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Taking the Bite Out of Fire Ants: Biologists battle destructive imported ants with vampire-like flies


Not far west of the University of Texas at Austin campus, a mosaic of neighborhoods stretches south for miles toward Town Lake, which runs through the heart of Austin. Tarrytown, Enfield and other neighborhoods in this area contain closely spaced homes that range from turn-of-the-century stone mansions to simpler, clapboard houses, all surrounded by stately oaks and well-manicured lawns. Less visible to passersby are low mounds of soil here and there on these lawns, evidence of what may be the best remaining stronghold of native fire ants that have been Texas residents for millions of years.

Female Brazilian phorid fly targets imported fire ants
Fire ants in North and South America are attacked by specific types of phorid flies based on ant size and chemical aroma. Here, a female Brazilian phorid fly targets imported fire ants.

“That area of Austin probably has the biggest patch of native fire ants left in the southern U.S.,” said Professor Larry Gilbert, who has taken on a foreign fire ant that is wiping out the natives’ territory throughout Texas and nearby states.

Born in Laredo to a Presbyterian minister who kept the family moving between small towns, Gilbert spent his youth outdoors, hunting, collecting butterflies and spending summers helping a grandfather tend beehives. His interest in nature led to a population biology degree at Stanford University, his post as director of The University of Texas at Austin’s two biology field stations and his fight against Brazilian fire ants that were first found in Texas in the 1960s.

Gilbert became concerned about imported fire ants while he was studying butterflies and focusing on keeping armadillos, native ants and other wildlife healthy at the College of Natural Sciences’ Brackenridge Field Laboratory on Town Lake’s northern shore.

“I’m a native Texan,” he said, “and I feel a great loyalty to the habitat in Texas.”

So when he saw native fire ant mounds there being rapidly replaced by those of Brazilian imports in 1981, he wanted to do something about it. The lanky ecologist has been digging deeper into fire ant ecology ever since.

The foreign ants that reached the southern U.S. in cargo boats in the 1920s don’t look or act very differently from native counterparts. But the Brazilian insects produce young more rapidly, which means more mouths to feed. As a result, the ants will even attack baby birds and other critters, such as Harvester ants favored as food by the state’s dwindling number of horned lizards. The imports also can harm livestock and objects such as underground circuit boxes near their nests, leading to an estimated $1.2 billion in damage annually in Texas.

Larry Gilbert inspects an imported fire ant mound at Brackenridge Field Laboratory
Integrative biologist Larry Gilbert inspects an imported fire ant mound at Brackenridge Field Laboratory, which abuts Town Lake, part of the Colorado River.

Taking the “know thy enemy” approach to heart, Gilbert and his postdoctoral students first determined what they were dealing with in Texas: a multiple-queen form of fire ant colonies with up to 200 queens. They also established how imported queen ants establish new colonies so successfully.

“My postdocs dug out their nests, separated out the queens and put fine, colored wire—unbelievably tiny stuff—around their waists to identify them at new mounds,” Gilbert said.

Using these marked queens, the researchers demonstrated the first example of ant colonies spreading by “budding.” Unlike imported fire ants in many southern states and in South America, these pests didn’t start new nests as the result of a single queen. In Texas, the imports spread like an advancing tank brigade, with several of a mound’s queens leaving home with a host of worker ants to set up new mounds.

Because pesticides can harm native ants as well, Gilbert became interested in more ecological approaches. He has now spent more than a decade fighting the Brazilian ants with a vampire-like fly.

A phorid fly hovers over a fire ant target until the fly can dart in and inject an egg through a gap in the ant’s outer armor. The egg grows into a larva, which eventually moves into the ant’s head. There, it consumes muscle tissue that causes the head to fall off within 10 days.

Since 1994, Gilbert has been raising and testing specific types of decapitating phorid flies in areas of Austin, southern Texas and other parts of the state. The fly from Brazil he has favored so far prefers to catch imported ants during fights at disturbed mounds.

Phorid Fly Expansion, Austin and Travis County, Texas, Fall 2003.  From three areas: Horsethief Hollow Ranch, Brackenridge Field Laboratory, Indiangrass Nature Preserve at Lake Walter E. Long
Gilbert and colleagues found Brazilian phorid flies at new sites in fall 2003 (yellow triangles). The flies were originally released at sites prior to February 2003 (white circles). Native fire ants still exist in part of Austin, based on 2002-2003 sightings (blue dots). Aerial photo: Texas Natural Resources Information System.

Years of low rainfall and temperatures that were hotter than those in their native homeland kept the experimentally introduced phorid fly populations from establishing and spreading. But Gilbert’s team kept releasing the flies each year, discovering along the way that part of this particular phorid fly’s difficulty was its preference to produce mostly female offspring from eggs laid in larger workers, an example of what is called environmental sex determination.

That process was unfortunate, because the multiple-queen colonies possess relatively few larger workers. This led Gilbert to suspect that this particular phorid in Texas is restricted to producing eggs that mostly become male flies, limiting mating opportunities.

Despite the challenges this biology presented to phorid reproduction here, Gilbert finally hit pay dirt in June 2003 after a string of rainy months. He and his team found dozens of the flies circling over imported ant mounds he kicked at Brackenridge Field Laboratory and at the City of Austin’s Indiangrass Nature Preserve and a private ranch along Bull Creek used for phorid release sites.

“Just their mere presence changes the scene,” he said, referring to the jittery behavior the imported ants develop around a predatory phorid fly.

Fire ant mounds in Texas cow pasture
Fire ants often establish mounds in sunny areas, such as this Texas cow pasture. Photo: Texas A&M University.

The imported phorid flies in the Austin area spread so well that they were estimated to cover from 70,000 to 150,000 acres last June. They also took hold in Caldwell, Texas, where colleague Charles Barr at Texas A&M University conducted independent releases. Gilbert is hopeful that enough survived the past mild winter to reach similar numbers this summer and fall. He continues to evaluate and release flies at multiple Texas sites.

But most of the native fire ants are already gone from Brackenridge, where he estimates only a single native-ant mound can be found on the 88 acres, unlike the old urban neighborhoods of Austin a few blocks away.

To make things tougher on the imported ants, Gilbert and his colleagues are working on other South American phorid flies that will attack the pest fire ants at sites other than disturbed mounds. Some come from more Texas-like climates in Argentina, and some produce more female offspring.

Larry Gilbert holds ants over a sieve used for sorting
In Larry Gilbert’s lab, imported fire ants are sorted using a sieve into proper-size targets for stocks of various phorid flies under study in Texas.

Just as doctors attack advanced cancers with multiple treatments, the strategy is to stress the imported ants with a flying menagerie of various phorids that work at different times of the day to disrupt food foraging and other activities.

That approach essentially mimics what occurs in different parts of South America, where phorid researchers have documented up to six types of phorid flies locally that keep their host ants scrambling for cover during daylight hours.

“It could take decades to work out the right combination of control agents in Texas, phorids and otherwise,” Gilbert said, noting that his main goal at age 61 is to be alive to see the biocontrol approach work.

Meanwhile, he hopes to find time to tackle remaining questions about the intricate biology of his favored subjects. For instance, he’s still curious why native ants find protection in Austin’s urban mosaic, where some feature—perhaps the roadblock real roads provide to the mass-invasion approach of imported fire ants—has kept them at bay.

“At Brackenridge, the University of Texas has gone through all this trouble to preserve the habitat, and yet the native fire ants, and most Texas harvester ant colonies have been wiped out,” he said, a bemused grin on his face. “The same is true on most ranches and Nature Conservancy reserves. Then we go over into Tarrytown, where everybody takes care of their yards and perhaps treats any ant mound they find with pesticides, and you’ve got the native fire ants surviving.”

The irony of that finding isn’t lost on Gilbert, but you can see the wheels turning as he envisions urban studies that could provide new clues for undoing the imported fire ants’ grip on his home state.

“I’m still optimistic that we haven’t gotten to the bottom of the barrel of possibilities,” he said. “We may just need a subtle shift in imported fire ant competitive advantage, reproductive rate and invasion potential to control them.”

Barbra Rodriguez

Photos of Professor Gilbert: Marsha Miller

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  Updated 2014 October 13
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