce the impact of fire ants. The idea was posed originally by a UT graduate student, Don Feener, in the late 1970s when he observed phorids that attacked one of our native ants at Brackenridge Field Laboratory.

Unfortunately, the answer is never. Imported fire ants are now permanent residents in the U.S. Eradication is possible from pesticides only temporarily and only at a local scale, but not at a regional level. However, we can hope for eradication of the pest status of the ant, assuming that we can find and successfully introduce effective biological control agents. Even assuming the best results with phorids, it could take years to reverse seven decades of spread and growth of RIFA populations. Although we expect phorids to be detrimental to populations of the RIFA, don't overreact to the media hype about phorids and expect an overnight solution.

The phorid flies that attack fire ants are tiny. Compare the end of Lincoln's nose and top of his lip on a U.S. penny to the length of this Pseudacteon phorid which attacks S. invicta. Only fire ants and a few dedicated biologists are likely to see phorids in action. They are not attracted to people as are some small flies, like gnats and mosquitoes.

The species of phorid flies that attack fire ants are specialists; many even attack only single species of fire ant. For example, our native Texas fire ants have 8 species of their own phorid flies, and they do not attack the imported fire ant - that's why we have to bring the specific phorid flies that attack the imported fire ant from the countries from which the ants were imported.

There are over 20,000 species of phorid flies. Most phorid flies are scavengers and some utilize corpses and are useful in forensics (the so-called coffin flies). Phorids that show up in houses typically breed in the sludge in sink drains but could be coming up from animal remains under the house (e.g. dead rats). Phorids that parasitize ants are a highly specialized minority that do nothing except attack and consume ants. The vast majority of people will never knowingly see one of these inconspicuous creatures.

Phorid flies of the genus Pseudacteon permitted for release have been through careful screening to identify those species most specific to the imported pest fire ant. Prior to the decision to release target-specific phorids, USDA APHIS produced an Environmental Assessment. This document contains a review of the biological qualities in fire ant-attacking phorids and assesses the potential benefits vs costs of releasing these exotic insects in North America. Aside from a small amount of nectar-feeding by adults, Pseudacteon flies are completely dependent on ants. Indeed, most species of flies under consideration for release as biocontrol agents of the RIFA are restricted to a single species or species group of fire ants. For example, 8 native phorid species attack our native fire ants in Texas (S. geminata and S. xyloni). The fact that in over seven decades, no switch by native phorids to the invading S. invicta has occurred is strong evidence of how extremely host-specific these flies are.

Studies of flies, genus Pseudacteon, that live, reproduce, and thrive on host fire ants in North America can reveal much about details of phorid natural history. Understanding the biology of these temperate-zone phorids could provide important guidance for developing optimum methods for introducing phorids from South America which would attack the pest fire ant species. A strain of our native species (S. geminata) the Tropical Fire Ant is an invasive pest in many other countries. Lessons we learn from studies in Texas may be applicable in areas where the Tropical Fire Ant is invasive.

Look at the ants. Fire ants include many opportunistic ant species of the genus Solenopsis. There are native and imported species. Native Texas fire ants are very similar to the imported pest, but actually help impede the spread of the imported species and should be spared if possible.

Both native and imported fire ants are small, dark orange/brown ants with workers of various sizes that quickly mobilize and sting en mass when their mound is disturbed . Other than the much larger "red harvester" or "Texas red ant" which has no variation in worker size and is conspicuous on its trails and around it's flat open mound entrance, most other stinging, ground-dwelling ants in Texas are encountered as solitary individuals.

Look at the nests. Fire ants live and do most of their foraging for food through underground tunnels. A nest consists of a network of tunnels and chambers that occupy a vertical column 12-18" in diameter and approximately 36" deep. After cool, rainy, weather in Spring and Fall, the ants clear blocked tunnels and expand chambers to create a conspicuous mound of loose soil above the nest. The colony dwells in this above ground extension when the temperature there is optimal for brood development. Though above-ground mounds harden and persist in some soil types, their absence does not mean fire ants are not present or receding.

You probably have imported fire ants if the following characteristics of the ants and their mounds are observed:

• Mounds of loose soil, resembling gopher diggings, are found above ground.
• Mounds are generally numerous and conspicuous.
• Worker ants are small, highly variable in size, aggressive, and sting relentlessly.
• Workers are generally dark with the gaster (abdomen) even darker than the rest of the body. Native fire ants are often uniformly reddish brown.Invasive, Large Worker
• Workers have the same body proportions from the tiniest to the largest.
• Head width of invasives never exceeds the abdomen width, even in the largest workers, while head widths of the larger native workers are wider than the abdomen.

Because not all fire ants are the pest species, distinguishing native from imported is an important first step before proceeding with chemical treatment. 

Along the Mexican border of Texas and in West Texas, there exist native fire ants (S. xyloni) that are difficult to distinguish from their imported cousins. However, if you can see four tiny teeth on the mandible or jaw of a fire ant, it is S. invicta. Native species possess three teeth.

Over East, Central and much of South Texas, the most common native fire ant, S. geminata, can be distinguished without examination of tiny details of anatomy. First disturb a mound by digging, then watch for the largest workers. If the heads of the largest workers are conspicuously wider than the gaster (abdominal segments), you are looking at the native species.

No, you have described the Texas red harvester ant, also known as the Texas red ant or "pogo" (genus Pogonomyrmex). These industrious ants collect grass seeds and store them in underground granaries. In fact, they have been shown to reduce rodent numbers by competing with them for grass seeds. Pogos are eaten by "horny toads" and are so conspicuous that people usually notice them before getting stung. Fire ants tend to eliminate this native Texan.

First of all, several kinds of fire ants are native to Texas. When people ask this question, they really mean imported fire ant species. Apparently the introductions of pest fire ants were accidental. Perhaps the soil of potted plants or ballast on ships arriving from South America to Mobile, Alabama contained invicta nests. Exactly when is not certain. There were invasions by two pest fire ant species. The first, the black imported fire ant from Argentina (Solenopsis richteri), was barely established and spreading when the red imported fire ant (Solenopsis invicta) arrived and proceeded to shove aside its cousin (which now survives in Mississippi and western Georgia). The original arrivals were probably in the 1930's or before. Professor E.O. Wilson, the famous ant biologist at Harvard, was first to discover the invasion while he was still a budding high school entomologist in Alabama. See the most recent USDA distribution map by county and time of arrival.

If you live in the northern U.S. where the ground freezes and you think you might have fire ants, check with a nearby university with a department of entomology. Though fire ants probably will not survive winters in northern states, these ants may have been accidentally introduced in potted plants and set up temporary residence during warmer months. Fire ants will locate mounds near heat sources (steam pipes, concrete walls, etc.) and conceivably could survive in colder areas than we expect.

They have recently been accidentally exported to Queensland, Australia and are following a spread similar to that encountered in the U.S.

Both biological wisdom and recent research indicate that fire ants, like other opportunistic organisms, become more "weedlike" and achieve pest status when introduced into regions free of their natural biological enemies, such as parasites and pathogens and where the native ant community does not have strong competitors that can withstand fire ants. A case in point is the fire ant species native to Texas. Our native fire ant species is a is a minor nuisance at home, but a major pest in India where it was inadvertently introduced. Likewise, the pest fire ant from Brazil is "just another ant" there and not a serious pest. Still, the Brazilian name for the ant, "lava pé" (translated "wash feet") reflects its status as a nuisance to those not watching where they stand.

Fire ants of the species S. invicta and its closely-related species of ants are not generally viewed as important pests in Brazil and Argentina. Occasional local outbreaks, which could result from temporary escape from normal controls due to habitat disruptions, have been reported in recently settled towns in the Amazon region. It is safe to say that fire ants native to South America are no more pests there than are native fire ants pests in North America.

First, Texas possesses extensive open grassland and pasture habitats favored by Solenopsis invicta. But also, in Texas we commonly encounter the multi-queen variety of fire ant which may have up to 100 reproductive queens per mound. Multiple-queen or polygyne S. invicta do not display the territorial conflict that promotes spacing between colonies in the single queen or monogyne form found in South America and in the SE United States. In Texas therefore, mound densities can be much higher than in regions dominated by the monogyne form.

Fire ant brood (immature workers) are sensitive to temperature and humidity. Worker ants move the brood up high when it is wet. That's why you see tall mounds after rains. When conditions are dry, they move the brood deeper to more humid chambers and you may see no mounds at all. Mounds can extend as much four feet below the surface.

Here is a thumbnail outline:

    1. egg laid by queen
    2. larva hatches and grows through 4 larval developmental stages or instars between which molts of larval skin occur
    3. at 4th molt a pupa is produced
    4. pupa hatched into adult ant.

There are two basic types of eggs.

    1. unfertilized eggs — become males with wings whose only function is to mate with queens
    2. fertilized eggs — become females which are either
            a. winged virgin queens or
            b. various castes of sterile workers.

How the colony feeds and cares for female larvae determines their caste; i.e., whether they behave as workers (all are sterile females) or queens. Male ants develop from unfertilized eggs and therefore possess only one set of chromosomes; i.e. they are haploid. Thus male ants have no father (but they have a grandfather). Females develop from fertilized eggs and are typical diploids. This type of life cycle occurs in other so-called eusocial insects including wasps, bees, and ants and is called "haplo-diploidy." Eusocial insects possess sterile castes that help queens by raising other siblings. Why some individuals give up the option to reproduce has been an interesting evolutionary dilemma since the time of Darwin but the work of people like the late W.D. Hamilton has largely answered the question.

Fire ants reproduce opportunistically when conditions are wet and warm. They are found in all types of soil, but they do better in open pastures and sunny, grassy places than in thick shaded woods. Grassy medians of freeway and mowed pipeline and powerline rights of ways provide prime "freeways" for the ants too.

Polygyne colonies (those with multiple queens/mound) can reproduce by budding off new colonies and spread by walking a few meters per year. Colony establishment by winged queens can occur miles beyond source populations. This mode of spread may be promoted by prevailing winds and is the only way that monogyne or single queen colonies reproduce. Judging from the spread across Texas, natural dispersal was on the order of 10-20 miles/year. Of course transport in nursery products spread the ants beyond the boundary of natural dispersal. Though fire ants may arrive in the NE U.S. and Canada via nursery products, nests in RVs, cars, vans, etc., they are not likely to become a problem because of the cold conditions in fall spring and winter.

When fire ants die, workers remove the bodies and body parts from nests. Nest hygiene is a key to disease prevention in social insects such as bees and ants. Trash piles, called middens, accumulate in underground chambers during weather that inhibits above-ground activity, and are then moved to the surface after spring and summer rains when ants rebuild galleries and clean house. Midden piles might increase in size and conspicuousness after a colony has had a territorial dispute with another colony of ants, after application of pesticides, or when a colony is experiencing higher rates of disease or parasitism.

This is another question on the minds of people who, as kids, enjoyed watching these great lizards and now lament their absence. The answer is either "probably yes" or "definitely no" depending on where you are. Horned lizards have definitely disappeared over most of the range now dominated by the red imported fire ant (RIFA). There might be other species of lizards or snakes that have suffered due to the presence of the RIFA, but the loss of the Texas horny toad has certainly been the most conspicuous one. In any case, the disappearance of horny toads in RIFA areas is probably due partly to the direct killing of the young lizard by foraging fire ants and partly to the fact that horny toad food, such as Texas harvester ants, have been decimated by the RIFA. However, horned lizards have also disappeared in many areas outside the range of RIFA. It is probable that the RIFA could still be at least indirectly responsible for the horned lizard demise even where the RIFA does not occur. For instance, the publicity about the RIFA has encouraged overuse of and proliferation of ant baits and poisons, killing all species of native ants they touch as well as the RIFA. Many people erroneously assume that the Texas red ant is the imported fire ant and unnecessarily treat their properties. Aside from thereby causing the demise of horned lizard, they are also removing dozens of native, friendly ants and increasing the opportunity for invasion by RIFA into a vulnerable system with lowered resistance. Agricultural activities such as shredding and other habitat destruction that prevent weeds from producing seed also can rob the Texas red harvester ant of its food source. No seeds = no ants = no lizards.

You might have seen pictures of massed fire ants killing hatchling quail chicks or stinging the noses and eyes of newborn deer. Without question RIFA can kill young birds, small mammals, and reptiles. However, in the case of quail, it could be that a more important impact on populations is the removal of insect food that would normally be available to quail chicks. Because quail populations fluctuate widely due to climatic changes, it is hard to pinpoint the extent to which RIFA might trigger population-wide collapse. Certainly if climatic conditions are unfavorable, the depredations by fire ants could become a major factor in affecting the survival of certain species of wildlife. In some cases, the loss of quail has followed a change in land management, where habitat quality has decreased and at the same time fire ants have increased.

Fire ants probably don't kill plants but they can diminish their health and vigor by tending and protecting aphids and mealy bugs on stems and roots. Fire ants feed on "honey dew" and other secretions of sap-sucking insects, and in doing so they may protect these pest insects against attack by native predators.

According to a study at Texas A&M University, fire ants cost the economy of Texas over $1 billion per year, while the overall cost to the US economy may amount to $6-8 billion.

One of the greatest economic impacts and dangers posed by RIFA results from short circuits and fires in electrical systems after fire ants move into circuit breakers, relays, motors, and other electrical devices. Why the ants are drawn to them is still something of a mystery, possibly the warmth and condensation or the soil disturbance along cable routes.

Being aware of your surroundings at all times is the way to avoid everything from car wrecks to rattlesnake bites. If fire ants do crawl onto your skin, they first bite with their mandibles in order to anchor for the thrust of the sting. As soon as you feel this pinching sensation, quickly sweep the ants off before they actually sting and you can avoid most of the damage! If you must work in proximity to fire ants, wear rubber boots and gloves powdered with talcum powder.

Though there have been no controlled trials of IFA immunotherapy, a reasonable support for the current clinical approach is Dr. Ted M. Freeman's "IFA Immunotherapy: Effectiveness of Whole Body Extracts", Journal of Allergy and Clinical Immunology, Vol 90 pgs 2150 ff, 1992. Conducted at the Wilford Hall Medical Center (Air Force hospital) allergy clinic in San Antonio, the study reports that 11 patients with IFA sensitivity refused immunotherapy. Six of the 11 received subsequent field stings and all six suffered anaphylaxis. However in the more than 60 people who had received immunotherapy, about 47 suffered field stings and only one reported a mild anaphylaxis. The p value for immunotherapy providing protection from subsequent sting reaction was <0.0001. Though not perfect, the results suggest that immunization can protect against anaphylaxis. During sting challenges of some 30 patients on maintenance doses of immunotherapy, only one person reacted to an IFA sting. He believes that pure venom would make a much better research tool than WBE (whole body extract). Unfortunately there is no commercial product available and extraction of venom to do medical research is tedious.

Pustules may result the day after being stung. Each pustule represents a separate sting.

There are two components of imported fire ant (IFA) venom: proteins and alkaloids. The immune response, especially IgE (an antibody, "immunoglobulin E"), is generally to the protein component of foreign substances - 4 in Solenopsis invicta (the red IFA) and 3 in Solenopsis richteri (the black IFA). There is evidence that people who are sensitive to IFA have IgE to the venom proteins. It is felt that the alkaloids are responsible for the local cell necrosis that produces the fairly typical and nearly pathognomonic (i.e., distinctively characteristic of a particular disease) "pseudo-pustule" of the IFA sting. This is "pseudo" because a true pustule is composed of an active neutrophilic infiltration fighting an infection. In the case of the IFA reactions, the infiltrate is just dead cells and there is no infection.

Fire ants bite and then sting! One feels a prickly sensation as the ants bite the skin's surface with their mandibles to get a grip. The bite does not inject venom, rather this gives them a solid anchor at the head end, while they jab the stinger (a hypodermic-like device modified from the ovipositor, the egg laying structure) into the victim, injecting a toxic alkaloid venom from a special gland and at the same time releasing an alarm pheromone (chemical signal) that excites additional attackers. Fire ants do not inject formic acid, as in the case in many ants. (Not all ants sting, but fire ants do and it is the venom injected by the sting that people react to. Some other ants that don't sting do bite and then squirt venom onto the area of the bite.)

Note: strychnine, nicotine, morphine, and caffeine are also alkaloids.

Immediately after being stung, wash off the area with alcohol, try not to scratch it so it doesn't get infected . Sometimes a white pustule will form the second day, but it will eventually be resorbed. Commercial preparations such as StingEze or MuscleRub, etc will numb the area for a while. A thick paste of baking soda and water can also help right after the sting. Careful application of ice will help decrease pain, but can burn the skin if left on too long. Meat tenderizer can also burn the skin. If the pustule becomes infected, apply an antibiotic and see your doctor. OTC Benedryl may help with local reactions: burning and itching. Follow label instructions carefully.

If other reactions occur soon after the stings, i.e., difficulty breathing, itchy rash, loss of consciousness, etc., get the person to an emergency room immediately. About 1% of the population have the potential for serious and dangerous reaction to fire ants. A physician can prescribe an EpiPen (single dose epinephrine auto injector device) to carry with you in case of subsequent ant stings and anaphylactic reaction.

If you still have native ants, keep doing what you have been doing (or not doing). If you see an invading fire ant mound in an otherwise RIFA-free area, verify that it is not the native species before attempting to kill it. If you do treat such a RIFA mound, be sure to confine treatment to that mound or its immediate surroundings.

First, put small pieces of hot dog as bait around the yard. Visually match up ants that come to bait with fire ants that you see by disturbing the mounds. If you have an area dominated by ants other than fire ants, avoid treating that. If you do have fire ants, either use one of the chemical products described below, or try a hot water treatment. To treat with hot water, open the top of the mound to expose the inner chambers, and then empty a five gallon bucket of very hot water into the mound. This will directly kill many workers and hopefully also the queens. However it is not possible to kill all the workers immediately and the treatment should be repeated after several weeks once the colony has regrouped.

An even more effective hot-water treatment utilizes a high-pressure water cleaner, with hot (160F) water. Use a long tubular nozzle inserted well into the fire ant mound to inject the water into the area with the queens and brood.

If you have a large yard with a high infection rate, then you could try using Amdro, Award, Logic or similar granule bait preparations. These don't kill instantly but give the workers a chance to take the bait back to the mound as food where its pesticides disrupt reproduction by hormonal control over queen ants. Fire ants forage out of underground tunnels that lead all around within 100' of a mound. Therefore use a broadcast spreader to evenly distribute the bait over your yard.

Pick a mild day on which you first determine that the ants will swarm a piece of hot dog. That means they will efficiently harvest the bait. Broadcast these granules all over the infested area on a nice day so that the fire ants get all of the bait. The worker ants will take the granules into the mound. Be patient because these baits take about 6 weeks to take effect; the mound will die. You should have control for many months and additional spot applications of the granule baits when you see small mounds restarting should keep things tolerable (or with just a mound or two, boiling water poured on the mound when it comes up after a rain is very effective).

If you coordinate with neighbors and use the same treatment area-wide on the same week, you will reduce the rate of re-invasion. We find that native ants increase after such treatments and that's good because they serve useful functions including helping to resist fire ant invasion. Imported fire ants are often the worst where native ants have been disrupted by soil disturbances that accompany home and road construction, or exterminated by broad spectrum pesticides.

Here's the suggestion: 3/4 lb. hydramethylnon in baited granules (under trade names "Amdro" or "Siege") mixed with 3/4 lb s-methoprene in baited granules (under trade name "Extinguish") broadcast applied per acre. A report published on trials with this mix is on the web: Amdro/Siege, a metabolic inhibitor, takes 3-6 weeks after ants consume it to show an effect and the effect lasts for several months until a re-invasion occurs. Extinguish is a growth regulator that takes longer to show an impact, but then can last a year or more. Since these products are not instantly toxic, workers can distribute each of them throughout the colony long before effects set in. While these compounds or breakdown products definitely would not be good for frogs or fish, if application occurs during a period when no run-off rains are anticipated, all of the active material will be taken into fire ant mounds within 30 minutes. Persistence in the environment is relatively short for both.

The answers to this question change frequently. Consult the TAMU extension service web site at http://fireant.tamu.edu/.

If you can avoid pets and kids, you could consider making these same baits listed above available in the house as well. Don't broadcast widely indoors but put baits in corners, under appliances and in closets. Indoors, I've also used boric acid (15% by volume) in peanut butter placed in bottle lids where ants have trails. Boric acid works in similar fashion, killing slowly after distribution among colony members (it also kills cockroaches when mixed with cornmeal and sugar). It will take about 2 weeks to completely control them, but is very effective. Treating the inside alone will never work since ants killed there are quickly replaced by a large population outdoors.

Other ants compete for food with fire ants and help keep them under control. If you kill all ants in your yard, you create an "ant vacuum" and after the next rain, it will be fire ant queens that land in your "safe" yard to begin new mounds unopposed by any other ants. Fire ants are better at colonizing and dominating newly disturbed habitat than the average ant species.

If native ants are not harming you but simply sharing their habitat with you, I would suggest leaving them alone. Often when one removes a native element of the ecosystem, something much worse fills the void. In Texas, our worst fire ant problems are in areas where people blasted the native system with pesticides and made invasion by introduced pests more likely.

Gasoline is as dangerous to the applicator as it is to a fire ant colony. That's a common, but inefficient and ill-advised, approach. Gasoline would be a very expensive, environmentally harmful method of killing fire ants.

Carpenter ants (genus Camponotus) are termite predators and move into old termite galleries or into wood softened by fungus. As a result they are accused, perhaps unjustly, of creating serious structural damage. We view them as a symptom rather than a cause. Fire ants will eat both Camponotus larvae and termites, but not wood. Fire ants also transport dirt into walls, circuit boxes, etc. but do not damage wood structures directly.

Many observant folks notice that armadillos dig into fire ant mounds and correctly assume that they are eating the ants. Armadillos also dig into mounds in droughts when more favored foods are hard to find. Actually they do this when the colony inhabits the uppermost parts of the mound, the portion of the approximate 3 ft deep nest that offers the best temperature for developing brood. Developing brood is what armadillos eat. So yes, armadillos probably do have an effect on fire ants in Texas as well as in in Brazil, where they are also found. Research should be carried out to assess the effect predation by armadillos has by comparing fire ant densities in areas with and without them. However, if armadillos alone could stem the tide of RIFA, these ants would not have become a problem. Actually, since some phorid flies attack fire ants when mounds are disturbed, armadillos and phorid flies could be a winning combination. Until we examine that idea more carefully, we should all avoid running over these little fire ant munchers.

After a phorid fly has laid an egg into the thorax of a fire ant worker, the egg develops into a larva (maggot), which feeds on the body fluids and squeezes its way into the head space of the ant where it continues to feed. About two weeks after the egg has been laid, the infected ant leaves the colony, in what has been termed a "zombified" stage. The ant wanders outside the nest, apparently orienting to a moist area such as leaf litter which is likely to be a good microhabitat for pupation. Next the fly larva releases an enzyme to decapitate the ant, then it finishes eating the head contents before pupating in the protective head capsule. After about two more weeks, the emerging fly crawls from the head capsule through the mouth space of the ant head.

The initial study describing this "zombie" phenomenon was completed by researchers at Louisiana State University (Henne, D.C. and Johnson, S.J. 2007. Zombie fire ant workers: behavior controlled by decapitating fly parasites. Insectes Sociaux, Vol 54(2) 150-153.