Red imported fire ant

red imported fire ants

The red imported fire ant (Solenopsis invicta), or simply RIFA, is one of over 280 species in the widespread genus Solenopsis. Although the red imported fire ant is native to South America, it has become a pest in the southern United States, Australia, Taiwan, Philippines, and the southernChinese province of Guangdong. There are also reports of ant hills in Macau, the former Portuguese enclave that borders the province of Guangdong. RIFA are known to have a strong, painful, and persistent irritating sting that often leaves a pustule on the skin.

In the 1930s, colonies were accidentally introduced into the United States through the seaport of Mobile, Alabama. They were first identified in the Mobile area by the then 13 year-old E.O. Wilson. Cargo ships from Brazil docking at Mobile unloaded goods infested with the ants. They have since spread from Alabama to almost every state of the American South, from Texas to Maryland. Since the 1990s, infestations have been reported in California in the West and New Mexico in the Southwest, but probably via ship or truck (rather than overland migration) in the case of California.

In a similar way, the ants were accidentally introduced into Australia in 2001.


RIFA are more aggressive than most native ant species and have a painful sting. A person typically encounters them by inadvertently stepping into one of their mounds, which causes the ants to swarm up the person's legs, attacking en masse. The ants respond to pheromones that are released by the first ant to attack. The ants then sting in concert, often inflicting death on smaller animals by overloading their immune systems[citation needed].

RIFA compete successfully against other ants, and have been enlarging their range, notably in the United States, where they have gradually spread north and west despite intense efforts to stop or eliminate them. Today they are found in most of the southern states: Alabama, Arkansas, Arizona,California, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, New Mexico, Oklahoma, South Carolina, Tennessee, Texas, and Virginia. The U.S. Department of Agriculture also lists the entire island of Puerto Rico as infested (USDA Quarantine Map). In addition, there are reports of RIFA in the San Francisco, California area.


RIFA are still on the move, often traveling from one area to another in turf, root balls of nursery plants, and other agricultural products. They are a pest, not only because of the physical pain they can inflict, but also because their mound-building activity can damage plant roots, lead to loss of crops, and interfere with mechanical cultivation. It is not uncommon for several fire ant mounds to appear suddenly in a suburban yard or a farmer's field, seemingly overnight. Their stings are rarely life-threatening to humans and other large animals, causing only 80 documented deaths as of 2006. However, they often kill smaller animals, such as birds. They sometimes kill newborn calves if the calves do not get on their feet quickly enough. The sting of the RIFA has venom composed of a necrotizing alkaloid, which causes both pain and the formation of white pustules that appear one day after the sting.

Red imported fire ants are extremely resilient and have adapted to contend with both flooding and drought conditions. If the ants sense increased water levels in their nests, they will come together and form a huge ball or raft that is able to float on the water, with the workers on the outside and the queen inside. Once the ball hits a tree or other stationary object, the ants swarm onto it and wait for the water levels to recede. To contend with drought conditions, their nest structure includes a network of underground foraging tunnels that extends down to the water table. Also, despite the fact that they do not hibernate during the winter, colonies can survive cold conditions as low as 16 °F (−9 °C).

At present, RIFA in the United States can be controlled, but not eradicated. A number of products are available, which can be used on a mound-by-mound basis to destroy ant colonies when they appear. With all such efforts, it is important to reach and kill the queens, which may be as far as six feet (2 m) underground; otherwise, some queens may simply move a short distance away and quickly re-establish the colony. However, during the past few years, even this technique resulted in failure to control colony growth. Scientists then discovered that an adaptation in RIFA had resulted in many populations having multiple queens in mounds over a large area.

RIFA were the first clearcut case discovered of a green-beard gene, by which natural selection can favor altruistic behavior.


See also general ant morphology

Red imported fire ants have both a pedicel and postpediole. In other words, they belong to a group of ants that have two humps between the thorax and abdomen. The workers have ten antennal segments terminating in a two-segmented club. It is often difficult to distinguish between the red imported fire ant Solenopsis invicta and some other species in the genus. A number of characters are used, but are not always consistent between the black imported fire ant (Solenopsis richteri) or hybrids between the two species. Positive identifications can be made using high performance liquid chromatography (HPLC) to distinguish differences in the cuticular hydrocarbons.

Ecological impact

The RIFA's introduction to the United States was in the late 1930s. Traveling in soil used as ballast on cargo ships, they came in through the seaport of Mobile, Alabama. They build mounds, usually no larger than 46 cm (18 in) in diameter and 46 cm (18 in) in height, although in Alabama some mounds have been reported to be over 60 cm high (2 ft) and larger, especially in fields where cattle graze. They build on soil close to homes and other buildings, and sometimes forage indoors for food and moisture. They are a nuisance and can threaten sleeping or bedridden individuals and pets. Occasionally they feed on vegetable plants in home gardens. The worst damage usually occurs during hot, dry weather when they invade flowerbeds while seeking warmth and moisture. If disturbed, fire ants bite and sting the intruder.

They are apparently attracted to electrical equipment and crawl into air conditioning units and the electrical wiring of stop lights, shorting them out. This is the leading cause of traffic light shorts in Texas, where the ants cause more than US$140 million in damage each year. Several ant species, including fire ants, have been shown to contain ferromagnetic nanoparticles that may contribute information about the geomagnetic field for orientation during foraging or migration. However, it has not been found that electric or magnetic fields attract the ants. Rather, when wandering ants cause electrical shorts, they attempt to sting the wire and produce powerfulsemiochemicals, including defensive and recruitment pheromones. The chemical signals draw additional ants to the short. The only effective protection is to bar ants from the equipment physically or with insecticides.

The FDA estimates that more than US$5 billion is spent annually on medical treatment, damage, and control in RIFA-infested areas. Further, the ants cause approximately US$750 million in damage to agricultural assets, including veterinary bills and livestock loss, as well as crop loss.

Fire ants are excellent natural predators and are biological controls for pests such as the sugarcane borer, the rice stink bug, the striped earwig, aphids, the boll weevil, the soybean looper, the cotton leafworm, the hornfly, and many other pests harmful to crops. However, they also kill beneficial pollinators such as ground-nesting bee species. Seeds, fruits, leaves, roots, bark, nectar, sap, fungi, and carrion are all fire ant prey, and they are not shy from creating their own carrion either. They are proficient enough at overwhelming intruders that they can virtually clear an area of invertebrates, lizards, and ground-dwelling birds.

Since September 2004, Taiwan has been seriously affected by the red fire ant. A few people are reported to have succumbed to venom from the ant stings. A large campaign to kill the ants has been partially effective, but it has not been able to eliminate all of them.

In China in January 2005, a controversy arose when it became known that Guangdong's provincial government had suppressed all information about the spread of fire ants in the province since the middle of 2004. Hong Kong newspapers, including AppledailyMingpaoHong Kong Economic TimesSingtao and Takungpao (the latter funded by the Chinese government), have also reported that the ants have been found in both Shenzhen and Wuchuan in Guangdong province.

According to a press briefing of the Agriculture, Fisheries and Conservation Department of Hong Kong, the city authorities have also located several ant-hills of Solenopsis invicta in an artificial wetland in Hong Kong's north-western section.

There have also been reports of colonies in Metro Manila and the Province of Cavite in the Philippines since July 2005, however since early 2007 they have spread now as far as the Bicol Region.

An outbreak of RIFA in Queensland, Australia, was discovered on 22 February 2001. It is believed that the ants were present in shipping containers arriving at the Port of Brisbane from the United States. Anecdotal evidence suggests fire ants may have been present in Australia for six to eight years prior to formal identification. While the outbreak is restricted to a small (800 km2) region of southeast Queensland in and around Brisbane, the potential social, economic, and ecological damage prompted the Australian government to respond rapidly. The initial emergency response was followed by the formation of the Fire Ant Control Centre in September 2001. Joint state and federal funding of A$175,000,000 was granted for a 6-year eradication program involving the employment of more than 600 staff and the broad-scale baiting of approximately 678.9 km2 between 8 and 12 times, followed by two years of surveillance. Following the completion of the fourth year of the eradication program, the Fire Ant Control Centre estimated eradication rates of greater than 99% from previously infested properties. The latest (May 8) Federal budget confirmed that the Program will receive extended Commonwealth funding of approximately $A10m for at least another two years until June 2009 to treat the residual infestations found most recently and to fund validation of the overall treatment and surveillance program. (see:) As in previous years the States have agreed in principle to match the Federal funding. That decision is set to be ratified in June 2007.

Counter measures

Red imported fire ants have virtually no natural biological control agents in the United States, China, Philippines, or Australia. Many scientists and agencies are attempting to develop methods to stop the spread of the RIFA.

Biological methods

Traditionally, control of RIFA has been achieved through pesticide use, but current research is introducing natural enemies of the ant. The microsporidian protozoan Thelohania solenopsaeand the fungus Beauveria bassiana are promising pathogens. Solenopsis daguerrei, a parasitic ant, invades RIFA colonies to replace the queen in hopes of gaining control of the colony. For this reason, its use as a biological control agent is also being explored.

Pseudacteon tricuspis and Pseudacteon curvatus are parasitoid phorid flies from South America which parasitize the ants. The female flies each lay an egg at the junction of head andthorax of their victims, prompting a jerky dance manoeuvre by the ants. The larva then slowly consumes the contents of the head, decapitating the ant in the process, and using theexoskeleton as a pupal case.

Phorid flies have been introduced in many places in southeastern United States, and are slowly reproducing and spreading to cover the entire RIFA range. The amount of actual damage done to the ants by phorid flies is minimal, but the ants appear to be aware of the hovering flies, losing their social organization and ceasing foraging. In addition, phorid flies are very species-specific, and should in theory leave native ant species (the fire ants' prime competitor) unmolested.

Scientists at the Agricultural Research Service also have been able to infect phorid flies with Kneallhazia solenopsae, a spore-producing insect pathogen, in order to control the population of red imported fire ants.  The flies are unharmed by the pathogen and serve as vectors in transmitting the disease to the ants. The pathogen is able to reduce red imported fire ant colonies from 53-100% and may serve as an effective biological control for the ants. 

A virus, SINV-1, has been found in about 20 percent of fire ant fields, where it appears to cause the slow death of infected colonies. It has proven to be self-sustaining and transmissible. Once introduced, it can eliminate a colony within three months. Researchers believe the virus has potential as a viable biopesticide to control fire ants.

In some cases, hastily adopted biological control agents can do more harm than good (such as the mosquitofish in Australia), and it remains to be seen how much success biological control of the red imported fire ant will have.

Physical methods

Researchers have also been experimenting with extreme temperature change to exterminate RIFA, such as injecting liquid nitrogen or pressurized steam into RIFA nests. Besides using hot steam, pouring boiling water into ant mounds has been found effective in exterminating their nests. Folk remedies have often sought a rapid increase in temperature by soaking the nest in gasoline or kerosene and lighting it on fire, but this is potentially dangerous and should not be attempted. Further, the burning of the nest is ineffective due the tendency of queens to be several feet underground. This confusion stems from the observation that fuel vapor has a near instantaneous lethal effect on the ants.

In Brisbane, Australia, colonies are being eradicated or effectively controlled by ground baiting with food laced with contraceptives (that render the colony's queen infertile) and toxicants. Mass baiting was undertaken following detection of the ants around the port of Brisbane and in south western Brisbane in 2001. Widespread public reporting of suspect colonies (by sending in samples of ants for identification) allowed mapping of the ant's locations. This was combined with satellite imagery to determine the vegetated habitats most likely to be infiltrated by the ants and the baits were targeted in these areas. Known infested areas were declared high-risk (Restricted Areas), and any material being moved from these areas which could harbour ants (soil, mulch, potted plants, potting mix, hay bales, construction machinery, etc.) had to be inspected prior to disposal or movement and bulk waste sent to transfer stations for examination, treatment and disposal. The infestation was initially thought to cover 270 km2 with a density of up to 600,000 colonies/km2 on highly infested sites. As program activity refined data on the infested area, overall size grew to around 80,000ha by 2006/7. At mid-2007 in the on-going nationally-funded eradication campaign, fewer than 100 active colonies were located in the entire South-East Queensland area during the 6 months between September 2006 and February 2007. The focus of delivering eradication has now switched largely to surveillance, while control and validation measures are expected to continue until 2009. The six year eradication campaign has cost A$175 million to date and has just secured funding in principle for a minimum of two more years.


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