Red imported fire ant
The red imported fire ant is an invasive South American ant, established in the southern US since around the 1930s, that builds mounds in sunny lawns, pastures and verges and stings in numbers when disturbed. Texas A&M AgriLife recommends the Two-Step Method: broadcast a fire ant bait over the whole yard in spring and autumn, then treat individual mounds only where they are a hazard. Baits are slow but reach the queen. Grits, club soda and most other home remedies failed when tested. Phorid flies released through research and USDA programmes now attack fire ants across much of the South, but they are not a product you order, and they do not reduce ant numbers much on their own.
Red imported fire ant (Solenopsis invicta) workers on the surface of a disturbed lawn mound. Photo: pending.
Identification
Workers are small: about 1/8 to 1/4 inch long by one Texas A&M page, 1.5–4 mm by another. They are reddish brown with a darker abdomen. The mix of sizes in one colony is the tell. Other ants' workers mostly look alike; fire ant workers range from tiny to large. Workers from multiple-queen colonies are smaller on average than workers from single-queen colonies.
The mound is the easiest thing to recognise. It is a dome of loose, worked soil. In undisturbed pasture it can reach 18 inches (45 cm), depending on the soil, but in mown turf it is usually only a few inches tall. There is no opening in the centre, as most ant mounds have. The ants come and go through underground foraging tunnels that open some distance away. New mounds often appear a few days after heavy rain. Break into one and you will see cream-coloured brood (eggs, larvae and pupae) among the ants. Texas also has native fire ants, including the tropical fire ant (S. geminata) and S. xyloni (called the desert fire ant in one Texas A&M sheet, the southern fire ant in another), and these sting too. The white pustule is the reliable sign of the imported species.
Easily confused with
- Other mound-building antsHarmlessFire ant mounds have no central opening, the workers vary in size, and they swarm up grass stems when disturbed. Many other ants are all one size.
- Red harvester antAlso a pestHarvester ants are larger (1/4–1/2 in) with square heads, and often clear a bare circle 3–6 ft across around one central nest opening.
Damage symptoms
Fire ants do little to the grass itself. The harm is the mounds, which get in the way of mowing and play, and above all the stings. Workers defend the nest fast. When a mound is disturbed they climb anything vertical, legs included, and can begin stinging within 10 to 20 seconds. Each ant grips with its jaws and can sting several times, and a person who stands on a mound may be stung by hundreds of ants at once. If ants are on you, rub them off briskly by hand or with a cloth. They grip with their jaws, so running water will not wash them off. Try not to scratch the pustules, because broken skin can become infected. Estimates of severe allergic reactions (anaphylaxis) vary, from fewer than 1% to about 6% of people stung. Texas A&M says anyone with chest pain, nausea, severe sweating, loss of breath, serious swelling or slurred speech after a sting needs emergency medical care straight away. For anything else, including how to treat stings, ask a doctor or pharmacist. This page does not give medical advice.
- Domed mounds of loose soil in sunny lawns, verges and bed edges, with no entry hole on top
- New mounds springing up a few days after heavy rain
- Ants swarming up grass blades and shoes within seconds of a mound being disturbed
- A burning sting, then a white fluid-filled pustule at the site within a day or so
- Foragers gathering at pet bowls, bins and spilled sugary drinks
Lifecycle & degree days
A colony has one queen or many. The average colony holds 100,000 to 500,000 workers and up to several hundred winged males and queens. Single-queen colonies usually build 40 to 150 mounds per acre. Multiple-queen colonies can build 200 or more mounds, with 40 million ants, per acre. Winged males and queens fly to mate, most often in spring and autumn, usually in the afternoon shortly after rain. Each mated queen then starts a new colony. The species arrived from South America around the 1930s, reached Texas in the 1950s, and now covers the eastern two-thirds of the state.
- Egg to adult
- About 22–35 days (7–10 egg, 6–10 larva, 9–15 pupa)
- Colony size
- 100,000–500,000 workers
- Mounds per acre
- 40–150 (single queen); 200+ (multiple queen)
- Mating flights
- Mostly spring and autumn, after rain
- Queen lifespan
- 2–5 years, or 7 years or more (sources differ)
Host crops
Fire ants do not feed on the grass. They nest in open, sunny ground, which in a garden means the lawn, bed edges and the strip beside paths and drives. They also forage wherever there is food and water, and colonies can move into wall voids and buildings in very hot, dry spells or after flooding. According to Texas A&M, the red imported fire ant infests more than 260 million acres across nine southern states, from Texas and Oklahoma east to Florida and South Carolina. That figure is undated, and the federal quarantine area has since grown.
Monitoring & action thresholds
Texas A&M gives no mound count that triggers treatment in a home lawn; the decision is about where people and pets are. Its Two-Step page says the method works best in fully infested areas (five or more mounds per quarter-acre) or where native ants are not a concern. Its organic-methods sheet says it is not suggested for previously untreated areas with many native ants and few fire ant mounds (15 to 20 mounds per acre or fewer), because broadcast bait also kills the native ants that compete with fire ants.
Walk the lawn a few days after heavy rain, when new mounds show up, and note any in play areas, by doors and around pet bowls. Check for foraging before you bait. Texas A&M advises baiting when workers are actively looking for food, usually in the late afternoon or evening. To test, put a small pile of bait next to a mound and see whether the ants find it within 30 minutes. Baits are slow, and control of 80 to 90% takes weeks to months.
Biological control
There is no biological control a gardener can buy that works across a yard. Mites and parasitic nematodes have been sold as single-mound treatments, but they are costly and any effect stays in the treated colony. Phorid flies (Pseudacteon spp.) are tiny parasitic flies from South America, about 1/16 inch long. A female lays an egg inside a worker ant. The maggot feeds inside the ant for about three weeks, and then the ant's head falls off. Six species have been released and established in the US, in part through a cooperative rearing and release programme that USDA-APHIS began in 2001. By 2008, P. tricuspis covered about half of the fire ant quarantine area. The flies were spread by these release programmes; no source we checked lists them for sale. They kill only about 1–3% of workers, and their main effect is to make foraging ants hide, which gives native ants more of the food. Texas A&M notes that even in South America, where these flies and other natural enemies are present, fire ants remain abundant in disturbed land. A strain of the fungus Beauveria bassiana infects fire ants, but no commercial fire ant product had reached the market when Texas A&M last reviewed it. Native ants, which compete with fire ants and kill young queens, are the natural enemy a garden can protect.
Cultural controls
The Two-Step Method from Texas A&M AgriLife aims to keep individual mound treatments to a minimum. Step 1 treats the whole yard with bait; Step 2 treats only the mounds you cannot live with.
- Step 1: broadcast a fire ant bait over the whole yard with a hand-held seed spreader. Texas A&M gives late August to mid-October; Harris County AgriLife gives late April–May and late September–October for the Houston area
- Use fresh bait from an unopened container, on dry grass, with no rain expected for 24–48 hours, in the late afternoon when ants are foraging
- Do not mix bait with fertiliser or seed
- Step 2: treat individual problem mounds by play areas, doors and paths with a labelled mound drench, granule, bait or dust
- For a low-chemical Step 2: wait at least 3 days after baiting, then pour about 3 gallons of very hot water on the mound. This kills the colony about 60% of the time, and it can scald the grass and plants it touches
- Skip grits, club soda, baking soda, cinnamon, coffee grounds and molasses. Tested, none controlled fire ant mounds
- Never use petrol (gasoline) or other fuels. They are dangerous, kill the surrounding plants and pollute soil and groundwater
- Shovelling one mound onto another does not make the colonies fight. Colonies that are regularly knocked about may move, but they are not eliminated
Chemical, last resort
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
Texas A&M AgriLife, Texas Imported Fire Ant Research & Management Project, Two-Step Method (fireant.tamu.edu)
Texas A&M AgriLife, fire ant Biology, Identification and FAQ pages (fireant.tamu.edu)
Brown, Natural, Organic, and Alternative Methods for Imported Fire Ant Management, AgriLife ENTO-009 (2014)
Drees & Puckett, Potential Biological Control Agents for the Red Imported Fire Ant, AgriLife ENTO-008 (2014)
Drees, Medical Problems and Treatment Considerations for the Red Imported Fire Ant, FAPFS023 (2002)
Callcott et al., Fire ant decapitating fly cooperative release programs (1994–2008), J. Insect Sci. 2011
Harris County AgriLife Extension, Red Imported Fire Ants
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →