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Research technician Arinda Canales examines a container holding a newly hatched phorid fly and the decapitated ant head where it developed.

UT study to reduce fire ant population by creating "zombie ants"

UT researchers are studying how parasitic flies can reduce the fire ant population injecting them with eggs, which migrates to the ants' heads and motivates them to leave the colony, creating a so-called “zombie ant,” according to a research associate.
Research technician Arinda Canales examines a container holding a newly hatched phorid fly and the decapitated ant head where it developed. Image provided

Researchers at the University of Texas at Austin are studying how parasitic flies interact with fire ants in hopes of helping to bring the ant population under control.

The goal of the research, directed by Dr Larry Gilbert, is to produce a “suite” of phorid flies that can be released into the environment to help stabilize the red imported fire ant’s population, said Rob Plowes, a research associate at the phorid fly facility at UT.

The red imported fire ant is a major pest for most of the southern United States, where it was accidently introduced from Argentina around the 1930s. Their impact ranges from damaging fields with their anthills, to decreasing local biodiversity, to causing painful, and in some allergic individuals, deadly stings.

The annual economic effect of the ants on Texas alone is 1.2 billion dollars and the national cost of the ants is estimated to be about $6 billion, according to research done by Texas A&M University.

“Economically they stand out as one of the most serious invasive pests,” Plowes said.

Since the ants are native to Argentina, the natural predators and competitors that may help to keep the ant population under control in their homeland aren’t around in the United States, Plowes said, making easier for the ants to spread and multiply.

In Argentina, phorid flies may play a key role in keeping the fire ants in check, a relationship that the research is trying to recreate in Texas and beyond, said Bart Drees, a professor at Texas A&M who conducts red imported fire ant management research and works with the UT lab.

The flies are miniscule, smaller than Lincoln’s nose on the penny. But the three-pronged way they affect fire ants could have a big impact on an ant population that has gotten out of proportion, said Plowes.

One of the effects the flies have on the ants stems from a bizarre parasitic relationship between the flies and ants, with the flies using ants as hosts for their eggs. The cycle starts out with a fly seeking out an ant and injecting it with a single egg. This egg goes on to develop into a maggot, which after about two-weeks of feeding on the ant’s fluids, migrates to the ant’s head and “motivates it to leave the colony,” Plowes said, creating a so-called “zombie ant.”

Once the ant is in a suitable spot, the maggot promotes the release of a chemical, which decapitates and kills the ant. The maggot continues to develop inside the ant’s head cavity and emerges a few weeks later as a fully developed fly.

Although this whole cycle ends up killing an ant, it’s not very efficient on its own, with most exposed ant colonies usually having less than five percent of their inhabitants infected.

However, researchers have found that a second key effect of just having a few flies around terrifies the ants to the point where they’re more concerned about evading a fly-attack than foraging for food, which can take a toll on the health of the colony at large.

“ Dr. Sanford Porter [a phorid fly and ant researcher] always tells a story that if you’re in South America and you drop a hotdog at a picnic and you look back 15 minutes later all the ants are hiding under the hotdog trying to stay away from the flies,” Drees said.

The third possible mechanism for fly impacts is not as easily observable as ants losing their heads or running for cover, but research suggests that the flies also could also impact the ants by introducing diseases into the colony, which the ants could spread to one another while sharing food, said Plowes.

“The flies may be vectoring and transmitting pathogens between fire ant colonies,” Plowes said. “Think of it as malaria for fire ants.”

There are a number of fungal, viral and bacterial pathogens that have been studied by researchers as having potential for biological control. Much work centers on the need for these diseases to be host specific, and to find ways to transmit the diseases between colonies. If flies act as disease carriers, this would add to their value.

Since getting the go ahead from the USDA in the mid-1990s, four different species of phorid have been released into the wild in Texas. Two have expanded their range across the state, while the other two species expansions are still at an early stage. In total, the UT lab hopes to introduce eight to ten species to Texas.

All the phorids being studied exclusively use the red imported fire ant as their host, but how they locate and target ant colonies is highly differentiated between species, with some flies preferring ants of a specific size or ants that release a certain pheromone, said research technician Christopher Nelson.

However, even with the successful establishment of phorid populations, Plowes said that it is hardy to quantify the exact effect the flies have on the red imported fire ant population because of the multiple factors affecting the ant population at any given time.

“ It’s a big problem for us to measure impacts, since we have to see if changes are because of the flies or because of drought or other causes,” said Plowes.

Plowes said that it will take up to decade to know if the phorid flies are directly responsible for any decreases in the fire ant population since colony turnover and weather cycles occur across many years, but even without any conclusive results at the moment, he said that he feels the phorid flies can mean nothing but good news for fire ant control.

“We’re confident that there isn’t going to be a detrimental effect, so this has to be at least part of the solution,” Plowes said, “and it’s a huge success having them established and having them spread so far.”

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