Invasion of the fire ants! (Blame Dixie)

U. FLORIDA (US) — The worldwide explosion of red imported fire ants can be traced all but entirely to the southern United States.

The findings could prove helpful in finding new ways to control Solenopsis invicta, researchers say. Americans spend more than $6 billion a year to control the ants and offset damage they cause, including medical expenses and $750 million in agricultural losses.

Native to South America, the ant had been contained there and in the southeastern U.S. before turning up in faraway places in the last 20 years—including California, China, Taiwan, Australia, and New Zealand.

Red imported fire ants are highly aggressive, have a painful sting, and are often only discovered after someone steps on a mound.

“Fire ants are very annoying pests, and they cause people to suffer,” says Marina Ascunce, a postdoctoral associate at the University of Florida. “People who are allergic can die (from ant stings).”

Details of the study are reported in the journal Science.

Using several types of molecular genetic markers to trace the origins of ants in nine locations where recent invasions occurred, researchers traced all but one of the invasions to the southern U.S.

The exception was an instance where the ants moved from the southeastern U.S. to California, then to Taiwan.

“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.,” says Ascunce.

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

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, says DeWayne Shoemaker, a U.S. Department of Agriculture scientist affiliated with the University of Florida.

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.

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.

“I really think our power to distinguish…hinged on us having such a large data set,” Shoemaker says. “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.

Researchers from the University of Georgia, National Taiwan University; and the University of Lausanne in Switzerland contributed to the research.

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