Invasive Fire Ants Have Established Themselves in the U.S.—And They’re Not Stopping Here

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Red invasive fire ants

I’ve written a few times in the past about invasive Asian carp, the Chinese natives who were imported for fish farms in the Midwest, only to escape and make their way up the Mississippi River. They’re now knocking on the door of the Great Lakes, and a few of them may have even slipped past defenses and made it into Lake Michigan. The carp themselves aren’t really dangerous to humans—though when startled, they can turn into unguided surface to surface missiles—but they’re still the target of a $78.5 million defense program by the federal government. They eat and eat and eat, so much so that scientists worry they could crowd out other species if they managed to establish themselves in the Great Lakes.


But Asian carp are Cub Scouts compared to another fearsome invasive species: the fire ant. The aggressive, stinging ants arrived in the U.S. from South America in the 1930s, and they’ve gradually established themselves throughout the South. They are nasty creatures—they inject an alkaloid venom when they sting, creating painful, itchy pustules in human victims. About 5 million people in the U.S. are stung each year, with 25,000 people requiring medical help, often due to allergic reactions. The attacks usually occur when an unwary victim disturbs a mound, causing dozens or even hundreds of worker ants to swarm out of the nest and bite exposed skin. The fire ants are dangerous to the bottom line too—the economic impact of attacks cost $6 billion a year.

Like many invasive species, the fire ants came as stowaways aboard a ship, landing in the Gulf seaport of Mobile, Alabama and spreading out from there. It turns out, however, that the fire ants aren’t done campaigning. A new study in Science charts the history of fire ant invasions from around the world, highlighting incursions in California, Taiwan, China and Australia over the past 20 years. The researchers—led by Marina Ascunce of the University of Florida—looked at 2,144 fire ant colonies from 75 locations around the world, and found out something surprising. The ants weren’t coming from their original habitat in South America. These ants were invading the world from their U.S. beachhead. As Ascunce told the Associated Press:

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.

Indeed, tests of the fire ants in other parts of the world show that they share more genetic information with American-established ants then the originals in South America. Nor are these new invasions a one time campaign—the study shows that fire ants have been introduced to California, Asia and Australia no fewer than nine times in recent years, after decades when they were largely confined to the southern U.S. The repeated introductions of invasive species from a single source population is called the “invasive bridgehead” effect, and it has some unsettling implications for the future of the fire ant, as the study makes clear:

For instance, our finding of repeated successful introductions of S. invicta from the southern United States suggests that particular population traits associated with its success there may have pre-adapted these ants for ready colonization of other areas. Alternatively, the repeated introductions could reflect higher propagule pressure from the United States relative to native areas, given that the probability of introductions increases with escalating traffic flow in global transportation networks (7).

Translation: as globalization pulls us all tighter, invasive species will be on the march, hitchhiking aboard container ships or plant exports or even as exotic pets. In the U.S. alone, invasive species cost an estimated $120 billion worth of damage annually, with the government spending hundreds of millions to combat them—and often losing the fight. The good news is that studies like this one will help scientists pinpoint exactly where invasive species are invading from, which should help policymakers devise smarter containment programs. Think of it as intelligence in the never-ending war against invasive species. But we better not let our guard down—the Science study also shows that fire ants could be capable of establishing themselves across almost half the planet.

But there’s one silver lining—at least they’re not giant space ants: