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Alien Flies to Extinguish Alien Fire Ant Invasion?

John Roach
for National Geographic News
June 18, 2004
 
Importing aliens to attack invading aliens may sound like a plot for a science fiction movie, but scientists believe the scheme is the best way to combat Brazilian fire ants that have taken over the southern U.S.

The red imported fire ant (RIFA), scientifically known as Solenopsis invicta, first arrived on U.S. shores over 60 years ago as stowaways aboard cargo ships from South America. Since then they have spread throughout the South, out-competing the native fire ants in their paths.


"RIFA has such high densities relative to its native counterpart and other native ants probably because it has escaped most of its specific natural enemies, mainly pathogens and parasites," said Larry Gilbert, an ecologist at the University of Texas at Austin.

With no natural enemies in the South, RIFAs have been on a rampage. They are stinging to death baby animals born in the fields, posing a health risk to humans, and laying nests in underground circuit boxes, where the ants cause short circuits and fires.

According to researchers with the U.S. Department of Agriculture, the invading ants ring up an estimated 1.2 billion dollars' worth of damage each year in Texas, where the RIFA takeover is particularly fierce.

Gilbert is exploring ways to control the fire ants. One solution that he believes shows promise is importing other insects from Brazil—parasitic flies known as phorids. Phorids kill the ants by laying eggs in them.

Fire Ant Control

A female phorid fly hovers over ants at disturbed mounds or as the ants forage along a trail. She selects an individual in which to lay her egg. She then dives in, injects her egg inside the ant, and darts away. The egg develops in the ant's throat, killing the ant in about ten days.

The phorid larva then moves into the ant's head, which falls off. There, the larva develops into a pupa, more closely resembling an adult phorid. About 40 days later the phorid fly emerges into the world from a pile of dead ants.

Although the flies only kill a small fraction of ants this way, the ants seem to be so afraid of the flies that their mere presence is thought to disrupt the ants' foraging behavior. These interruptions slowly reduce the rate at which new RIFA colonies form.

Gilbert likens this process to a caterpillar eating the leaves of an oak tree. While the caterpillar will not kill the oak tree, the loss of leaves means the trees produce fewer acorns. Thus, over time, new oak trees sprout less frequently than do competing trees.

"I view ants in a parallel way and thus anticipate that the impact of phorids mostly likely will be a slow decline of fire ants relative to native ants," Gilbert said.

Donald Feener is an ecologist at the University of Utah in Salt Lake City who studies the relationship between parasitic flies and ants. He said that while he has concerns about the research methods of Gilbert and his colleagues, their concept "is a great idea."

"In ecological terms, the flies act as indirect effects in ant communities because they change how the ants interact with one another, especially when ants compete with one another," Feener said.

Finding Flies

There are more than 20,000 different species of phorid flies, about 50 of which attack fire ants.

Some of these phorid species attack the fire ants of the geminata group, which includes the fire ant species native to the southern U.S. Others attack fire ants in saevissima group, which includes the RIFA and other species from South America.

According to Feener, most of the species that attack fire ants in the geminata group do not attack fire ants in the saevissima group, and vice versa. But the flies do attack more than one species within a given group.

Illustrating that phorids will only attack one ant group, Gilbert said the flies that attack the geminata group in the U.S. are dwindling as their host ants dwindle. The flies do not and will not attack the RIFAs, which are taking over the land.

"We are pretty certain that chemical signals specific to these ant species are used by the flies to locate and discriminate host ants," Gilbert said. "That accounts for the high degree of specificity."

Challenges the researchers face include finding, importing, and rearing phorid flies that attack RIFAs. The researchers then must successfully introduce these phorids to the wild, where the phorids, according to theory, will control RIFA populations.

One of Feener's main concerns with Gilbert's team's work is that Gilbert and company are conducting field trials with phorid flies that attack RIFAs at disturbed mound sites, not while they are out foraging for food.

This is a problem, Feener says, because phorids need to disrupt the RIFAs where they directly compete with the native fire ants: out foraging. "So, it is not clear how these flies would act to regulate RIFA populations," he said.

Also of concern to Feener is a study he performed with Gilbert and Woody Benson at the University of Campinas in Brazil. The study found inconclusive evidence that phorid flies help regulate fire ant populations in their native Brazil.

"This is not to say that the flies do not help regulate the fire ant populations, but we can't say for certain that they do, because we don't have the data," Feener said.

Gilbert believes the phorids do indirectly regulate the fire ant populations. His long-term hope is to replicate in the U.S. the communities of phorids found within fire ant populations in South America.

Five to seven specialized phorid species are native to South America, he said. Each phorid species prefers a different type of ant (big or small), style of attack (mound or trail), and season for attacking.

"We will see full deployment of phorid biological control only as we can replicate that multilayered system of RIFA enemies here in the U.S.," Gilbert said. "That's the current focus. Even if it works, population impacts are expected to be gradual."

Both Gilbert and Feener said that if the phorid introduction fails, the picky flies are unlikely to undergo a population explosion and turn to attacking the native fire ants.

"While flies released in the U.S. will probably not attack other ant species, I would be troubled if the program does not work, because it adds to the increasing homogenization of the world's biota, which may well have unanticipated, unintended consequences down the line," Feener said.

For more ant news stories, scroll down.

Photograph courtesy USDA APHIS PPQ Imported Fire Ant Station Archives,
Invasive.org,
Image #1148020,
http://www.invasive.org/browse/detail.cfm?imgnum=1148020,
Downloaded June 18, 2004
 

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