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Global travelers. Red imported fire ant workers tend their brood and their queen (inset).

Kenneth G. Ross

Fire Ants Using U.S. as Staging Ground for Global Invasion

For a tiny insect, red fire ants pack a nasty sting. Stepping on a nest can put you in the hospital. The ants originally hail from South America and spread to the United States 75 years ago. Now genetic studies show that the southern United States, not these native lands in South America, is the source of multiple new invasions around the globe. Pinning down where these ants are coming from may help countries slow the spread of the insects before they do too much damage.

In the southern United States, fire ants have been a menace since they snuck into North America in the 1930s from Argentina, most likely as cargo stowaways. They spread rapidly in Florida, Louisiana, and other southern states, driving out other ants and disrupting the landscape with prolific, aggressive colonies. With no natural enemies, their densities reached 10 to 100 times that of Argentina. Each year, they exact a $6 billion toll from property damage, medical treatments, and control costs.

For many years, the fire ants seemed confined to states such as Florida and Texas. But in the 1990s, they made it to California, hitchhiking on the pallets used to transport honeybees moved from farm to farm to pollinate crops. And recently, fire ants have shown up in Australia, Taiwan, China, and the Caribbean. They have the potential to spread over half the land in the world, seemingly limited by dry climates and freezing weather.

Entomologist Kenneth Ross of the University of Georgia, Athens, and his colleagues wanted to know whether fire ants in Asia and elsewhere also came from South America or whether they were transplants from the United States. The researchers collected ants from 2144 colonies in 75 places around the world. They then analyzed the genetic material of one ant from each colony to classify the populations into related groups.

The analysis revealed the existence of more than 322 groups of fire ants. Most of those groups were found only in South America, where the ant was native. There, the ants have existed long enough to have evolved this diversity. Just three of the 322 groups made up the majority of the ants in the United States. But those same three groups were found in newly invaded areas, such as Taiwan and Australia, Ross and colleagues report online today in Science, indicating that the ants came from these southern states, not South America.

The analysis also revealed that fire ants successfully emigrated from the southern United States at least nine times. They invaded Taiwan, China, and California twice, and they settled successfully three times in Australia. It looks like California was a steppingstone for one invasion of Taiwan, the group reports.

"The fact that they have been repeatedly introduced from the U.S. to new areas halfway around the world, all within the last decade, is rather alarming," says Ed Vargo, an entomologist at North Carolina State University in Raleigh. "I hope [the U.S.] government will work together with other countries to identify how they are being transported and develop ways to prevent further spread."

Fire ants often stow away in dirt in nursery stock or on pallets or earth-moving equipment. They can persist for months without food and will even eat their young to prevent starving. And they thrive in areas where the natural vegetation has been disrupted, which is where they are likely to land once transported.

Heavy global trade and travel have increased the likelihood that they can catch a ride abroad. There, the chances of surviving are low, but the red imported fire ants now thriving in the southern United States have already shown that they have what it takes to move to new territory, says Walter Tschinkel, an entomologist at Florida State University, Tallahassee.

Given these attributes, he's not optimistic about control measures. "It is hard to imagine what kind of draconian policy would prevent or reduce their spread," says Tschinkel. "It would be better to use resources for conserving, protecting, and rehabilitating native habitats." Healthy ecosystems, he says, are not as susceptible to fire ant invasions.