Ascunce said the scientists were surprised by the findings.

“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.

The study results show the problematic side of a robust global trade and travel network.

DeWayne Shoemaker, a U.S. Department of Agriculture scientist affiliated with UF’s Institute of Food and Agricultural Sciences who was senior author and lead investigator on the grant that funded the study, said pinning down precise origins for the ants is a huge win because it helps scientists know where to look to find the most effective biological control agents, such as phorid flies.

Since the late 1990s, scientists have been releasing phorid flies to help control the ants while reducing use of pesticides. The flies hover over mounds before injecting an egg into an ant. When the egg hatches, the maggot develops in the ant’s head, eventually decapitating it. The maggot turns into a fly and the cycle repeats.

Shoemaker, a key member of the research team that sequenced the complete genome of the red imported fire ant earlier this year, said the team collected ants from 2,144 colonies at 75 geographic sites. From there, they used multiple genetic tests—including some similar to human paternity tests — to determine the ants’ origin with high confidence levels.

“I really think our power to distinguish … hinged on us having such a large data set,” he said. “I don’t think we’d have had the statistical power to come up with these kinds of conclusions otherwise. All of these conclusions are highly supported by data.”

It is widely believed the red imported fire ant first entered the U.S. in the 1930s through the port of Mobile, Ala., on cargo ships, possibly in dirt used as ballast.

Other team members included Cheng-Jen Shih and Wen-Jer Wu, both of National Taiwan University; Jane Oakey of Biosecurity Queensland, Australia; Luis Calcaterra of the USDA-ARS South American Biological Control Laboratory in Argentina; Jérôme Goudet, of the University of Lausanne in Switzerland and Kenneth Ross of the University of Georgia.