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Invasive Asian needle ants thriving, spreading in U.S.

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Asian needle ants look innocent but they pack a painful sting (Image: Benoit Guenard)

There's a new invasive ant species on the block and it could mean big trouble for people and the environment. The Asian needle ant has been in the U.S. since the 1930s, but their population has exploded in the past 8 years. These stinging ants are spreading rapidly and displacing another invasive ant species, the aggressive Argentine ant, in forests and backyards across the country.

Researchers from North Carolina State University published their findings in the journal PLOS ONE. The study began in 2008, when lead author Eleanor Spicer Rice noticed a number of Asian needle ants (Pachycondyla chinensis) living and working near an Argentine ant supercolony in Raleigh, North Carolina. The observation surprised Spicer Rice, since Argentine ants (Linepithema humile) are territorial and aggressive toward intruders. She decided to investigate further.

Over the next four years, Spicer Rice and Jules Silverman found that Argentine ants seemed to ignore Asian needle ants, and the Asian needle ants used this to their advantage, displacing significant portions of the Argentine ant population. Between 2008 and 2011, the researchers observed a drop in the presence of Argentine ants at their study sites, from being present in 99% of the sites down to 67%, while the Asian needle ant expanded from 9 percent to 32 percent of the sites. Both species overlapped in about 15 percent of the sites.

The Argentine ant has been a very successful invader. The aggressive, territorial ants — which can live in super-colonies consisting of thousands of queens and millions of workers — easily displaced native species as they spread across the U.S. No ant species has successfully pushed back — until now.

So what gives Asian needle ants an edge over the competition? Spicer Rice and Silverman believe the needle ant's ability to tolerate cooler temperatures is a major factor in their success. During the winter, both species become dormant and limit their activity, halting reproduction and driving their populations down temporarily. However, the Asian needle ants wake up and become active again much earlier. They get a head start, beginning to reproduce, forage for food, and build new nests in Argentine ant territory as early as March, while the Argentine ants take another one to two months to wake up.

Asian needle ants have other behavioral traits that work to their advantage in gaining new territory. One of these is flexibility. Asian needle ants are not picky about where they live. In forested areas, they nest in logs or under rocks and leaves. In suburban and urban neighborhoods, they can nest anywhere from potted plants to piles of mulch, or even underneath doormats. Colonies can range from a few dozen ants to a few thousand, and larger colonies can live in one big nest or several small ones. Asian needle ants are also not fussy when it comes to food. They love termites, but will also scavenge for other ants, dead or dying insects, and even human garbage.

This behavioral versatility is allowing the Asian needle ant to move into forests, rural areas, and urban environments at the same time. And because it can tolerate cooler temperatures, it could spread into a broad range of territory. Already, Asian needle ants have been found in Alabama, New York City, and Washington, among other areas.

The booming population of Asian needle ants is bad news for more than just Argentine ants. The invaders appear to be driving out native ant species in forests. These native species play important roles in the ecosystem (for instance, by dispersing seeds) and the loss of such ant species can pull the forest apart at the seams. Asian needle ants don't play well with other ants: when they move in, they eat other ants, devour their food sources, and take up their nesting spaces.

The invasive ants can also be a pain for the humans they live among — literally. Asian needle ants have venomous stings that can cause allergic reactions in some people. At Dr. Eleanor's Book of Common Ants, Eleanor Spicer Rice describes her first encounter with an Asian needle ant sting:

"At first, I felt a slight burning sensation right where she stung me. About an hour later, the burn spread out to an area about the size of a quarter around the sting, and it began to feel a little like being stabbed with pins. This flash of sharp pain followed by a dull nerve ache continued for the next two weeks every time I touched the area around the sting. For those of us not allergic to Asian needle ants, that's the worst part of Asian needle ant stings."

Spicer Rice says people need to be aware of what Asian needle ants are and that they could be closer than they think. The ants look innocuous: slender, shiny, black with lighter orange legs, and only about 0.2 inches long. Spicer Rice and Silverman are also the co-authors, along with Jonathan Shik, of a paper showing that toxic baits are effective at killing Asian needle ants. If widely used, this strategy could slow their spread.

And Asian needle ants are spreading at an alarming rate. Spicer Rice works on a citizen science project called School of Ants where people send in ants collected from their neighborhoods to North Carolina State University for identification. Asian needle ants have been coming in from all over the U.S. and are now the most common ants sent in to the project.

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Asian needle ants look innocent but they pack a painful sting (Image: Benoit Guenard)
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Mary Bates, Ph.D.