A typical fire ant colony produces large mounds in open areas, and feeds mostly on young plants, seeds, and sometimes crickets. Fire ants often attack small animals and can kill them. Unlike many other ants, which bite and then spray acid on the wound, fire ants bite only to get a grip and then sting (from the abdomen) and inject a toxic alkaloid venom called solenopsin, a compound from the class of piperidines. For humans, this is a painful sting, a sensation similar to what one feels when burned by fire—hence the name fire ant—and the after effects of the sting can be deadly to sensitive individuals. The venom is both insecticidal and antibiotic.
Fire ants nest in the soil, often near moist areas, such as river banks, pond shores, watered lawns and highway shoulder. Usually, the nest will not be visible, as it will be built under objects such as timber, logs, rocks, or bricks. If there is no cover for nesting, dome-shaped mounds will be constructed, but these are usually only found in open spaces, such as fields, parks and lawns. These mounds can reach heights of 40 cm (15.7 in), and can also be as deep as a metre and a half (five feet). Colonies are founded by small groups of queens or single queens. Even if only one queen survives, within a month or so, the colony can expand to thousands of individuals. Some colonies may be polygynous (having multiple queens per nest).
Although most fire ant species do not bother people and are not invasive due to biological factors, Solenopsis invicta, known in the United States as the red imported fire ant (or RIFA) is an invasive pest in many areas of the world, notably the United States, Australia, the Philippines, China and Taiwan. The RIFA was accidentally introduced into the United States aboard a South American cargo ship that docked at the port of Mobile, Alabama, in the 1930s, and came to infest the majority of the Southern and Southwestern United States.
In the US the FDA estimates more than US $5 billion is spent annually on medical treatment, damage, and control in RIFA-infested areas. Furthermore, the ants cause approximately $750 million in damage annually to agricultural assets, including veterinarian bills and livestock loss, as well as crop loss. Over 40 million people live in RIFA-infested areas in the southeastern United States. Between 30 and 60% of the people living in fire ant-infested areas are stung each year. Since September 2004 Taiwan has been seriously affected by the red fire ant. The US, Taiwan and Australia all have ongoing national programs to control or eradicate the species, but, other than Australia, none have been especially effective. In Australia, an intensive program costing A $175 million had by February 2007 eradicated 99% of fire ants from the sole infestation occurring in south-east Queensland.
In just seventy years, according to a study published in 2009, lizards in parts of the United States had developed longer legs and new behaviors to escape the ants, which can kill the lizard in under a minute
The venom of fire ants is composed of alkaloids such as piperidine (see Solenopsis saevissima). The sting swells into a bump, which can cause much pain and irritation at times, especially when caused by several stings in the same place. The bump often forms into a white pustule, which can become infected if scratched, but if left alone will usually flatten within a few days. The pustules are obtrusive and uncomfortable while active and, if they become infected, can cause scarring. Some people are allergic to the venom, and as with many allergies, may experience anaphylaxis, which requires emergency treatment. An antihistamine or topical corticosteroids may help reduce the itching. First aid for fire ant bites includes external treatments and oral medicines.
External treatments: a topical steroid cream (hydrocortisone), or one containing aloe vera.
Oral medicines: antihistamines
Severe allergic reactions to fire ant stings, including severe chest pain, nausea, severe sweating, loss of breath, serious swelling, and slurred speech, can be fatal if not treated.