Red imported fire ant
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| Solenopsis invicta Buren, 1972 |
The red imported fire ant (Solenopsis invicta), or simply RIFA, is one of over 280 members of the widespread genus Solenopsis. Although the red imported fire ant is native to South America, it is best known in the United States, Australia, Taiwan, Philippines, and the southern Chinese province of Guangdong. In January 2005, several ant-hills belonging to fire ants were found in northern Hong Kong. Later, after a thorough search for the ant was conducted there, several hundred ant-hills were found in different parts of Hong Kong. There were also reports of ant hills in Macau, the former Portuguese enclave that borders the province of Guangdong. They have recently been introduced in the Philippines in July 2005 when a cargo plane coming from Texas via California arrived in Manila.
Colonies were accidentally introduced into the United States in the 1930s through the seaport of Mobile, Alabama. Cargo ships from Brazil docking at Mobile unloaded goods infested with the ants; they have since spread from Alabama to the coastal plain and piedmont of almost all of the southeastern states, as well as into California. The ants were accidentally introduced into Australia in 2001, in a similar way.
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[edit] Overview
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 swarm and immediately sting when any movement is sensed.
RIFA are efficient competitors to other ants, and have been successful at enlarging their range, notably in the United States, where they have gradually spread north and west despite intense efforts to stop them. Today they are found in most of the southeastern states, including Texas, Oklahoma, Louisiana, Arkansas, Mississippi, Alabama, Tennessee, Georgia, Florida, North Carolina, South Carolina, Maryland, and Virginia. It is not uncommon for several fire ant mounds to appear suddenly in a suburban yard or a farmer's field, seemingly overnight. Even in the San Francisco, California area, there are large numbers of red imported fire ants.
(At least one community uses the presence of fire ants as a publicity opportunity: Marshall, Texas hosts an annual fire ant festival.)
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 because their mound-building activity can damage plant roots and lead to loss of crops. Their stings are rarely life-threatening to humans and other large animals, causing only 80 documented deaths as of 2006. 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 which appear one day after the sting.
Red imported fire ants are extremely resilient and have adaptations 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 and tunnels that extend 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.
[edit] Body structure
Fire ants are symmetrical along the body, meaning that the left part is a mirror image of the right. They have hardened exoskeletons, a tough integuments for support and protection. They breathe through tracheae, like other terrestrial insects.
Their bodies, like their relatives' (the bees and wasps), are divided into three major parts: head, mesosoma, and metasoma. They have three pairs of legs, and a pair of elbowed antennae. As in other ants, the worker and soldier ants are sterile females. The queen is responsible for laying thousands of eggs. The number of males is low because they are only produced during a brief period of the year when mating swarms are emerging.
Contrary to what most people believe, a fire ant does not just “bite.” They use their jaws to anchor themselves and then, like a bee or wasp, inject venom using a sting at the tip of their abdomen. Using their strong mandibles to attach themselves to the victim’s flesh, they can sting many times continuously. They are extremely aggressive and have the ability to deliver venom in unison.
[edit] Life cycle
The life cycle starts when the queen mates with a male. The sexual male and female workers have wings which allow new colonies to disperse. The sexuals (known as alates) fly into the air, sometimes as high as 100 feet (30 m) in a large cloud and mate on the wing. After inseminating the female with sperm, the male dies. The female then searches for a suitable nest site, drops her wings, and begins laying eggs. Until her first batch of workers (known as minums) are able to forage for her, the queen survives on internal fat stores and energy gained from the breakdown of her now useless flight muscles. This first group of approximately 10 eggs takes around 10 days to hatch. It is approximately 20 more days before the larvae become worker ants. The first worker ants are very small in size. Gradually, the size and number of workers increases as the queen is fed with more nutrients.
[edit] Spread
Since their accidental importation from South America into North America in the 1930s, the RIFA population has increased its territorial range to include most of the Southern United States. They build mounds, no larger than 46 cm (18 in) in diameter and 46 cm (18 in) in height, 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 somehow attracted to electrical fields[citation needed] and crawl into air conditioning units and the electrical wiring of stoplights, 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.
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 vet bills and livestock loss as well as crop loss.<ref>McDonald, Maggie (February 2006). "Reds Under Your Feet (interview with Robert Vander Meer)". NewScientist 189 (2538): 50. </ref>
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 Appledaily, Mingpao, Hong Kong Economic Times, Singtao 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 northwestern section.
There has also been reports of ant-hills being reported in Metro Manila and the Province of Cavite in the Philippines since July 2005.
An outbreak of RIFA in Queensland was discovered on 22 February 2001. It is believed that the ants were present in shipping containers arriving to 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 region of southeast Queensland near 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 estimates eradication rates of greater than 99% from previously infested properties.
[edit] Countermeasures
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.
Traditionally, control of RIFA has been achieved through pesticide use, but current research is introducing natural enemies of the ant. The microsporidian protozoan Thelohania solenopsae and the fungus Beauveria bassiana are promising pathogens. Pseudacteon tricuspis and Pseudacteon curvatus are parasitoid phorid flies from South America (more below). Solenopsis daguerri (Santschi), 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.
A few species of phorid flies, found in the ants' native habitat in South America, parasitize the ants. The female flies each lay an egg at the junction of head and thorax of their victims, prompting a jerky dance maneuver by the ants. The larva then slowly consumes the contents of the head, decapitating the ant in the process, and using the exoskeleton 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 by phorid flies is minimal, but the ants appear to be aware of the hovering flies, losing their social organization and ceasing foraging, thus causing much greater damage in the long run. In addition, phorid flies are very species-specific, and should in theory leave native ant species (the fire ants' prime competitor) unmolested.
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 that the biological control of the red imported fire ant will have.
Researchers have also been experimenting with extreme temperature change to exterminate RIFA, such as injecting liquid nitrogen or pressurized steam into RIFA nests.
[edit] See also
[edit] References
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