The Argentine ant has spread to every continent except Antarctica, overwhelming native ants with its sheer numbers and fierce battle tactics. But this invader may have met its match in a recent U.S. arrival: the Asian needle ant. The cross-species face-off, a surprise to entomologists, could harm ecosystems where the battle lines are drawn.

Although invading ants make up just a handful of the more than 12,400 described ant species in the world, they have an outsized impact on ecosystems, economies and human health. Invasive ants often kill, eat or outcompete native ant species that play key roles in their environment. Whereas many native ants are gardeners, tilling the soil and planting seeds, alien ants do not usually pick up the jobs of those they push out. The Asian-Argentine rivalry is a rare opportunity for researchers to observe an invasion in progress.

Scientists at North Carolina State University stumbled on the unfolding ant war several years ago. Eleanor Spicer Rice, an entomology graduate student at the time, was tracking a network of Argentine ant nests in an office park in Morrisville, N.C., and found a few nests of Asian needle ants. She remembers thinking that it was “really weird that another ant could be nesting within the Argentine territory.” (Argentine ants do not tolerate competition.) Weirder still, the Asian ants were driving the others back.

The researchers wondered how the outnumbered Asian ants were gaining a leg (or six) up. Cold-tolerance tests in the laboratory hinted that they were better adapted to the temperate North Carolina climate than the tropical Argentine ants; whereas the Asian needle ants shook off their winter sluggishness as early as March, the Argentine ants did not resume activities until late April or early May, the entomologists reported on February 8 in PLOS ONE. If so, Asian needle ants may be winning the war in North Carolina simply because the state is at the northern limit of the Argentine ant's range.

An Asian needle ant takeover would be bad news for Argentine and native ants and for humans. The Asian ant's burning sting can induce a life-threatening allergic reaction in sensitive individuals. “More people are allergic to Asian ant stings than to honeybee stings,” Spicer Rice says.

To figure out how far the Asian ants are spreading, North Carolina State researchers have turned to citizen scientists. Their project, School of Ants, asks volunteers to find insects in cities and suburbs and send in specimens. To date, they have found Asian ants from New York City to Washington State.

As climate change, travel and disrupted habitats offer more opportunities for invading ants, the species wars will continue.