Fire Ant Sting Identification: The White Pustule

If a red sting site raises a white, fluid-filled pustule a day or two later, Texas A&M's Imported Fire Ant Research and Management Project says that is characteristic and that only fire ant venom causes this symptom. The sting itself produces burning and itching first; the pustule follows in a day or two.

That makes fire ants unusual among biting and stinging insects, most of which leave marks that cannot be told apart. The CDC, for comparison, describes bed bug bites as resembling mosquito or flea bites. Here there is a specific, delayed, visible sign.

Texas A&M also notes the practical downside of that sign: the stings are not usually life threatening, but they are easily infected and may leave permanent scars, so the pustules should not be broken open.

Texas A&M describes what happens when a mound is disturbed: dozens to hundreds of reddish-brown worker ants crawl up vertical surfaces, such as grasses and other objects, on and around the mound. The same source notes few native ants charge up vertical surfaces like this.

A shoe or an ankle standing next to the mound is one of those vertical surfaces. That is why a single misstep produces a ring or cluster of stings rather than one, and why the marks are usually on the feet, ankles and lower legs of someone who was working in a yard.

If you did not see ants, a cluster of stings on the ankles is not proof; compare with flea bites, which also concentrate low on the leg but are described as small, red, hardened welts rather than a white pustule.

Texas A&M describes the mound as fluffy worked soil with no opening in the centre, unlike most ant mounds — the ants enter and leave through underground tunnels. In pastures a mound can reach 18 inches tall; in turfgrass it is usually only a few inches high.

The workers are small, about 1/8 to 1/4 inch long and reddish-brown, and the source points out something easy to check: variation in size within a single colony is itself a distinguishing characteristic of imported fire ants, because many other ant species are uniform in size.

Do not kick or probe a mound to test it. Photograph a few workers on a plain surface from a safe distance and the app identifies them.

Texas A&M's advice is to treat stings with an insect bite remedy containing benzocaine or other ingredients that deaden pain and protect against infection, and notes that some people become sensitive to fire ant stings and should seek the advice of an allergist.

The American College of Allergy, Asthma and Immunology includes fire ants alongside honeybees, hornets, wasps and yellow jackets among the insects that cause sting allergies. It describes a large local reaction as swelling that extends beyond the sting site — a sting on the forearm making the whole arm swell — which usually peaks two to three days after the sting and can last a week or more, and is not life threatening.

What is life threatening is a systemic reaction. Both sources point to the same signs and the same response: get emergency help.

Texas A&M says to seek immediate emergency medical attention if a sting causes chest pains, nausea, severe sweating, loss of breath, serious swelling or slurred speech. Its fire ant FAQ adds excessive swelling, itching or redness, shortness of breath, thickening of the tongue and sweating to the list of reasons to seek medical attention.

The American College of Allergy, Asthma and Immunology describes the serious reactions as hives, itching and swelling in areas other than the sting site, tightness in the chest and difficulty breathing, a hoarse voice or swelling of the tongue or throat, dizziness or a sharp drop in blood pressure, and loss of consciousness or cardiac arrest. Injectable epinephrine is often prescribed as emergency rescue medication for people at risk, who should carry it at all times and seek medical attention immediately after using it.

This page reports published guidance and is not a diagnosis.

You identify fire ants by three things together: workers of mixed sizes in one mound, a reddish-brown body with a darker rear, and a mound of loose soil with no opening on top. Texas A&M describes them as medium-sized red and black ants whose workers range from 1/16 inch to almost 1/4 inch; Wikipedia gives 2.4 to 6.0 mm, red and somewhat yellowish with a brown or black rear section (gaster). The University of Tennessee notes that one colony has small, medium and large workers.

With a hand lens, the University of Tennessee lists two confirming features: antennae with ten segments ending in a two-segmented club, and a waist of two segments.

• Mound: dome-shaped in undisturbed ground and lacking a central opening (University of Tennessee); Texas A&M says mounds are rarely more than 18 inches across.
• Behaviour: when a mound is disturbed, hundreds of workers pour out and run up vertical surfaces to sting (University of Tennessee).

Yes. In Tennessee, the University of Tennessee lists the red imported fire ant (*Solenopsis invicta*), the black imported fire ant (*Solenopsis richteri*) and a hybrid of the two. Texas A&M adds the native tropical fire ant (*Solenopsis geminata*), which looks almost identical except for a few larger workers with large, square, two-lobed heads.

Colonies also come in two forms: single-queen (monogyne) and multiple-queen (polygyne), which differ in colony size, how densely mounds are packed and how workers behave (Texas A&M).

The red imported fire ant comes from central South America. Wikipedia records that it reached Mobile, Alabama, by cargo ship between 1933 and 1945 and was first found in Queensland, Australia, in 2001; it is also established in parts of China, Taiwan, Hong Kong and the Caribbean, and has been eradicated from New Zealand.

Texas A&M's fire ant FAQ lists the usual mix-ups and the difference for each:

• Native fire ants: nearly identical; only a few larger workers with big, square, two-lobed heads give the tropical fire ant away.
• Termites (swarmers): the front pair of wings on an ant is larger than the hind pair, while a termite's four wings are about the same size. See termites.
• Carpenter ants: usually larger than most house-infesting ants, and they tunnel into wood. See carpenter ant identification.
• Harvester ants: much larger, and they clear a large bare area around a single entrance hole.
• Leafcutter ants: also much larger, with a dense cluster of mounds at the colony centre called a "town".

The behaviour test also helps: few native ants charge up vertical surfaces the way fire ants do when a mound is disturbed.

Texas A&M says imported fire ants should generally be controlled because of their aggressive behaviour. Texas A&M also notes that not every ant mound needs treating: native ants in mounds at the base of established trees are probably not harming the tree and may help by eating other insects. That is why identification comes first.

If you are unsure which ant you have, the University of Tennessee advises contacting your county Extension office for identification. Do not stand on or break open a mound to look.

In their native South America fire ants have several, and some have been brought to the US. Texas A&M says three species of phorid flies, which parasitise the ants, have been released and established in parts of Texas and are spreading on their own, but even in South America the flies affect only about 3% of the workers in a colony. Other biological controls it lists are predatory mites, parasitic nematodes and the fungus *Beauveria bassiana*.

Wikipedia lists general predators of the red imported fire ant: spiders and other arachnids, birds, and insects including other ants, dragonflies, earwigs and beetles.

Research and drafting for this page used AI assistance. Every figure, date and quotation was then checked line by line against the sources listed at the bottom of the page, each of which was opened while writing. Nothing here is a diagnosis. If a bite, sting or contact with an insect is followed by trouble breathing, swelling of the face or throat, spreading redness, fever or any symptom that worries you, get medical care; in the United States Poison Help (1-800-222-1222) is free, confidential and open 24 hours a day.