Genetic evidence now spotlights the United States as the source of recent fire ant invasions in the rest of the world.
The aggressive, stinging fire ants (Solenopsis invicta) aren’t native to the United States but rather to a broad swath of South America. Yet the southern United States, invaded by fire ants in the 1930s, has sent off at least eight separate waves of fire ant invasions to other countries in recent years, says entomologist Kenneth Ross of the University of Georgia in Athens. A ninth invasion probably hopscotched from the South to California before hitting Taiwan.
“It’s not good news,” Ross says. These waves of ants are now colonizing the Caribbean, Australia, New Zealand, Taiwan and China, including Hong Kong and Macau, he and his colleagues report in the Feb. 25 Science.
“This study is going to cause quite a stir,” says geneticist Michael Goodisman of Georgia Tech in Atlanta, who studies a different internationally invasive ant. The new fire ant study, he notes, “could have important trade and travel implications.”
Regardless of any furor, the study is a valuable step in dealing with the problem, says another invasive-ant biologist, Ben Hoffmann in Darwin, Australia, with the country’s CSIRO research service. “We need to know how invasions spread to be able to prevent spread and effectively manage invasions.”
Biologists had certainly considered this United States–bridgehead scenario of invasions, Ross says, “but without data, it was anybody’s guess.” To track the invasions, an international research team analyzed ants from 2,144 colonies in a total of 75 places in 11 countries and looked at several kinds of genetic information, including dozens of DNA markers.
“Most studies don’t come close to those numbers,” says Goodisman.
Ross explains that looking closely at fire ants in their native range in South America revealed 322 distinct genetic types. Only 11 of those types were found in the southern United States, including three that were very rare in the native range. Yet the populations from newly invaded territories had combinations of the three rare variants from those U.S. types, not the others left behind in South America. Additionally, the researchers ran computer models of how gene patterns in populations change as invaders bud off into new territories. The scenarios that fit the data best, alas, showed the United States as the source, Ross says.
This analysis raises the possibility that the rigors of invading the United States and then of moving on toward world domination have winnowed out weaklings and less invasive ants. Populations now erupting from the United States could be specially adapted as super-invaders, Ross says.
Even if the insects don’t have special adaptations, basic fire ant biology gives the species some tricks for traveling, Ross says. In the ants’ native range, they survive flooding by fleeing their nests with their young and gripping each other to create a living raft of ants that floats until the flood subsides. If they’re afloat for longer than they can survive without food, adults eat the young. Such a capacity for fasting allows fire ants to endure days or even weeks as international stowaways in any kind of cargo.
For in-country travel, young fire ant queens flying off to find mates may end up wafting onto trucks or other ground transportation that take the queens on detours. Aerial queens can reach substantial numbers, Ross says. “We can go into a parking lot and collect 5,000 of them in an afternoon.”
