The aggressive, stinging ants arrived from South America and got a foothold in the United States in the 1930s, gradually spreading across the South. Now, researchers say, the United States has become the springboard in the last 20 years for the ants to invade such distant locations as China, Taiwan, Australia and New Zealand.
Genetic tests on fire ants in the newly invaded countries found they came from the United States, rather than South America, an international team of researchers reports in Friday's edition of the journal Science.
The genetic profile of all these ants is closer to fire ants in the U.S., explained Marina Ascunce of the Florida Museum of Natural History at the University of Florida.
"I thought that at least one of the populations in the newly invaded areas would have come from South America, but all of the genetic data suggest the most likely source in virtually every case was the southern U.S.," she said in a telephone interview.
That discovery may help prevent further spread of the pests.
"By knowing where they are coming from, biological controls can be more focused," Ascunce said. "We can also improve screening or monitoring in source areas or on key transportation routes."
Larry Gilbert, director of the Brackenridge Field Laboratory at the University of Texas at Austin, said: "Although those of us who research fire ants are not surprised at the finding, this is the first hard evidence of the sources of worldwide fire ant spread.
"Various exotic invasive pests have been and are being transported to the U.S. by commerce, so it's ironic that one of our pests has used the southern U.S. as staging area, and then as a port of exit for worldwide destinations. This is a sad and disturbing result, but one that needs to be addressed, not only in the U.S. but in more recently infected countries," said Gilbert, who was not part of the research team.
He said the study results emphasize the need to find the most common ways ants hitchhike across deserts and oceans and to develop better ways to intercept the pests.
Ascunce noted that monitoring in South America has blocked shipments of from that region to Asia when fire ants were discovered.
Lead study researcher DeWayne Shoemaker, a U.S. Department of Agriculture scientist affiliated with the University of Florida, said the research team sequenced the complete genome of the red imported fire ant earlier this year. He said the team collected ants from 2,144 colonies at 75 geographic sites and used multiple genetic tests to determine the ants' origin.
Scientists think the red imported fire ant first entered the U.S. in the 1930s through the Port of Mobile in Alabama on cargo ships, possibly in soil used as ballast. They are easily transported, Ascunce noted.
Now, their economic impact on the United States alone is estimated at $6 billion annually, including agricultural damage, efforts to control the pests and medical costs, the researchers said.
The study was supported by the U.S. Department of Agriculture, Taiwan Council of Agriculture and National Science Council of Taiwan.