Aphaenogaster Ants — Aphaenogaster spp.

The ant nobody has a name for

Most of the ants in this library arrive with a name attached. Carpenter ant. Pavement ant. Odorous house ant. The name usually tells you the important thing about the animal.

This one does not have that. The regional extension key lists it simply as Aphaenogaster spp., because there is no single settled common name in use here. Different sources call them winnow ants, after the way they handle seeds, or spine-waisted ants, after the pair of spines on the back of the thorax. Neither has stuck firmly enough to be worth building a page around.

So the genus name it is, and the lack of a common name is itself informative: this is not an ant that has bothered enough people, often enough, to earn one.

Recognizing them

Size. Four to six millimeters, monomorphic — every worker the same. That puts them squarely between the small indoor ants and the large carpenter and thatching ants.

Color. Most species brown to reddish-brown.

The spines. This is the identifying feature. A pair of spines project from the epinotum — the rear upper surface of the thorax, just before the waist. Under a hand lens they are unmistakable, and they are what separates this genus from the large ants people worry about.

They are also generally long-legged and long-bodied relative to their bulk, which gives them a slightly leggy, deliberate look in motion compared with the compact pavement ant.

Why finding them is a moisture report

This is the part worth carrying away from the page.

The extension guidance says they nest in wood in various stages of decay, in cavities in sound wood, or in soil beneath stones and wood. That nesting list comes from PNW 624, the WSU Extension key to ant pests in this region.

Read that carefully, because it is the same distinction that governs the whole of this site’s approach to ants in structures. This species uses damage that is already there. It nests in wood that has already decayed, or in a void that already existed. It is not excavating sound structural timber the way a carpenter ant does.

So an Aphaenogaster colony in a piece of a building is telling you two things, and the second one matters more:

There are ants in that timber. Modest problem, easily dealt with.

That timber is decaying, or has a cavity in it. Much bigger problem, and the one worth spending money on.

That puts this species in the same diagnostic category as moisture ants: the insect is a symptom, and treating it while leaving the wet wood is treating a smoke alarm.

What they eat

Seeds, dead and live insects, and other arthropods — so a predator and a scavenger rather than a specialist.

The extension guidance adds that they have been reported feeding on household foods including cakes, peanut butter and meat, which covers sweet, fatty and protein. That breadth is why an occasional trail indoors is possible.

It is not, however, a species that typically establishes a persistent foraging line into a kitchen the way an odorous house ant does. Where there is a determined trail across a counter every evening, this is usually not the answer.

Colony structure

Colonies run to as many as three thousand workers with several queens.

Both halves of that are worth noting. Three thousand is a moderate colony — much larger than a thief ant or a ponerine ant colony, considerably smaller than a mature pavement ant or thatching ant nest.

Several queens means the colony has some resilience and does not collapse from losing one, which is a familiar pattern by now across this library and one of the reasons treating ants by killing visible workers so rarely accomplishes anything.

The swarm, which is when people notice

Reproductives swarm from late summer into fall, and that is when this species generates calls.

The extension guidance is explicit that these ants are usually only an incidental pest for homeowners when the reproductives swarm — which is a fair description of a whole class of insect complaint. An animal that has been living quietly in a stump or under a slab all year suddenly produces a cloud of winged forms, and the household concludes something has gone wrong.

Nothing has gone wrong. A swarm is a colony reproducing, and it is the one week of the year when an otherwise invisible ant becomes conspicuous.

The genuinely useful question when winged ants appear indoors is where they came from, not what they are. Winged ants emerging from inside a wall or a floor mean a colony in the structure. Winged ants that flew in through an open door or came to a light mean nothing except that it is the season.

The weak sting, and the honest scale of it

The guidance handles this well and it is worth reproducing the sense of it.

Workers have a weak sting and will bite if a nest is disturbed. The sting rarely penetrates human skin. Bites hurt only if inflicted in a sensitive area. Most people consider these minor annoyances.

That is about as low as insect hazard gets while still being non-zero, and it sits in useful contrast to two other pages in this library. The harvester ant, in the same subfamily, delivers a sting the same guidance calls rather severe. The ponerine ant, tiny and unremarkable, stings painfully.

Sting severity in ants does not track size or appearance, and this genus is a good demonstration of that — mid-sized, spiny, and essentially harmless.

Telling them from the ones that matter

Carpenter ant. Larger and polymorphic — a range of worker sizes in one colony — evenly rounded thoracic profile with no spines, usually black, and genuinely excavating sound wood. This is the important distinction, because it is the difference between an ant exploiting decay and an ant creating a structural problem.

Thatching ant. Larger again, frequently bi-colored, with a notch rather than spines on the thorax, and a mound of plant material outdoors.

Pavement ant. Smaller at 2.5 to 3 millimeters, with the head and thorax sculptured by fine parallel grooves, and characteristic piles of excavated sand along paving and slabs. Pavement ants also carry spines, which is the one point of genuine confusion here — the size difference sorts it.

Moisture ant. Small and yellow, and pointing at the same underlying condition as this genus without the spines or the size.

Around this county

Ordinary and largely unremarked. They are part of the general ant fauna in soil, under stones and boards, and in the decaying wood that a wet county produces in quantity.

Where they come to our attention is the same set of circumstances described on several other pages here: a stump left in the ground, landscape timber going soft at the base, an old deck frame, a woodpile that has been in place too long, and the punky wood at the bottom of a fence post.

Indoors they turn up where there is already a moisture problem — the sill of a crawlspace that has been damp for years, framing behind a leaking shower, the plate under a door that has been wetting for a decade. In every one of those cases the ant is the least of it.

What we would actually do

Identify it, because the spines make this quick and because the alternative answers — carpenter ant in particular — are much more expensive to be wrong about.

Find out what it is nesting in. If it is outdoors in a stump or under a stone, that is the end of the matter and there is nothing to sell.

If it is in the building, find out why the wood is suitable. Decaying timber or an existing cavity means a moisture source or a past defect, and that is what we would be looking for and reporting on.

Deal with the wood, not the ants. Replacing or drying the affected timber removes the nest site permanently. Treating the ants and leaving wet wood means another colony, or worse, finds it later.

Most visits for this end with a conversation about a downspout, a stump or a piece of framing rather than about an insect.

What we will not sell you

We will not treat an ant colony in a stump in somebody’s back garden.

We will not describe this species as damaging a structure. It uses wood that has already failed, and calling that structural damage would be borrowing the carpenter ant’s reputation to sell a job.

We will not sell a treatment for a late-summer swarm that will be over in days, where the ants came in through an open door.

And where we find them in a building, the invoice worth having is for the moisture repair, which is frequently not our work at all — and we will say so.

The spines, and the other ant that has them

Since the spines are the identifying feature, it is worth being precise about the one genuine point of confusion.

Pavement ants also have spines on the epinotum. Both genera sit in the same subfamily and both carry that pair of projections, so a hand lens showing spines does not on its own settle which of the two you are looking at.

Two things separate them and both are easy.

Size. Pavement ants are 2.5 to 3 millimeters. Aphaenogaster are 4 to 6 — up to twice as long, and the difference is obvious with the two side by side or against anything for scale.

Sculpture. A pavement ant’s head and thorax are marked with numerous fine parallel grooves, which is where the identification usually gets made in the field. Aphaenogaster do not have that patterning.

And the evidence on the ground. Pavement ants excavate soil and leave characteristic piles of fine sand along driveways, patio joints and basement slabs. Aphaenogaster do not do that.

If there are sand piles on paving, it is a pavement ant. If there is a colony in a decaying stump, it is more likely this.

Seeds, and where the name winnow ant comes from

The seed handling is worth a paragraph because it is where one of the two competing common names originates and because it explains something about where these ants live.

They collect seeds among the other things they eat, carry them back, and process them — and the handling of seed and chaff underground is what “winnow” refers to. It is not the specialized seed economy that harvester ants run, where whole colonies are organized around stored grain, but it is a real part of the diet.

Practically that means they forage over ground rather than following a fixed route to a single food source, which is another reason they are less visible than a trailing ant. There is no persistent line to notice.

It also puts them among the insects that move seed around a landscape, which for a native ant in decaying woodland is a genuine ecological role and one more reason a colony in a stump is better left where it is.

Where we actually find them

In practice, on jobs in this county, this genus turns up in a short list of places and almost all of them are outdoors.

Stumps and buried wood. A stump left in the ground after a tree came down is the classic, and it is usually where a colony is when somebody reports ants near a patio or a bed.

Landscape timber. Ties, raised beds, retaining sleepers and old deck framing that has gone soft at ground contact.

Under stones, slabs and boards. Anything flat on soil that has been in place long enough to create a stable, sheltered space beneath it.

Firewood stacks, particularly the bottom course that has been in contact with the ground for a season or more.

Fence posts at the soil line, where the decay is.

Indoors is much less common, and where it happens it is always the same condition: timber that has already been wet long enough to soften. A crawlspace sill above damp ground. A rim joist behind failed flashing. Framing under a shower that has been leaking quietly.

Why we would rather you called about the wood

Somebody finding this ant in a building has, without knowing it, done a useful piece of building inspection.

The insect is telling you where the moisture is. It has located a piece of softened timber in your structure and moved into it, which is information that would otherwise have cost somebody an afternoon with a moisture meter and a torch.

So the conversation we would rather have is not about the ants. It is: where is that timber, how long has it been wet, what is wetting it, and how far has the decay gone. Those questions lead somewhere. Killing a colony of a minor-annoyance ant and leaving the wet wood leads nowhere, and it leaves a building quietly getting worse behind a solved-looking problem.

That is the same argument this site makes about moisture ants, and it is worth making twice because it is the single most common way pest control gets sold in place of building repair.

What this genus contributes to the library

A page about an ant that rarely causes trouble might look like completeness for its own sake, and it earns its place for a different reason.

This library now covers all twelve of the principal pest ants in the regional extension key, and the value of that is comparative rather than individual. Somebody looking at an ant can work through a set that actually contains the answer, instead of being pushed toward whichever species has the most alarming page written about it.

That matters commercially as well as editorially. The ant most people fear here is the carpenter ant, and it is the most expensive to treat. A library that described only the frightening species would send every mid-sized dark ant toward that conclusion. A library that includes the harmless ones — this genus, the false honey ant, the mound builders outdoors — lets an identification land where it should.

So the point of this page is partly that it exists to be ruled out. An ant with spines, at four to six millimeters, nesting in a rotten stump, is not a carpenter ant, and having somewhere to say so is worth as much as any of the pages about things that genuinely need treating.

The short version

Four to six millimeters, brown to reddish-brown, all one size, with a pair of spines on the back of the thorax. Nests in decaying wood, in existing cavities, and in soil under stones and boards.

Weak sting that rarely breaks skin. No structural damage of its own. Noticed mainly when the winged forms swarm from late summer into fall.

Outdoors, in a stump or under a slab, the answer is to leave it alone. Indoors, the ants are pointing at wood that has already gone soft, and the wood is the thing worth dealing with.

If it has spines and it is nesting in something rotten, it is not a carpenter ant, and that is the most useful thing this page has to tell anybody.

A note on the swarm, and what to check

Since the swarm is the one time this genus is conspicuous, it is worth knowing what to look at while it is happening, because the answer is time-limited.

Winged ants indoors in late summer are worth thirty seconds of attention before anybody reaches for a vacuum. The question is not what species they are. It is whether they are coming out of the building or into it.

Ants emerging from a gap in a wall, a floor joint, a ceiling or around a light fitting mean a colony inside the structure, and that is worth investigating whatever the species turns out to be. Ants clustered at a window trying to get out, or arriving through an open door, mean a colony outdoors and a warm evening.

Watch for a minute and the difference is usually obvious. It is the single most useful observation available during a swarm, it costs nothing, and the opportunity closes within a day or two when the flight ends.