Argentine Ants: The Supercolony Ant and Why Sprays Backfire
Argentine ants are small, dull brown invasive ants from South America whose nests cooperate instead of fighting, so a whole neighbourhood can be one colony with hundreds of queens. They do not sting and are a nuisance rather than a health threat. Spraying the trails kills only foragers, which the colony replaces; University of California entomologists recommend slow-acting sweet liquid baits placed outdoors instead.
If a trail of tiny brown ants appeared on your counter after rain and came back a week after you sprayed it, this is very likely your ant. Linepithema humile is the most common ant in and around California homes, according to UC IPM, and one of the commonest nuisance ants in the Southeast.
What do argentine ants look like?
Workers are small, all one size, and light to dark brown. The reliable features are a single node (bump) in the waist, an uneven thorax seen from the side, no sting, and at most a musty smell when crushed. Long, busy trails of workers of identical size, often along edges, pipes and wires, are typical.
| Argentine ant | Odorous house ant | Pavement ant | Pharaoh ant | Red imported fire ant | |
|---|---|---|---|---|---|
| Worker length | 1/16 to 3/16 in; sources differ | 1/8 in | 3/16 in | 1/16 in | 1/16 to 1/5 in, mixed sizes |
| Colour | Dull light to dark brown | Dark brown to shiny black | Dark brown to black | Yellow to orange | Reddish, darker abdomen |
| Waist nodes | One | One | Two | Two | Two |
| Usual nest | Outdoors in shallow nests | Shallow mounds in soil or debris, or wall voids | Lawns, under stones, mounds along sidewalks | Wall or cabinet voids, behind baseboards | Mounds with several openings in soil or lawns |
| Food | Sweets, sometimes protein | Sweets, sometimes protein | Sweets, protein, grease | Fats, protein, sweets | Sweets, protein, oils |
Compiled by The Pest Control Brief from UC IPM Pest Notes: Ants (Publication 7411), Clemson HGIC and University of Georgia Extension Circular 926.
The odorous house ant is the usual look-alike. Utah State’s tip is that the Argentine ant’s single node has a sharp peak, and Clemson notes the odorous house ant gives off a rotten coconut smell when crushed, while an Argentine ant may smell musty. Of the five, the red imported fire ant is the one known for a painful sting; Clemson says Argentine ants have no sting at all. Our ant identification guide walks through the node test with drawings for every common household species.
The size figures are a genuine disagreement in the extension literature. Clemson says workers are about 1/16 inch (1.6 mm), UC IPM 1/8 inch (3.2 mm) and the University of Georgia about 3/16 inch (4.8 mm). UC San Diego researchers described them as about 2 mm. Workers in one colony are all one size, so judge uniformity, not length.
What is an Argentine ant?
It is a South American ant, native to northern Argentina around the Paraná River, according to UC Riverside’s Center for Invasive Species Research. In the United States it was first detected in New Orleans in 1891 and was established in California by 1907. Clemson says it probably arrived on coffee ships from Brazil and Argentina. It is now established in at least 15 countries on six continents, mostly in Mediterranean climates or places with mild winters and moderate to high humidity.
Are argentine ants invasive?
Yes. UC Riverside’s Center for Invasive Species Research calls it a globally distributed invasive pest of urban, natural and farmed land. In the US they are now found throughout most of the southern states and California, with isolated infestations elsewhere, according to Clemson. Their spread has been mostly by people: a 2001 PNAS study found human-carried jumps averaged about a thousand times farther than colonies spread on their own (see the table below).
The supercolony, and why argentine ants are so successful
In most ant species, neighbouring colonies fight. Introduced Argentine ants mostly do not. Nests swap workers and queens freely, so what looks like hundreds of separate nests in a neighbourhood behaves as one colony. UC IPM describes the colonies as linked into one large supercolony with multiple queens, and the University of Georgia estimates a mature colony can hold a million or more workers and hundreds of queens.
| Study | Where | Finding |
|---|---|---|
| Tsutsui, Suarez, Holway and Case, PNAS 2000 | California and Argentina | Nest pairs 60 to 800 km apart in California showed no aggression; every distant pair in Argentina fought. A genetic bottleneck at introduction is linked to the loss of aggression. |
| Giraud, Pedersen and Keller, PNAS 2002 | Southern Europe | Two supercolonies; the main one ranges over 6,000 km from Italy to the Spanish Atlantic coast, “the largest cooperative unit ever recorded”. The two supercolonies are highly aggressive to each other. |
| Van Wilgenburg, Torres and Tsutsui, Evolutionary Applications 2010 | North America, Europe, Asia, Hawaii, New Zealand, Australia | Ants from these main supercolonies accepted each other as if one global colony, but attacked ants from South Africa and from smaller secondary colonies in Hawaii and California. |
| Suarez, Holway and Case, PNAS 2001 | United States | Budding spread averaged 150 m a year at most; human-carried jumps averaged about a thousand times farther. |
| Human and Gordon, Oecologia 1996 | 450-hectare reserve, northern California | Argentine ants found and recruited to bait more consistently and in higher numbers than native ants, and displaced them in 60 per cent of fights at shared bait. |
Compiled by The Pest Control Brief from the peer-reviewed papers named.
New colonies form by budding: newly mated queens leave on foot with a group of workers rather than flying off alone, as UC IPM and Clemson both describe. Clemson adds that in winter colonies may merge into one favourable nest and split again in spring or summer. Workers that do not fight their neighbours reach densities native ants cannot match; the University of Georgia has measured trails of more than 350 feet.
The “Argentine ant war”
Real fighting happens only at the rare boundaries between supercolonies. Aggression between Europe’s two was “invariably extremely high”, while Tsutsui’s team needed extensive searching to find California nests that would fight at all.
Damage to native wildlife and gardens
Argentine ants displace most native ants where they invade; in the Stanford reserve study the invaded area grew by 12 hectares in a year. The effect runs up the food chain. Coastal horned lizards in southern California feed on larger native ants, and a 2002 study in Conservation Biology across 21 sites in four counties found lizard presence and abundance tied to the absence of Argentine ants. Laboratory work by Suarez and Case found hatchlings could not keep growing on a diet of Argentine ants.
The ants also farm aphids, scales and mealybugs for honeydew and drive off their natural enemies. UC Riverside’s invasive species centre reports that parasitism of California red scale, a citrus pest, fell two to five-fold where Argentine ants were present. Ant-tended aphid colonies are covered in our guide to getting rid of aphids.
When they come indoors: our iNaturalist data
UC IPM and Utah State both say these ants invade buildings when it is too wet or too dry outside, and the University of Georgia describes them moving into wall voids for winter warmth. Our analysis of iNaturalist records shows a distinct seasonal shape.
Argentine ant records hold their highest share from November to February, peaking at 170 in February (yearly mean 100), and drop to 39 in June and July. All ants combined do the opposite, peaking in March (129) and bottoming out in July (77). Most of these records come from California, where the winter is the rainy season. That fits extension descriptions of ants moving indoors in wet weather, though iNaturalist records do not say whether an ant was photographed inside or out. UC IPM still advises baiting in late winter and early spring, while colonies are building up again.
California holds 10,689 of the 12,069 US records in our pull; elsewhere the counts are small, so treat other states as indicative only.
Why sprays backfire
A spray kills the ants it touches. With Argentine ants that is the wrong target: the foragers on a trail are a small fraction of the colony, and UC IPM notes as few as 1 per cent of foragers can feed the queens and larvae left in the nest. UC IPM Pest Notes spell out the consequences of perimeter spraying: it “won’t provide permanent control”, the ants “quickly build back up”, and barrier sprays sometimes make things worse by trapping ants indoors.
| Approach | What it does to Argentine ants | Verdict |
|---|---|---|
| Spraying trails indoors | Kills visible foragers; colony untouched | Temporary; soapy water removes the scent trail as well |
| Pyrethroid perimeter spray | Kills foragers at the foundation; can trap ants inside | Short-lived; restricted in California because of runoff |
| Granules with cyfluthrin or permethrin | Contact insecticide that looks like bait | UC IPM says avoid; does not control the colony |
| Prepackaged borate stations (about 5.4 per cent) | Kills foragers before they carry much back | Kill foragers in the home; less effective on major infestations |
| Liquid borate bait, 0.5 to 1 per cent, in refillable stations | Slow enough to be shared through the colony | UC IPM’s choice for severe infestations |
| Professional fipronil treatments | UC IPM lists it among the more effective professional materials | Licensed applicators only; runoff concerns apply |
Compiled by The Pest Control Brief from UC IPM Pest Notes: Ants (Publication 7411) and UC IPM’s Argentine ant baiting guidance.
The runoff problem is not hypothetical. University of California and state investigations found urban creeks can carry monitored pesticide concentrations up to three times those in agricultural runoff, and since July 2012 California’s Department of Pesticide Regulation has restricted outdoor pyrethroid applications by professionals: no applications during rain except under eaves, no band sprays to walkways, patios, doors or windows, and nothing into storm drains or curbside gutters. Our own analysis of fipronil use and California streams found structural pest control accounts for 99.7 per cent of reported fipronil use in the state, and fipronil was detected in 16.9 per cent of 2,489 California stream samples since 2015.
How to get rid of argentine ants?
Bait outdoors, clean up indoors, and be patient. UC IPM says this approach should manage most home invasions.
- Wipe up the trail with soapy water or a window cleaner to remove the scent trail, and find where the ants enter.
- Seal the entry point with caulk, and store sweet foods and pet food in sealed containers.
- Set liquid borate bait outdoors in refillable stations at 0.5 to 1 per cent borate in sugar water. UC IPM suggests at least one station on each side of the house, in shade where ants trail, or stations every 10 to 20 feet around the foundation. Sweet baits attract Argentine ants all year; protein baits mainly in spring.
- Do not spray while baiting, and keep stations topped up. UC IPM says it can take 5 to 10 days to see fewer ants and several weeks or more for full control.
- Fix the outdoor magnets: trim branches touching the house, keep mulch back from the foundation, repair leaky faucets and sprinklers, and manage aphid-infested shrubs.
- For ants in a potted plant, UC IPM’s method is to submerge the pot for 20 or more minutes in 1 to 2 tablespoons of insecticidal soap per quart of water.
UC IPM lists borax and boric acid among the borates in these baits; our guide to borax for ants covers concentrations. If ants return after a season of baiting, call a licensed professional: Utah State calls control difficult, and professionals have materials homeowners cannot buy. Ask for baits and spot treatments rather than monthly perimeter sprays, as UC IPM suggests; our ant control cost guide covers typical prices.
Argentine ants vs sugar ants and fire ants
“Sugar ant” is a nickname, not a species, and the Argentine ant is one of the ants most often given it; our sugar ants guide sorts out the others. Fire ants are a different matter: they sting, build mounds, and are reddish with workers of mixed sizes. UC IPM notes that in parts of California competition from Argentine ants has slowed the spread of the red imported fire ant, and UC San Diego researchers note that in much of the South fire ants limit Argentine ants in turn.
Frequently asked questions
Do argentine ants bite?
Rarely, and harmlessly. They have no sting. Clemson says they may bite when provoked, while UC Riverside’s invasive species centre says they do not sting or bite humans and are not a direct threat to people, unlike the red imported fire ant. UC IPM notes that most ants can bite with their jaws but rarely do. If you are being stung on the lawn, you are dealing with a different ant.
Are argentine ants dangerous?
Not to people directly. The harm is indirect: Clemson notes they also feed on garbage, sewage and dead animals, so food they have swarmed over may need to be thrown out. Outdoors they protect aphids and scale insects, displace native ants, and have been linked to the decline of coastal horned lizards in southern California.
Why do argentine ants keep coming back after I spray?
Because the spray only killed foragers. The queens, brood and most workers stay in shallow nests outdoors, often on a neighbour’s property too, and replace the losses. UC IPM says perimeter treatments may knock numbers down, but ants quickly build back up and invade again. Slow-acting liquid baits work because workers carry them back and share them with the queens.
Where do argentine ants nest?
Mostly outdoors in moist spots: under mulch, leaf litter, patio stones, potted plants, logs and slabs, often just below the soil surface, according to UC IPM and the University of Georgia. UC IPM notes they nest deeper in dry conditions. In winter they often move into wall voids, according to the University of Georgia, and Utah State lists wall voids, under carpets and damp spots near sinks, tubs and leaks as indoor nest sites.
When is the best time to bait argentine ants?
Late winter and early spring, according to UC IPM, when colonies are small and just beginning to grow; Utah State gives the same advice. The University of Georgia adds that baiting through the warm season prevents the large late-season populations that cause winter invasions. Bait whenever ants are trailing indoors, but expect the best results from early-season treatment.
Sources
- UC Statewide IPM Program, Pest Notes: Ants, UC ANR Publication 7411 (Rust and Choe).
- UC Statewide IPM Program, Key to Identifying Common Household Ants: Argentine ant baiting guidance.
- UC Statewide IPM Program, Green Bulletin Vol. 3 No. 1, New Restrictions on Pyrethroid Insecticide Applications Protect Urban Surface Waters, November 2012.
- UC Statewide IPM Program, In the News: Ants take the bait for less toxic solution (Greenberg, Klotz and Rust).
- University of California, Riverside, Center for Invasive Species Research: Argentine Ant (Daugherty and Hung).
- Clemson Cooperative Extension, Home and Garden Information Center: Argentine Ants.
- University of Georgia Extension, Circular 926, Argentine Ants (Suiter).
- Utah State University Extension, IPM Notes: Argentine Ant.
- Tsutsui, N. D., Suarez, A. V., Holway, D. A. and Case, T. J., Reduced genetic variation and the success of an invasive species, PNAS 97: 5948 to 5953, 2000.
- Giraud, T., Pedersen, J. S. and Keller, L., Evolution of supercolonies: the Argentine ants of southern Europe, PNAS 99: 6075 to 6079, 2002.
- Van Wilgenburg, E., Torres, C. W. and Tsutsui, N. D., The global expansion of a single ant supercolony, Evolutionary Applications 3: 136 to 143, 2010.
- Suarez, A. V., Holway, D. A. and Case, T. J., Patterns of spread in biological invasions dominated by long-distance jump dispersal: insights from Argentine ants, PNAS 98: 1095 to 1100, 2001.
- Human, K. G. and Gordon, D. M., Exploitation and interference competition between the invasive Argentine ant, Linepithema humile, and native ant species, Oecologia 105: 405 to 412, 1996.
- Fisher, R. N., Suarez, A. V. and Case, T. J., Spatial patterns in the abundance of the coastal horned lizard, Conservation Biology 16: 205 to 215, 2002; and Suarez, A. V. and Case, T. J., Bottom-up effects on persistence of a specialist predator: ant invasions and horned lizards, Ecological Applications 12: 291 to 298, 2002, as reported by UC San Diego, February 2002.
- Welzel, K. F. and Choe, D. H., Development of a pheromone-assisted baiting technique for Argentine ants, Journal of Economic Entomology 109: 1303 to 1309, 2016.
- Tay, J. W., Hoddle, M. S., Mulchandani, A. and Choe, D. H., Development of an alginate hydrogel to deliver aqueous bait for pest ant management, Pest Management Science 73: 2028 to 2038, 2017.
- The Pest Control Brief, California’s Fipronil Is 99.7 Percent Pest Control, and It Reaches Streams, October 2026.
- The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.
