Urban ant collectors across temperate North America are undoubtedly familiar with the pavement ant Tetramorium caespitum*. This small brown insect is as common as dirt along sidewalks.
The pavement ant is not native here. Rather, it is a European species that proliferates in the novel habitats where Americans added cement and paving stone to previously uncapped, pavement-free soils. Since we love our sidewalks and our asphalt, we have created a lot of ant habitat and a lot of pavement ants.
Identification of the pavement ant in North America was straightforward until recently. Tetramorium caespitum is a small, blocky, brown ant with a squareish head, a two-segmented waist, a series of lengthwise ridges on the head, two nubbin-like spines on the propodeum, and an antennal socket with a distinct ridge as described here.
This diagnosis failed in the 1980s when an extremely similar species was introduced to St. Louis. The newcomer, the Japanese pavement ant Tetramorium tsushimae, is so similar in appearance to its European congener that correct identification even under high magnification involves measuring several body parts on a sample of workers and performing statistical analyses. On average, the new introduction is slightly smaller and with slightly larger propodeal spines. Your chances of nailing the ID based on a single worker aren’t great.
There is one easy identification trick that works pretty well at low magnification in the field, though. The trick is worth learning, because Tetramorium tsushimae appears to be more aggressively invasive than the common pavement ant and may become more common as it spreads from Missouri and Illinois.
Here’s the trick:
Colonies of the Japanese pavement ant usually host a great deal more color variation in the workers.
While older T. tsushimae are uniformly dark, the same as their European counterparts, younger workers are strongly bicolored, with a light thorax, giving colonies a more varied appearance. This difference should be visible in the photograph above.
Now that you can spot the difference, keep an eye out for T. tsushimae. It could show up many places where T. caespitum is currently king.
*sometimes called “Tetramorium sp. E.”, for reasons too lengthy to discuss here.
source: Steiner, Florian M., Birgit C. Schlick-Steiner, James C. Trager, Karl Moder, Matthias Sanetra, Erhard Christian, and Christian Stauffer. 2006. Tetramorium tsushimae, a New Invasive Ant in North America. Biological Invasions 8(2):117-123.