Pavement Ant

Tetramorium immigrans

What does a pavement ant look like, and what is it not?

Small, dark, stocky, and sculptured rather than smooth — that last word does most of the work.

Pavement ant workers are a uniform brown-black, all much the same size within a colony, and shorter and thicker in build than the other small ants that come indoors here. Held under any magnification, the surface of the head and the top of the thorax carry fine parallel grooves running front to back, like very shallow machining marks. Nothing else you are likely to find in a New York kitchen has that texture. The abdomen, by contrast, is smooth and glossy, so the insect looks two-toned in finish even though it is one color.

Two more characters settle it.

The waist has two bumps. Between thorax and abdomen there are two distinct raised nodes, one behind the other. That immediately rules out the carpenter ant and the odorous house ant, which both have a single node — and in the odorous house ant’s case a node that is flattened out of sight altogether.

There is a pair of small spines at the back of the thorax, pointing rearward. They are short and easy to miss without magnification, but they are there, and the odorous house ant has nothing of the kind.

The two-node waist is shared with the pharaoh ant, so if the ant is very small and pale amber-yellow rather than dark, you have the wrong species and a very different problem — pharaoh ants are an indoor-only insect that must never be sprayed, and confusing the two is one of the more consequential identification errors in this trade.

The crush test that identifies odorous house ants does not work here. Pavement ants have no notable odor.

Winged reproductives appear in late spring and early summer, and they cause a predictable round of alarmed phone calls because they emerge in quantity, often indoors from a crack in a cellar slab. They are the same dark color, with the pinched waist and elbowed antennae that mark any ant, and they are frequently mistaken for termite swarmers. The distinction is set out under the eastern subterranean termite profile, and it is worth thirty seconds of your time because the two events happen in the same season and the same rooms.

This species is not native. It arrived from Europe long ago, spread through ports and cities, and is now one of the most abundant ants in the paved parts of the region — which is exactly where it thrives.

Why does this ant live under concrete?

Because a slab is a superb roof, and the ground under it is better habitat than the ground beside it.

Consider what sits below a sidewalk flag, a patio, a garage floor or a driveway apron. There is a bed of sand or stone and then soil, all of it protected from rain, from foot traffic, from predators and from the drying effect of sun and wind. The slab above absorbs heat during the day and releases it slowly, which extends the season at both ends and keeps a nest workable in shoulder weather when open ground has gone cold. Moisture beneath a slab is buffered — it does not flood the way a shallow nest under mulch floods, and it does not bake dry the way exposed soil bakes. And the slab has joints in it, which are ready-made doors.

That is why this species is so completely tied to hardscape. In a Long Island subdivision, the productive nesting is under the front walk, the driveway apron where it meets the garage, the patio slab, the pool surround, the stoop, and the concrete pad under an air conditioning condenser. In New York City, it is under the sidewalk itself, under areaway paving, under a cellar slab, and beneath the flags of a brownstone rear yard.

Excavation is continuous, and the spoil has to go somewhere. It gets carried up and deposited at the nearest joint, producing the small conical craters of loose soil along a paving seam that most people have seen a thousand times without registering what they are. Those craters are the best possible evidence, because they sit directly over the nest and they cannot be faked by a foraging trail. Sweep one away and it is back within a day or two if the nest is active.

There is a slow structural consequence to this that gets underplayed. Removing soil from under a thin slab or a paver bed, year after year, contributes to settlement, and a flag that has dropped a quarter of an inch at one corner has usually been undermined at least in part. Homeowners tend to see it the other way round — the crack came first, the ants exploited it — and both directions are real.

What happens through the year, and what is the fight on the sidewalk?

The visible calendar has three events on it.

Spring. Colonies come back to strength as the slab warms, and this is when the sidewalk battles happen. Workers from two adjoining colonies mass along the boundary between their territories and grapple, sometimes in the thousands, across a patch of concrete for hours at a stretch. It is a genuinely striking thing to watch, it happens on residential walks and city sidewalks alike, and it does not need intervention. What it tells you is that there are two separate nests under that stretch of sidewalk, which is useful when the work is being scoped.

Late spring into early summer. The reproductive flight. Winged ants emerge, and where the colony is under a cellar slab they emerge indoors, coming up through a floor crack, an expansion joint or the gap where a pipe passes through the floor. A cloud of winged ants in a finished basement is one of the more common panicked calls we get, and it is very often this species rather than anything structural.

Summer. Peak foraging. Trails extend, indoor complaints rise, and the diet leans heavily toward greasy and protein-rich material as well as sugars — which is a practical detail, because it changes what bait is offered.

Autumn and winter. Activity contracts under the slab. Colonies near a heat source — beside a boiler, along a heated floor loop, next to a hot water line run under the floor — stay active through the cold months and produce winter complaints in an otherwise quiet season. Localised winter activity in one corner of a cellar is usually a thermal story, and mapping where the heat is in that floor is the shortcut to finding the nest.

Rain matters here too, though differently from the odorous house ant. A slab sheds water to its edges, and a downspout discharging onto a driveway apron or a patio saturates the bed underneath it. Colonies shift away from the saturated zone and toward the building, which is why a wet spell followed by indoor ants in a basement is a drainage finding as much as an ant finding.

What is actually at risk?

The house is not, in any structural sense. What is at risk is food, surfaces and, in a commercial building, an inspection result.

Pavement ants do not excavate timber, do not feed on wood, and do not carry disease in any way that matters in a residential setting. They can sting, and the sting is so slight that most people do not notice it. As household pests go, they are a volume problem: a trail that runs from a cellar slab crack up through a floor joint into a kitchen, delivering a steady line of ants onto a counter, is intolerable long before it is dangerous.

Two settings raise the stakes.

Food premises. A restaurant kitchen or a bakery in a ground-floor city space almost always sits on a slab, frequently one with old expansion joints, floor drains and utility penetrations. That slab is nesting habitat directly under the operation, and ant activity on a prep surface is a finding. The relevant program work sits under restaurant pest control and food facility IPM.

Multi-unit buildings. In a co-op or condo with a slab-on-grade cellar, a single colony under the floor can produce complaints in the ground-floor units, the trash room and the laundry simultaneously, and each one gets logged separately. Treating them as three unrelated calls is how a building spends a summer on the same nest.

There is also a diagnostic value to the ants that is worth capturing. A slab that supports a large colony directly beneath it usually has moisture under it, and persistent moisture under a slab is worth knowing about for reasons that have nothing to do with insects.

Where do they come through into a building?

At every break in the concrete, and the list is short enough to be checked properly.

Expansion and control joints. The deliberate breaks in a slab, and the most productive entry on most buildings. A joint filler that has shrunk, cracked or been dug out is an open route.

The cold joint at the slab perimeter. Where a floor slab meets the foundation wall there is a junction, not a bond. On older construction it is often open, and on many finished basements it sits behind a stud wall where nobody has looked in decades.

Shrinkage cracks. Ordinary cracking in a garage floor, a cellar slab or a patio.

Plumbing and conduit penetrations. Where a waste line, a supply line, a floor drain or electrical conduit passes through the slab, there is an annulus. Very few of them are sealed. Around a sunken tub, a shower drain or a toilet flange, the gap is generally concealed by the fixture itself.

The slab edge at a threshold. Sliding door tracks, garage door thresholds and the transition at a walkout basement.

Under sill plates on a slab-on-grade house. The sill sits on the slab edge with a gap behind the exterior finish, and that gap is continuous with the ground.

In city buildings, the areaway slab, the vault under the sidewalk, the joint where a cellar floor meets an old rubble foundation, and the base of a party wall in an attached house.

Indoors, the activity is usually concentrated along one run rather than distributed. That is not accidental — it is telling you which part of the slab the nest is under. Following the concentration to its densest point, and looking for the joint or crack at that spot, narrows a whole-floor problem to a few linear feet.

Why does a finished basement make this so much harder?

Because the finish covers exactly the surfaces that hold the answer.

A finished cellar in a Long Island house typically has a stud wall built inboard of the foundation, insulation behind it, a floor laid over the original slab, and trim covering the junction between the two. Every one of the entries listed above — the cold joint at the slab perimeter, the shrinkage cracks near the wall, the annulus at a floor penetration — is now behind finished material. What the occupant sees is ants emerging at a baseboard, at a carpet edge, or from under a piece of trim, with no visible crack anywhere.

That produces two recurring mistakes. The first is treating where the ants appear rather than where they enter, which puts material at the end of a route rather than at its start. The second is opening the wrong section of wall, which is expensive and disheartening.

The way through it is to be methodical about the concentration. Watch several evenings, mark on the floor where ants are emerging and in what density, and look for the densest run. Check whether the emergence follows a straight line — a joint under the floor covering — or radiates from one point, which suggests a penetration. Check the exterior directly opposite: if there is a patio, an apron, a walkway or an air-conditioning pad against that wall, the nest is very likely under it, and the exterior is the better place to work.

Where the finish has to come off, it is usually a short length of base trim and a section of floor covering rather than the wall itself, and knowing which few feet to lift is what the survey buys you.

The same logic applies in reverse in an unfinished cellar or a garage, which is why those spaces are so much quicker to resolve. Everything is visible, the joint is right there, and the sealing step is straightforward carpentry and sealant rather than a demolition question.

How is it treated?

By working the transition rather than the room.

Confirm the species first. The method here is straightforward, but a misidentified pharaoh ant treated as a pavement ant is a serious error, and a small carpenter ant treated as either wastes a season. Specimens, not descriptions.

Locate the nest from the spoil. Outdoors, the craters do this for you. Indoors, the concentration of activity along a joint, plus any spoil appearing at a floor crack, does the same. Where the nest is under an exterior slab, the exterior is where the work goes.

Bait at the trail and at the entry. Placements go where the ants are actually crossing, in several small locations rather than one large one. This species will take both sugar-based and grease- or protein-based material depending on the time of year, and offering more than one and seeing which is taken is faster than theorising about it.

Targeted application into the entry itself. Treating the joint, the crack or the penetration that the trail is using puts material exactly where the traffic is, without broadcasting anything across a floor or a food-preparation area.

Exterior perimeter work where it is appropriate, focused on the slab edges, joints and the ground immediately alongside the hardscape, rather than as a blanket band around the property.

Then the physical work. Rout and properly seal the failed expansion joint. Fill the shrinkage cracks. Seal the annulus at plumbing and conduit penetrations. Close the cold joint at the slab perimeter where it is accessible. Fit a proper threshold seal at the garage and the walkout door. This is the step that decides whether the same colony is back through the same joint next June, and it is a small piece of the structural exclusion discipline applied to a floor rather than a wall.

Fix the water. A downspout discharging onto the driveway apron, a patio pitched back toward the house, an irrigation head soaking the walk, a hose bib dripping onto a slab edge. Saturated bedding under a slab moves colonies toward the building.

Sequence matters here as it does everywhere else. Sealing the joints while a large colony sits underneath simply redirects it to the next opening, so population reduction comes before closure.

What keeps them from coming back?

Sound concrete, dry bedding, and a perimeter that is not doing the ants any favors.

Keep joints and cracks in walks, patios, aprons and cellar floors filled and maintained; this is ordinary hardscape upkeep that happens to be pest work. Make sure water leaving the roof lands well away from paving rather than onto it. Check that the patio and the apron still pitch away from the house — decades of settlement often reverse the original fall. Where a paver or flag has dropped, reset it and address the void beneath rather than just levelling the top.

Keep a clean margin between the building and any bedding material, and avoid stacking stone, pots and boards against the foundation, which give a colony a second option when the slab route gets closed.

Indoors, the usual food discipline applies: dry goods in rigid sealed containers, grease cleaned from behind and beneath a range, no crumbs in floor joints, drains kept clear, pet bowls not left down overnight. It does not stop a colony from entering, but it removes the reason the trail keeps running to the same place, and it makes bait considerably more competitive than the alternatives on offer.

For a building with a slab-on-grade cellar and old joints throughout, the honest position is that this is a recurring seasonal pressure rather than a one-off, and the way to change that is joint by joint over a couple of seasons rather than by treating harder.

What can you check before anyone arrives?

Four things, and all of them make the visit shorter.

Walk the hardscape and look at the joints. Every walk, patio, apron, stoop and pad. Note where you find the little craters of loose soil, and note which side of the joint they are on. Photograph them with something for scale. That map is the most valuable thing you can hand over.

Note where indoors the activity is densest, and whether it runs in a line or spreads from a point. Mark it with tape rather than trying to remember.

Look at where the water goes. Stand outside during rain if you get the chance. A downspout emptying onto a patio, a walk that ponds, a pad that stays wet for a day after everyone else’s has dried — each of those is a finding, and each of them changes where a colony sits under the concrete.

Do not spray anything, and do not scrub the trails away. The trail is the route we treat, and a strong residue on it costs time. If you have already used something, say so when you call — it changes what we do first rather than being a source of embarrassment.

One more thing, which applies to the swarm in particular. If winged ants appear indoors in late spring, catch a few in a jar or tape a couple to a card before vacuuming. The difference between a pavement ant flight and a termite flight is a thirty-second examination of a specimen and a very large difference in what happens next. That comparison is set out in the eastern subterranean termite profile.

Related work

Treatment, materials and the seasonal bait question are covered under ant control, which is the service page this profile belongs to.

Read it alongside the odorous house ant, which is the species most often confused with this one and which needs a very different approach because its nests are dispersed rather than fixed under concrete. The oriental cockroach profile is also worth a look if your activity is coming out of a cellar floor or an areaway, because the same slab defects, drainage faults and damp bedding produce both, and one survey usually answers both questions.

If you have craters along a walkway joint or a line of ants coming out of a basement floor, get in touch and say where the concentration is. Our coverage across Nassau, Suffolk and New York City is set out under locations.

Treatment for this pest is covered under Ant Control.