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Invasive fire ants could be controlled by viruses, scientists say

A small study suggests pathogens could provide an alternative to chemical pesticides, which are used widely around homes and public places

An imported red fire ant queen surrounded by immature and worker ants. (Steven M. Valles/USDA Agricultural Research Service)
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ATLANTA — Fire ant mounds are an unwelcome herald of spring across a widening swath of the American South and West, but scientists say experiments with species-specific pathogens offer hope for restoring balance to ecosystems affected by the highly invasive insect.

A study published in the Journal of Invertebrate Pathology found that a virus known as solenopsis invicta virus 3 had a significant effect on red imported fire ant colonies. Researchers saw a sevenfold decrease in the number of nests, as well as a correspondingly significant decrease in the size of nests, over the course of the evaluation.

The study, although small and limited to one species, could have implications for how infestations are treated. Chemical pesticides are the first-line defense and are widely deployed around homes and public spaces, including schools, parks and playgrounds.

“Using insect pathogens as natural biopesticides … has advantages over the chemical insecticides in that, in general, the microbial agents are much more safe to the environment and to humans,” said David Shapiro-Ilan, a research leader with U.S. Department of the Agriculture and the editor in chief of the journal that published the study.

Another potential advantage of the virus is that it appears to persist and spread naturally in the environment once introduced, he said.

“You’re getting more bang for your buck,” Shapiro-Ilan said. “You don’t have to apply it to every single mound, every single season.”

Scientists have not introduced a new virus to North America — the virus is already present here at low levels. It also underwent extensive analysis to ensure it infects only the targeted species, said Steven Valles, the lead author of the study and a research entomologist with the USDA’s Agricultural Research Service Imported Fire Ant and Household Insects Research lab in Gainesville, Fla.

“We’re quite pleased that it contributes to the natural control of fire ants,” Valles said.

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He quashed hopes that the virus would devastate fire ants to such an extent that Southerners could walk outside barefoot without fear of being stung. But he did express cautious optimism that, when used in concert with other natural control methods under study, including parasitic flies and microsporidia, imported fire ant populations could be noticeably affected.

“It’s important to continue to release things that will naturally control the ant to give it more ecological balance with the native ants so that they can compete,” Valles said.

Damage caused by fire ants

Domestically, fire ants inflict more than $6 billion a year in economic damage, according to the USDA. More than 80 people have died as a result of being stung by fire ants, and countless others have had to seek medical attention.

Colonies of fire ants were spotted clumped together and moving through floodwaters left by Tropical Storm Cindy in Galveston Island, Tex., and Mandeville, La. (Video: The Washington Post)

In addition to the economic damage and the nuisance they cause to humans, fire ants are also a threat to wildlife, particularly insects and ground-nesting reptiles and birds.

“The fire ants pretty much eliminate everything,” said Dini Miller, an entomologist at Virginia Tech. “The biodiversity can be a wipeout.”

Curbing imported fire ant populations could help native species that have been hurt by them rebound, said Edward LeBrun, a research scientist and ecological entomologist at the University of Texas at Austin.

Those species include the endangered Attwater’s prairie chicken, but also the somewhat less sympathetic native fire ant, which most people would be hard pressed to tell apart from the imported kind.

“Preserving diversity is a good thing,” LeBrun said in defense of the native fire ant.

“There is some basic minimum amount of diversity that we need to support life on earth,” he said. “We don’t know what that minimum is, right? So the deeper we carve into it, the more likely we are to get below it.”

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Imported red and black fire ants are native to South America. They are widely believed to have been accidentally introduced to North America through the port of Mobile, Ala. in the 1930s.

Having escaped the natural enemies of their native range, fire ant populations exploded. Studies suggest they are at least four times more abundant here than in their native habitat.

And they are evolving.

A hybrid fire ant

Since their introduction, the red and black fire ants have hybridized into a new ant, unique to North America, that shows signs of speciation. The hybrid is more cold tolerant, and is supplanting its parent species in some regions.

“The hybrid has the greatest potential to spread northward in the U.S., especially with the added effect of climate change,” Karen Vail, an entomologist at the University of Tennessee Institute of Agriculture, wrote in an email.

The hybrid also appears to be resistant to the viruses that infect its parent species, including the one that was the subject of Valles’s paper. He is working with scientists in Tennessee to test another variant they are hoping will be effective against the hybrid.

Michael Kaspari, professor of geographical ecology at the University of Oklahoma, called the ant-virus study “underwhelming” because of its small size and methodology, but nonetheless he saw promise.

“Caveats aside, any time you can introduce a pathogen that attacks only the target species — moreover one that persists and even spreads through the environment on its own — then this is a result that promotes cautious optimism,” he wrote in an email. “So much nasty, costly stuff has been done to combat this fire ant — notably pesticides that kill native species and must be reapplied in perpetuity — that this virus is hopeful.”

Kaspari said he imagined a future scenario in which a homeowner finds a fire ant nest while weeding her garden.

“She heads to the hardware store, buys a tube of ‘FireAntBGone,’ mixes up the paste with some sugar water, pours it on the colony, and she’s inoculated her yard without hurting anything else,” he wrote. “One can dream, right?”

Even if pathogens prove to be effective at controlling imported fire ant populations, experts said they are unlikely to stop the ants’ global trajectory. Over the past few decades, they have spread to Asia and Australia.

“Every time something invasive pops up, somebody thinks they can eradicate it and they can’t,” said Daniel Suiter, a professor of urban entomology at the University of Georgia. “I can’t think of a situation where an invasive popped up and it was eradicated. Once it gets in and establishes, from that point forward you just kind of have to deal with it.”