Tiny Indoor Ants – Pavement Ants

2010 April 3

Back in early March, I was walking through one of the buildings on campus, and happened to notice that somebody had dropped a tortilla chip on the floor. One of the “spicy” kinds, I think. It looked a bit odd, though, as if somebody had liberally coated it with black pepper. And then I noticed that the black dots weren’t pepper – they were moving. What’s more, they weren’t confined to the chip. There was a line of them going about a foot across the floor, to a tiny hole in the concrete at the base of the wall. It was, of course, tiny little ants. So, I put the ant-covered chip into a small bottle that I keep handy for such purposes, and brought them home to photograph.

And when I say tiny, I do mean tiny. These little ladies were maybe 2-3 millimeters long, tops. They were obviously after the grease (and maybe the salt) in the chip, because they weren’t carrying back any crumbs and tortilla chips aren’t noted for their sugar content.

And they were certainly numerous. Even though a lot of them jumped ship from the Dorito when I picked it up, I still got more than a dozen of them on a chip that was not much bigger than my thumbnail.

This is the best picture I got of one of their heads, and while it is blurry, we can see that they have the general-purpose ant mandibles, good for managing a lot of different foods.

These could be native Michigan ants, but the way things go with insects around here, it’s pretty likely that they are some non-native species. For one thing, the building I found them in is the one that houses the Biology department, and there is a greenhouse up on the top floor. I’ve never been in that greenhouse, but I’m willing to bet that it contains all sorts of little accidental introductions. I’ve seen similar tiny ants infesting other greenhouses, like the simulated rainforest at the Frederik Meijer Gardens and Sculpture Park in Grand Rapids.

I don’t know which kind of tiny indoors-infesting ant they are, there are many kinds. They look too dark to be Pharaoh ants, Monomorium pharaonis, although the size, body shape, and lifestyle is consistent. The hole in the wall that they were coming out of looked pretty well-trafficked, so this colony has probably been there for a while. I can probably get some more specimens out of it to look for specific features, if anyone has suggestions of things I should look for that aren’t visible in these pictures.

Update: OK, the consensus (Thanks, Eric Price and Cupi, and Alex Wild concurs on his site), is that they are Pavement Ants, Tetramorium species-E. And yes, as I rather expected, they are a non-native species from Europe. The theory of how they got here was that they were in ship ballast[1], and once in place they just spread from the seaports all through North America. They are referred to as “Species-E” because there are a bunch of very similar, hard-to-distinguish species in their native Eurasia, and there is argument over which one of them it was that took the trip across the ocean. the species that made the trip over turns out to be one that had no name because it had never been described before!

For much, much better pictures, see Alex Wild’s image gallery.

When they are outdoors, these ants like to burrow under sidewalks, under rocks, and similar places. They sometimes move indoors, living in walls and under floors, and that’s when they are a problem. They eat most types of food, and get into everything. And, since they evidently have multiple-queen nests, they get very numerous. If you have them inside and want to get rid of them, it looks like the recommended method is boric acid ant baits. This doesn’t kill them right away, which is actually important for control. The foragers carry it back to the nest, where the concentration of boric acid in their food gradually increases until it kills the queens after a few weeks. A fast-acting poison would be less effective, because it would immediately kill any foragers that stumbled across it. The rest of the colony would never be exposed, and would be unaffected.

[1] Ships are designed to be most stable when they are full of cargo. Empty ships float too high in the water, making them prone to capsizing. So, when a ship is not fully loaded it needs some sort of heavy ballast to make it float right. It used to be common just to shovel in dirt as ballast, and then shovel it out again on arrival so that it could be replaced with cargo. This, of course, meant that any little things living in the soil got shipped along with the ballast, and so this was a rich source of invasive non-native species. These days, ships mostly just pump in water to use as ballast, which means that currently most of the non-native species coming in are aquatic, like zebra mussels. So, between the old ballasting method and the new ballasting method, pretty much all of the available pest species in both dirt and water have been shipped around the world. Which is why people working to keep out invasive species really dislike ship ballast.

7 Responses
  1. Eric Price permalink
    April 3, 2010

    Maybe Pavement ants?

  2. April 4, 2010

    Tetramorium sp.
    Maybe Tetramorium caespitum or T. impurum

  3. April 4, 2010

    Drat! I was going to guess Argentine ants. They seem to be unstoppable.

  4. April 5, 2010

    It looks like Argentine ants would be a reasonable guess in California, but I don’t think they have gotten this far north. Bug Guide shows them in California and in a couple of southern states, but not elsewhere. Also, looking at the Argentine ants pictured on BugGuide, the body shape and coloration is different: for one thing, they have larger, brown abdomens with a waist consisting of just one segment, while the ones I found have small, black abdomens and 2-segmented waists.

  5. April 5, 2010

    A slight correction: they’ve done an excellent job (using DNA evidence and morphology) figuring out which of the Eurasian Tetramorium species made the trip over to become our pesty pavement ant.

    But, it turns nobody had formally described that species. From what I understand they have a paper in the works to provide a new name. Until then, it’s “species E”.


  6. April 5, 2010

    Correction made. Thanks, Alex!

  7. Katie permalink
    April 23, 2010

    Pretty sure those are pavement ants. I had problems with them last summer at my house.

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