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  National Geographic 21 Dec 05
USDA Fights Invasive Fire Ants With Flies
Maryann Mott for National Geographic News

Not all good things come in small packages. The red imported fire ant--a mere quarter of an inch (less than a centimeter) in size--is a 6-billion-dollar-a-year problem in the United States.

The reddish-brown venomous insects short-circuit appliances, damage traffic lights, sting people, and threaten endangered bird and reptile species.

To get the burgeoning ant population under control, entomologists at the U.S. Department of Agriculture's (USDA) Imported Fire Ant and Household Insects Research Unit in Gainesville, Florida, are using a tiny winged warrior: the phorid fly.

The flies swoop down on ants, depositing an egg underneath their skin. Within days fly maggots are born and release an enzyme that decapitates their ant host. The fly then completes its development using the ant's head as a safe hideaway. When the ants see the adult flies, they run and hide, freeze, or twist upside down to avoid getting stung, says Sanford Porter, an entomologist at the USDA's research unit in Gainesville.

Formidable Foe

Red imported fire ants (Solenopsis invicta) were accidentally carried over on ships from South America in the early 1900s. The invaders now infest 320 million acres (129.5 million hectares) in 14 states, including Texas, California, New Mexico, and Puerto Rico.

Fire ants are a formidable foe because of their high reproductive rate. A single-queen colony can have 250,000 workers. In some states there are up to 10 colonies per acre, each producing 5,000 offspring annually.

Robert Vander Meer works with Porter at the USDA's insect unit in Gainsville. In Florida, he said, the most common reason air conditioners break down is because fire ants short-circuit the electrical components. They also gnaw on wires for sprinkler systems and traffic lights.

Scientists are not sure why the ants are drawn to electrical systems, but the strange attraction wreaks havoc and is considered a major economic impact of the insects' spread.

The aggressive ants also swarm over people, stinging multiple times with venom that induces a painful, fiery sensation. A white, fluid-filled pustule or blister develops a few days after the sting. Some 500,000 people in the Southeast U.S. are at even greater risk because they are highly sensitive or allergic to the ant's venom, Porter said.

Fire ants feed on almost any plant or animal material, including vulnerable reptile and ground-bird hatchlings. Fire ants are tied with snakes as the number one predator of eggs of the black-capped vireo, a ground-nesting bird, Vander Meer said. A large population of the endangered bird is located at the U.S. Army's Fort Hood military installation in Texas.

"We have been working with military personnel there to develop repellents that will keep the ants away from the nests, and so far that looks like it's been very effective," he said.

Safe Biocontrol

USDA scientists are working on improving current bait systems and developing novel population suppression methods as alternatives to costly and environmentally damaging pesticides.

Porter first discovered phorid flies in 1989 while doing research in South America, where fire ant population levels are about five to ten times less than in the United States because of natural enemies.

In all, about 20 different species of phorid flies are known to attack the ants. A single species is unlikely to produce the desired impact on fire ant populations, Porter said, so a variety of the barely visible insects will be introduced.

Since 1997 three fly varieties have been released in almost all Southeastern states. Porter plans to introduce a fourth within the next month.

The flies are not a danger to animals or people. "They're about as safe a biocontrol agent as you can get," Porter said. He hopes to start seeing a reduction in fire ant populations within five years. "If we can bring natural enemies from South America here, we can reduce the ant population by 80 percent," Vander Meer said. "And that would be without using any chemical control at all."
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