"It's just exploded in the last 10 years," said one of the researchers, University of Georgia entomologist Kenneth Ross.
Working with former UGA student DeWayne Shoemaker, Ross and other scientists compared genetic material from red imported fire ants in the United States to fire ants that have showed up over the past decade in Australia, Taiwan, Hong Kong and China.
They also looked at genetic material from colonies in the ants' native territory, Argentina.
The researchers concluded that the fire ants now infesting Asia and Australia came from the American South, with one exception. One type of fire ant that has become established in Taiwan actually came from California - but the California ants had been accidentally imported from the Southern United States earlier, so they're ultimately from the South, too.
Fire ants recently discovered in New Zealand are almost certainly from the United States as well, Ross said.
Shoemaker, Ross and other scientists took genetic material from more than 2,000 ant colonies in 11 countries to track the stinging ants' global travels.
The study demonstrates the power of recently developed methods of genetic analysis, Ross said. Scientists are using similar techniques to map waves of human migration across the globe over the centuries.
But the scientists also see a frightening side effect from swelling global trade at the end of the 20th century and the beginning of the 21st, he said.
The fire ant seemed hunkered down in Georgia and other parts of the American South for decades after it was accidentally brought to Mobile, Ala., on a South American cargo ship in the late 1930s.
Just in the past 10 years, the United States has become a transportation hub for the fire ant, sending it across the Pacific to at least three and probably five countries.
"I've had some people suggest to me that it's bioterrorists, or even malicious tourists. But it's almost certainly the case that it's large-scale shipping," Ross said.
The fire ant is by no means the only unwanted animal or plant species the U.S. has accidentally exported over the past decade or so. But the stinging ant is one of the most striking examples, Ross said.
"Once they get established, they move around easily, by their own dispersal abilities but also by human activities," he said.
Hidden in a cargo container, the ants can survive a long time with just a little moisture. If they get hungry enough, the adults will cannibalize the young.
"They're really well adapted to surviving long, arduous journeys under difficult circumstances," Ross said.
Observers reported floating rafts of fire ants in the floods that followed Hurricane Katrina - another well-documented survival mechanism that lets fire ants move to dry land and safety during floods, Ross said.
As global trade accelerates, opportunities are rising for fire ants and other invasive species to move from country to country, Ross said.
"It's a very difficult situation, even if we were all in agreement that we wanted to deal with this issue - and it's not clear everyone does," Ross said. "The question becomes, how in the world do you do it with the scale of global commerce?"