crazy ants

How to identify Nylanderia fulva, the hairy crazy ant

Nylanderia fulva

Those of you paying attention to emerging pests probably know about the hairy crazy ant, Nylanderia sp. This little invader has been challenging fire ants as the primary nuisance ant along the U.S. gulf coast, first becoming dominant in Houston, Texas about 10 years ago and now expanding rapidly across the region. Crazy ants don’t sting, but their colonies have the habit of building up to disconcertingly large numbers of individuals. Some invaded sites are simply covered in ants (video!).

Writing about this insect can be difficult for nomenclatural reasons both vernacular and technical. The species variously goes by the names hairy crazy ant, caribbean crazy ant, and rasberry crazy ant, as well as an unsettled slate of scientific names: Nylanderia fulva, N. pubens, or N. sp. near fulva or pubens. Ongoing research by John LaPolla is currently sorting out the correct name. For reasons that I won’t delve into here, I call it Nylanderia fulva, the hairy crazy ant.

Nomenclature aside, our newly pesty North American form appears to be a single species. As this ant’s importance grows, so too does the importance of correct diagnosis. Thus, I have written this post as an informal tutorial to identifying these insects. A more rigorous account can be found at Joe MacGown’s site.

To use this guide, you’ll need a microscope powerful enough to count hairs on the back of a small ant.

Nylanderia are small ants, less than 4mm in length, with long, thick hairs arranged in rows on their backs.

Step 1. Verify that your ant belongs to the genus Nylanderia. These are small (2-4 mm) formicine ants, and their waists have one, rather than two, constricted segments. Within the formicines, Nylanderia is one of two genera with stiff, upright, bristly hairs arranged in rows on the back of the thorax. Many ants are hairy, but only Nylanderia (and Paratrechina– more on this below) sport their hairs in pairs or lines.

Nylanderia fulva/pubens has a dense coat of hair on the mesopleuron (Image: modified from

Step 2. Verify that your ant is hairy enough. Most of the couple dozen Nylanderia species in North America are rather shiny in appearance. Nylanderia fulva, in comparison, is more dull as it carries a dense coat of fine hairs matted along much of its cuticle. In particular, only N. fulva and one other non-native species, Nylanderia bourbonica, show this coat along the mesopleuron (the side of the middle of the thorax) as indicated in the figure above.

Nylanderia flavipes is another non-native Nylanderia, but like most species its flanks are shining (Image: modified from
The black crazy ant Paratrechina longicornis has similarly long hairs to Nylanderia, but it is much more slender. It also lacks hairs on the mesopleuron. (Image: modified from

If your ant is hairless or only sparsely hairy on the side of the thorax, it’s not a hairy crazy ant.

Nylanderia bourbonica is also unusually hairy for the genus, but it is much darker in color than the hairy crazy ant, and the sides of its thorax usually have a somewhat sparser coating of hairs. (Image: modified from

Step 3. Check the color: Reddish-brown, or brownish-black? Nylanderia bourbonica– not the hairy crazy ant, but often found in similar places- is darker in color. If you’ve got hairy crazy ants, they will be a light to medium reddish brown.

In the field, hairy crazy ants are reddish-brown and rather pudgy.

There you have it. If your mystery ant is in the southern United States, is reddish-brown, is 3-4mm long, bears rows of stiff hairs on its back, and has a dense coat of fine hairs on its sides, you’ve likely got Nylanderia fulva.

Additional resources:

Myrmecology makes Slate Magazine

Scott Solomon, who researches fungus-growing ants, has a brief piece in Slate Magazine on the Paratrechina Crazy Ants invading Houston.

I’m not convinced that this ant is anything different from Paratrechina fulva, a common South American species and the oldest name in that species complex. People have been calling the Houston invader “P. cf. pubens“, but the taxonomy of the genus is so poor that it’s difficult to say anything with confidence. Do you folks who’ve looked at specimens (that’s you, James) have any thoughts about the identity of this ant?