Using fungus to fight crazy ants

US-based researchers have successfully used a naturally occurring fungus to control invasive crazy ants at a state park in Texas. The breakthrough has been described in a paper published in Proceedings of the National Academy of Sciences.

The tawny crazy ant (Nylanderia fulva) originates from South America and is an invasive pest in the southern United States. In Texas, the ants have been known to swarm and kill animals, including native species – and to take over homes, interfering with electrical devices such as sewage pumps and air conditioners.

The species is also known as the Rasberry crazy ant – and no, that’s not a misspelling of “raspberry”, but the surname of an exterminator who first discovered the ants’ presence in Texas in 2002. The “crazy” moniker comes from the ants’ rapid and erratic movement patterns.

Since that first sighting in 2002, the tawny crazy ants have been steadily spreading across Texas counties, causing havoc for homeowners and native animals. The ants form huge “supercolonies” that make them nearly impossible to eradicate. They have even been shown to be able to neutralise the venom of one of their main competitors, fire ants.

Back in 2015, scientists at the Brackenridge Field Laboratory at the University of Texas in Austin reported that they had discovered a pathogenic fungus which could hold the key to controlling the crazy ants. They called the fungus Myrmecomorba nylanderiae.

Researchers Nicholas LeBrun and Robert Plowes had noticed that some crazy ants they had collected in the field looked a bit odd – their abdomens were swollen with fat.

They found that the infected ants’ abdomens were full of spores from a microsporidian fungus. These fungal pathogens essentially take over the ants’ fat cells and use them to produce fungal spores.

Microscope image showing microsporidia spores
Microsporidia spores collected from a tawny crazy ant at Pace Bend Park in central Texas. Credit: Edward LeBrun / University of Texas at Austin.

The team started tracking the fungus in 15 crazy ant populations in Texas. They found that over eight years, every population that had the fungus became smaller, and 62 per cent of the fungus-infected populations died out completely.

This was a welcome surprise.

“You don’t expect a pathogen to lead to the extinction of a population,” LeBrun explains.

“An infected population normally goes through boom-and-bust cycles as the frequency of infection waxes and wanes.”

He thinks the affected crazy ant colonies in Texas may have collapsed because the early death of worker ants makes it more difficult for the population to survive through winter.

Importantly, the fungus seemed to specifically target tawny crazy ants without noticeably affecting other species, making it an attractive option for targeted biocontrol.

So far, things were looking promising. The team began exploring how the fungus could be used to control invasive crazy ant populations.

Then, in 2016, the opportunity came to test the new biocontrol strategy in the real world.

A middle-aged white man kneeling in a forest collecting a sample of crazy ants from a log on the ground
Researcher Edward LeBrun collecting tawny crazy ants at a field site in central Texas. Credit: Thomas Swafford / University of Texas at Austin.

LeBrun was contacted by workers at the Estero Llano Grande State Park in Weslaco, Texas. Animals in the park, from insects and reptiles to mammals, were falling like dominoes in the path of a terrifying infestation of tawny crazy ants.

“They had a crazy ant infestation and it was apocalyptic; rivers of ants going up and down every tree,” LeBrun says.

“I wasn’t really ready to start this as an experimental process, but it’s like, okay, let’s just give it a go.”

The research team introduced fungus-infected crazy ants into the park, positioning them in nest boxes near the existing nesting sites. They placed a bait – hot dogs – around the nest boxes to attract the local ants and bring the populations together, with the aim of spreading the fungus.

The experiment showed very encouraging results. The fungus spread quickly through the park’s crazy ant population. Within two years of the infected ants being introduced, the population had plummeted.

The same team has since used the M. nylanderiae fungus to successfully eradicate tawny crazy ants from a second site, this time in Austin, Texas. It hopes to continue to test the biocontrol mechanisms in other areas of the state in the coming months.

“I think it has a lot of potential for the protection of sensitive habitats with endangered species or areas of high conservation value,” LeBrun says.

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