How to Identify the Argentine Ant, Linepithema humile

The Argentine Ant (Linepithema humile), a small brown ant about 2-3mm long, is one of the world's most damaging insects. This pernicious ant is spreading to warmer regions around the world from its natal habitat along South America's Paraná River. Linepithema humile can drive native arthropods to extinction, instigating changes that ripple through ecosystems. In California, horned lizard populations plummet. In South Africa, plant reproduction is disrupted. Worldwide, the Argentine ant is a persistent house and crop pest. This is not a good ant.

My Ph.D. dissertation, completed a few years ago, dealt with the taxonomy and evolution of the 20 or so mostly obscure species in Linepithema, the broader group from which the Argentine ant emerged. The project had many different aspects, but most people will only care about one small part: I figured out how to reliably identify the Argentine Ant.

The taxonomy of the Argentine Ant has been a particularly difficult problem in South America where several frustratingly similar species co-occur. Researchers who try to study L. humile in its native habitat occasionally end up working with the wrong species. That's an embarrassing error, and one that has resulted in misinformation about the Argentine Ant spreading through the literature. I won't bore you with the details of the thousands of Linepithema specimens I looked at to determine exactly what was and wasn't an Argentine ant. But I will share the result, a system for separating Argentine ants from related species.

Here is how to tell if you've got an Argentine Ant.

Step 1. Check your location.

While Argentine ants can arrive almost everywhere in the world with human commerce, their survival outside of warm regions is low. If you live in North Dakota, for example, the chance that the ant you just collected is an Argentine Ant is pretty slim. On the other hand, if you live in coastal California, Chile, or Portugal you'll be hard-pressed to find any urban ant that's not an Argentine ant. The map above is a rough indication of places where Argentine ants are most frequently encountered.

Outside of the occasional greenhouse, the other 20 species of Linepithema generally do not travel around the world. If you are not in South America, Central America, or the Caribbean, then any Linepithema species you see will almost certainly be an Argentine Ant. Only in the Neotropics, where the Argentine ant co-occurs with some frustratingly similar relatives, does identification to species become tricky.

Step 2: Make sure you have the right genus.

You'll need a microscope for this and all subsequent steps. All Linepithema species share a distinct pattern of teeth on their mandibles and an unusual shape to the clypeus, that bit of exoskeleton that forms the front margin of the head. If your ant shares the combination of a clypeus that is broadly concave in the center (as shown by the smooth red line in the photo above) and mandibular teeth that are arranged as a series of small denticles interspersed with larger single teeth (shown by the jagged red line), then you have a Linepithema species. Outside of Central and South America, your ant will likely be L. humile, the Argentine ant.

Step 3. Check to see that your Linepithema is relatively hairless.

So you have a Linepithema. How do you know that it is L. humile? Check the hairs.

Linepithema humile is among the least hairy species in the genus. It lacks standing hairs on the dorsal surface of the mesosoma and on the first two segments of the gaster. These hairs can be subtle, so you'll need to check carefully with the right lighting. In L. humile you won't see any standing hairs until the third gastric segment. Any ant that has standing hairs on the first two dorsal segments of the gaster will be a different species.

Step 4. Check to see that your Linepithema has large eyes.

The final check to confirm an identification is to look at the eyes. By a slim margin the Argentine ant has the largest eyes of any Linepithema, containing more than 90 ommatidia. Counting ommatidia is tedious, so I find it easier to just get a sense for what a larger eye looks like. This takes a bit of practice. The above photo contrasts the Argentine ant (at right) with its relative L. gallardoi. It's not a huge difference, but it turns out to be consistent.

You'll also notice from the photo that the Argentine ant has a longer first antennal segment than L. gallardoi. The long antennae of the Argentine ant is a helpful trait, but only to a point. There are a few other species that also have long antennae, so it is still best to check for the standing hairs and the large eyes.

See how easy that was? Just kidding. Separating Argentine ants from closely-related species is challenging, even for me. If you need more information, the two papers I've written on the taxonomy of Linepithema are linked below. These papers provide additional characters, as well as information on identifying the other Linepithema species.

Sources:

Wild, A. L. 2004. Taxonomy and distribution of the Argentine ant Linepithema humile (Hymenoptera: Formicidae). Annals of the Entomological Society of America 97(6): 1204-1215. [download PDF]

Wild, A. L. 2007. Taxonomic revision of the ant genus Linepithema (Hymenoptera: Formicidae). University of California Publications in Entomology 126. [download PDF]

Specimen images courtesy of April Nobile at www.antweb.org.