Tapinoma sessile (Say, 1836)

Odorous house ant

Crush one and it smells like rotten coconut, the native ant that moves indoors every spring.

Order: Hymenoptera
Family: Formicidae
Group: Ants
Ontario status: Native, established provincewide

Why it matters

A persistent food contaminator and one of the most common spring invaders in Ontario homes. Does not sting, bite, or damage structures, but it is relentless.

Origin

Native to North America, it lived in forests long before houses, nesting under logs and stones.

Behavior

Forages day and night in strong trails, especially after rain. Colonies merge and split with the seasons, huge in summer, fragmented in winter.

Living conditions

Nests in wall voids, under floors, in insulation, and outdoors under mulch and stones. Moves indoors in spring seeking sweets and moisture; colonies relocate often.

Signs of presence

Small dark ants in trails, especially in kitchens and bathrooms in spring
Rotten-coconut smell when ants are crushed
Ants appearing after rain

Weaknesses

Colonies relocate frequently, so nests are findable by following trails
Strongly attracted to sweets, easy to monitor with sweet baits
Outdoor nests under mulch and stones are accessible

Myths and facts

Myth The coconut smell means the house has a gas leak or rot.
Reality The smell is the ant's own defensive chemistry (methyl ketones), it is the identification test, not a house problem.

Research notes

Morphology

Workers 2.4-3.25 mm, dark brown to black. Thorax uneven in profile (no spines); single flattened, inconspicuous petiole node. 12-segmented antennae, no club. Gaster with slit-like anal opening (dolichoderine character).

Diagnostics

Confirm by rotten-coconut odor when crushed + uneven thorax + single flat node. The odor test is diagnostic in the field.

Easily confused with

Pavement ant, Tetramorium caespitum Pavement ant has two waist segments, grooved head sculpturing and no odor; odorous house ant has one flat node and the coconut smell.
Little black ant, Monomorium minimum Little black ant has two waist segments and no odor; odorous house ant has one node and the diagnostic smell.

Life cycle

Adult: Workers live several months; queens live multiple years.
Egg: Eggs hatch in ~2-3 weeks at warm temperatures.
Egg to adult: ~34-40 days at warm temperatures
Generations per year: Continuous brood in season; colonies overwinter and rebound
Nymph or larva: Legless larvae; naked pupae.

Development

Complete metamorphosis: egg → larva → pupa → adult.

Reproduction

Multiple queens (polygynous), multiple interconnected nests (polydomous); colonies bud. Supercolony-scale networks (100,000+ workers) documented.

Distribution

Global: North America
Native range: North America
Ontario: Native; established provincewide, common in structures

Ecology

Highly adaptable native tramp; nests opportunistically indoors and out, relocating with moisture and food. Tends honeydew-producing insects outdoors. Foraging peaks after rain.

Resistance

Limited published data.

IPM implications

Colonies relocate readily, so monitoring must be repeated, a nest mapped in May may have moved by July. Trail-following to nest sites is the primary technique; sweet-based monitoring exploits its strong sugar preference. Because colonies bud and merge seasonally, single-nest thinking fails; treat the network. Outdoor harborage (mulch against foundations) is the preventable reservoir.

Chemical ecology

Defensive methyl ketones produce the diagnostic rotten-coconut odor; dolichoderine anal secretions used in defense and trail marking.

Genetics and DNA

No reference genome published as of 2026.

Taxonomy

Order Hymenoptera, family Formicidae, subfamily Dolichoderinae. Described by Say, 1836. Native North American tramp species.

Open questions

Drivers of seasonal colony fusion/fission cycles; supercolony genetics in urban Ontario.

References

Hölldobler, B. & Wilson, E.O. 1990, The Ants, Harvard University Press
Mallis Handbook of Pest Control