Pavement Ant

Tetramorium immigrans — the pavement ant — is one of the most familiar urban ants in eastern North America and a flagship species of urban entomology. The species is native to Europe (where it has been present for millennia) but was accidentally introduced to North America in the 1700s-1800s via trans-Atlantic shipping (almost certainly in soil ballast or potted plants brought by European immigrants), and has spread aggressively across NA over the past 200 years to become one of the most common urban ant species across the continent. The species is also called Tetramorium caespitum in older literature — the Tetramorium taxonomy was recently revised by Wagner et al. (2017) and the NA invasive populations were renamed T. immigrans (literally 'the immigrant ant' — referring to the European-immigrant introduction history) to distinguish them from the native European populations of T. caespitum. Workers are 2.5-3.5 mm long, dark brown to nearly black, with the typical formicid body plan. Pavement ants nest under sidewalks, building foundations, paving stones, and other urban hardscapes — colonies build extensive subterranean galleries beneath pavement and frequently throw out small piles of excavated soil between sidewalk cracks (the diagnostic visible sign of pavement ant infestation). Workers forage in long trails across pavement and walls, finding food via pheromone trails that lead other workers to discovered food sources. The species is famous for the spectacular ANNUAL 'PAVEMENT ANT WAR'. Every spring (typically late April through May in the northeastern US), neighboring colonies engage in MASSIVE MULTI-DAY BATTLES on sidewalks. Thousands of worker ants from rival colonies meet at colony boundaries and engage in MASS COMBAT — grappling with each other, biting with mandibles, decapitating opponents, and dragging away dead and wounded ants. The battles can last several hours per day for 5-7 days, with millions of ant-encounters and thousands of casualties per battle. The wars establish territorial boundaries between colonies for the coming year; once colonies have determined boundaries, fighting ceases until the following spring's renegotiation. The wars are one of the most-photographed urban arthropod events in eastern NA cities and are a flagship subject in urban entomology research. Pavement ants are minor household pests (they enter homes seeking food, but rarely cause structural damage) and are harmless to humans (no venomous sting, no significant bite). The species is a flagship example of cosmopolitan urban arthropod success.