Linepithema humile (Mayr, 1868)

Argentine ant

A small brown ant that forms massive supercolonies, dominant in California homes.

Order: Hymenoptera
Family: Formicidae
Group: Ants
Ontario status: Not established in Ontario, dominant household ant in California and the Gulf Coast US

Why it matters

One of the world's worst invasive species, displaces native ants across entire landscapes and disrupts ecosystems. In homes it's a relentless kitchen invader; in agriculture it protects honeydew pests.

Origin

Native to the Paraná River basin of South America (Argentina, Paraguay); shipped worldwide in soil and plants, now on six continents.

Behavior

Forms foraging highways, dense trails of thousands of workers. Unlike most ants, neighboring nests don't fight; they merge into supercolonies spanning entire regions.

Living conditions

Nests in shallow soil, under stones, in mulch, wall voids and potted plants. Loves moisture and honeydew, tends aphids and scale insects for their sugary secretions.

Signs of presence

Dense, fast-moving trails along baseboards and pipes
Ants tending aphids on garden plants
Nests under stones, in mulch, in potted plants
No fighting between ants from different nests, all one colony

Weaknesses

Not established in Ontario, cold winters are the barrier
Workers are tiny and desiccate quickly in dry air
Dependent on honeydew and moisture; drying a site out weakens it
Colonies are diffuse, but every fragment needs queens and moisture to survive

Myths and facts

Myth Kill the queen and the colony dies.
Reality There are hundreds of queens per colony, and spraying workers just makes the colony split into more colonies.
Myth They're just like regular ants.
Reality The supercolony, millions of workers, hundreds of queens, zero internal aggression, has no equivalent among native ants.

Research notes

Morphology

Workers monomorphic, 2-3 mm, slender, dull light-to-dark brown. Twelve-segmented antennae without a club; single upright petiolar scale; no sting; sparse erect hairs. Queens ~4-6 mm.

Diagnostics

Confirm by size (2-3 mm) + dull brown + single upright petiolar scale + lack of aggression between nests + trailing behavior. No coconut odor.

Easily confused with

Odorous house ant, Tapinoma sessile Similar size but emits a rotten-coconut odor when crushed; Argentine ant has no such odor.

Life cycle

Adult: Workers live weeks to months; queens live a year or more.
Egg: Tiny white ovals tended by workers; hatch in ~2-4 weeks.
Egg to adult: ~6-8 weeks at warm temperatures
Generations per year: Continuous in warm climates, colonies grow year-round
Nymph or larva: Legless larvae fed by workers over ~3-6 weeks.

Development

Complete metamorphosis: egg → larva → pupa → adult.

Reproduction

Polygynous, hundreds of queens per colony, each laying; colonies reproduce by budding (queens plus workers walk to new sites).

Distribution

Global: Six continents; Mediterranean climates, California, southeastern US
Native range: Paraná River basin, South America
Ontario: Not established, winters too cold

Ecology

Unicoloniality arises from reduced cuticular-hydrocarbon diversity in introduced populations, nestmates are recognized as kin across kilometers. Dominates by numerical superiority and interference competition; mutualism with honeydew producers.

Resistance

Insecticide resistance reported in some populations; bait aversion (behavioral resistance) documented.

IPM implications

Colony budding means disturbance-based tactics backfire, killing foragers fragments the colony into more colonies. Monitoring follows honeydew sources (aphids/scale on vegetation), since carbohydrate supply drives colony growth; bait logic must account for hundreds of queens, not one.

Chemical ecology

Trail pheromones from the pygidial gland organize mass foraging; cuticular-hydrocarbon homogenization underlies the loss of inter-nest aggression.

Genetics and DNA

Draft genome published 2011, one of the first ant genomes; noted reduced immune-gene diversity consistent with unicoloniality (Smith et al., PNAS).

Taxonomy

Order Hymenoptera, family Formicidae, subfamily Dolichoderinae. Described by Mayr, 1868. Unicolonial: mutually non-aggressive across vast populations.

Open questions

Genetic mechanisms maintaining unicoloniality; limits of cold tolerance.

References

Smith et al. 2011, PNAS (genome)
Hölldobler & Wilson 1990, The Ants
Silverman, J. & Brightwell, R.J. (2008). The Argentine ant: challenges in urban pest management. Annual Review of Entomology, 53, 231-252.