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Crawl Space: Invasive Ant Armies Clash on U.S. Soil

One of the most aggressive invasive ants in the world seems to have met its match in North Carolina--but if the arthropod challenger prevails, don’t expect it to play nice



By Benoit Guenard

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

Invading ants make up just handfuls of the more than 12,400 described ant species in the world, says Jes Søe Pedersen, an associate professor at the Center for Social Evolution at the University of Copenhagen in Denmark. Yet, their impact on ecosystems, human health and the economy far outstrips their Lilliputian size. Red fire ants can endanger the lives of those who unwittingly stumble on a nest. Some invasive species are agricultural pests or “farm” voracious plant-eating aphids to milk them for their sugar-laden excrement.

Invasive ants often kill, eat or outcompete native ant species—the latter of which play key roles in the ecosystems where they make their homes. Many native ants are gardeners—they till the soil and plant seeds. Alien ants that come from a different environment do not pick up the jobs of those they push out.

The invaders can also create an ecosystem meltdown: Yellow crazy ants that invaded Christmas Island aggravated native birds so much that they have changed their eating habits and feast on the island’s famous red crabs. Food competition from the ants, combined with other invasive species such as feral cats and black rats, may have led to the extinction of the Christmas Island pipistrelle bat.

Figuring out an invading species’s impact on an ecosystem is tricky. Researchers rarely observe an invasion in progress. But this time may be different. The opening salvos between the newly recognized invasive Asian needle ant and the more notorious Argentine ant offer just that opportunity, Pedersen says.

Researchers at North Carolina State University in Raleigh unexpectedly stumbled on the unfolding ant war several years ago. Eleanor Spicer Rice, a graduate student studying entomology at the time, was tracking a network of Argentine ant nests in an office park in North Carolina and found a few nests of Asian needle ants. “It is really weird that another ant could be nesting within the Argentine territory,” Spicer Rice says. Argentine ants do not tolerate competition. Typically, they are able to push other ants out of an area by attacking the rivals and dominating food sources. Originally from South America, they form massive interrelated networks of nests called super colonies.

The situation seemed unprecedented. The largest ant colony in the world is an Argentine ant super colony spanning more than 6,000 kilometers in the Mediterranean region. For some reason, across a few square miles of North Carolina the Argentine ants’ world-conquering strategy was not working. The Asian needle ants were, in fact, gaining ground. In March 2009 Spicer Rice and her colleagues found Argentine ants at 90 trees, sharing nine of them with the Asian ants. By June 2011, the Argentine ants had been driven back to 67 trees, were neighbors with the interlopers in 15 and the Asian needle ants had taken over 17 of the sites.

To see if the Asian needle ants were driving the other species away, the researchers selectively poisoned the Asian needle ants in some areas. The Argentine intruders returned, indicating that predators, resource availability or other factors were not behind their initial retreat.

Although the Asian needle ants did kill more Argentine ants once they had claimed territory, the researchers wondered how the less numerous insects were gaining a leg—or six—up. Cold-tolerance tests in the lab hinted that Asian needle ants were better adapted to the temperate North Carolina climate than the tropical Argentine ants. The Asian needle ants shook off their winter sluggishness as early as March whereas the Argentine ants did not resume activities until late April or early May, the researchers reported on February 8 in PLoS ONE.

An Asian needle ant takeover would not only be bad news for Argentines but also native ants—and for humans as well. Their burning stings 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.

The fierceness of the Argentines wouldn’t allow them to be pegged as pushovers and suggests that understanding how invasive species react to new climate is important to predict their spread. The Asian needle ant may be winning the war in North Carolina simply because the state is at the northern limit of the Argentine ants’ range, Spicer Rice says—or it could be poised to become the next threatening invasive species.

Another project led by N.C. State researchers could reveal if Asian needle ants are the next big ant threat. The project, School of Ants, asks citizen scientists to help catalogue the species in urban areas by sending in the insects they find. The researchers are particularly interested in the spread of the Asian needle ant and have discovered specimens from locations as widely separated as New York City and Washington State.

The first record of Asian needle ants in the U.S. dates from the 1930s in Louisiana, indicating the insect was probably a ship stowaway. Like the Argentine ant, the Asian needlers form colonies with thousands of workers and many queens, Pedersen says. Unlike other species that fly to new locations, invasive ants are able to mate within a single colony. As a result, a single cup of soil, perhaps stored with vegetables or flowers, can hold enough individuals to gain a foothold on a new continent.

Climate change, human movements and disrupted habitats offer new opportunities for invading ants, Pedersen says. “In general we will see more ant species being invasive, and here we already have a new member of the crew—the Asian needle ant,” he says.

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