Crazy ants carry the key to their own population control

Tawny crazy ants (Nylanderia fulva) are native to South America but have become an invasive pest in the US. The first record of tawny crazy ants in this country was from Brownsville, Texas, in 1938, and since then the species has spread to all states on the Gulf Coast of the US. This invader occurs in colonies that stretch to enormous numbers of individuals and it causes destruction to local insect communities and dismay to residents in urban areas where it has become established. 

Called “crazy” because of its rapid and unpredictable movements, this species of ant nests in almost any hole, nook or cranny it can find. In urban areas, tawny crazy ants often nest in electrical equipment, such as pump units, electrical distribution boxes or fuse breaker boxes. This can cause short circuits and the failure of the electrical equipment. The ants displace local ants and other insect species, and are not easily treated with pesticides. Each colony has multiple queens so it is almost impossible to eradicate an entire colony. 

Scientists at the University of Texas, Austin, have been studying tawny crazy ants over the past few years and have made an encouraging discovery. They have found that a naturally occurring fungus parasitizes these ants and can lead to the decrease or total demise of ant populations. The findings of their study are published today in the journal Proceedings of the National Academy of Sciences.

“I think it has a lot of potential for the protection of sensitive habitats with endangered species or areas of high conservation value,” said Edward LeBrun, a research scientist with the Texas Invasive Species Research Program at Brackenridge Field Laboratory and lead author of the study.

The ants have raised alarm across the southeastern US in the past 20 years, as they have spread from state to state. They are carried inadvertently by animals, vehicles and humans – for example in trash and on plants that are translocated from one site to another. They totally overwhelm local insect species and have also been known to seek moisture in the eyes of mammals and birds.

LeBrun and colleague Lawrence Gilbert, also from Brackenridge Field Laboratory, collected some tawny crazy ants from Florida, about eight years ago, and they noticed that some of them had abdomens swollen with fat. When they looked inside the ants, they found spores from a microsporidian fungal pathogen, and identified a species of this type of parasite that was new to science. Microsporidian pathogens commonly hijack an insect’s fat cells and turn them into spore factories for their own benefit. 

Although the researchers did not know where the pathogen had come from, or whether it was perhaps indigenous in the ants’ home range in South America, they began to notice that ants infected with the fungus occurred at sites across Texas. Observing 15 local populations for eight years, the team found that every population that harbored the pathogen declined – and 62 percent of these populations disappeared entirely.

This meant that the researchers had inadvertently uncovered a natural control method for keeping populations of tawny crazy ants in check. They wondered whether they could use the pathogen to induce population decrease in areas where the ants were a pest or were destroying sensitive habitats. 

“This doesn’t mean crazy ants will disappear,” LeBrun said. “It’s impossible to predict how long it will take for the lightning bolt to strike and the pathogen to infect any one crazy ant population. But it’s a big relief because it means these populations appear to have a lifespan.”

“You don’t expect a pathogen to lead to the extinction of a population. An infected population normally goes through boom-and-bust cycles as the frequency of infection waxes and wanes.”

The researchers got a chance to test out the use of the fungal pathogen when LeBrun got a call from Estero Llano Grande State Park in Weslaco, Texas, in 2016. The park had an infestation of tawny crazy ants and was losing its insects, scorpions, snakes, lizards and birds to the invader species. Even baby rabbits were being blinded in their nests by swarms of acid-spewing ants seeking the moisture in their eyes.

“They had a crazy ant infestation, and it was apocalyptic, rivers of ants going up and down every tree,” said LeBrun. “I wasn’t really ready to start this as an experimental process, but it’s like, OK, let’s just give it a go.”

The team used crazy ants that they had collected from other sites and that were already infected with the microsporidian pathogen. They introduced nest boxes of infected ants near to nesting sites of ants resident in the state park, hoping that the two populations would mix and the infected ants would carry the pathogen into the nests of the resident ants. The researchers placed hot dogs around the exit chambers to attract the local ants and merge the two populations. 

The experiment worked spectacularly. In the first year, the disease spread to the entire crazy ant population in Estero. And within two years, crazy ant numbers crashed. Now, they are nonexistent and native species are returning to the area. The researchers have since eradicated a second crazy ant population in this way at another site in the area of Convict Hill in Austin.

The researchers do not know exactly why the ant colonies collapse and theorize that the pathogen may shorten the lifespan of worker ants, meaning that the colony struggles to collect enough food to survive through winter. Whatever the reason, the fungus seems to infect only the crazy ants, leaving other, native species of ants and other insects unharmed.

The team plans to test their new biocontrol approach this spring in other sensitive Texas habitats that are infested with crazy ants.

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By Alison Bosman, Earth.com Staff Writer