Tawny crazy ants can seem unbeatable. These invasive insects march through warm climates, killing wildlife, invading buildings, and even shorting out motors and electrical devices with their huge swarms. Pesticides barely slow them down. Yet they sometimes vanish mysteriously. Now, researchers know why: A funguslike pathogen called Myrmecomorba nylanderiae can wipe out entire populations.
“This work has huge potential beneficial implications,” says Corrie Moreau, an entomologist and evolutionary biologist at Cornell University, who was not involved in the work. The new study showed that both natural and experimental infection will cause ant colonies to collapse. “What makes this work so brilliant is the authors have harnessed the power of nature to solve this problem.”
Ants are among the most worrisome of invasive species, because they can cause so much ecological and economic damage. Supercolonies, which can stretch for hundreds of kilometers, are particularly bad. That’s because instead of wasting resources fighting one another, ants from multiple nests coexist peacefully and march together into new territory. Some supercolonies are prone to boom-and-bust cycles, but researchers have never figured out why.
Tawny crazy ants (Nylanderia fulva) go through such volatile cycles. Like other crazy ant species, they’re named for their unpredictable movements while foraging. Native to Brazil and neighboring countries, tawny crazy ants spread to the Caribbean by the 20th century, and to the southern United States by the late 1990s. Once in a new place, the ants hitch a ride in landscaping supplies, for example, or in RVs. After a nest is established, the ants can terrorize nearby homeowners and businesses.
The ecological impact is even more drastic. Tawny crazy ants take over the nesting sites of other ant species, including dominant fire ants. They also kill larger arthropods such as crickets and scorpions and drive away lizards, snakes, and tree-nesting birds. Infested areas are rife with ants that stream up and down tree trunks, says Edward LeBrun, an ecologist at the University of Texas, Austin’s Brackenridge Field Laboratory and an author of the new paper. “There’s no insect noise and there’s no bird noise,” he says. ‘They have really profound ecological impacts.”
LeBrun has tracked 15 populations of tawny crazy ants in Texas for more than 9 years. In 2013, the ecologist received some dead ants from Florida and noticed their abdomens were swollen. He and his colleagues soon discovered the ants had been sick with a funguslike pathogen called a microsporidian. After being ingested, this pathogen fires a harpoonlike filament and injects its cellular machinery into a host cell to produce more spores. Infected ants die within months. The microsporidian infecting the Florida ants was one scientists had never seen before, and in 2015 researchers including LeBrun dubbed it M. nylanderiae.
LeBrun checked the bodies of Texas ants stored in his lab freezer, and he discovered the pathogen had already infected more than one-third of local populations. Fieldwork revealed it was spreading to more and more nests in southeastern Texas. “Just seeing it pop up from nowhere was shocking,” he recalls.
To figure out how the pathogen destroys colonies, LeBrun brought samples of nests into the lab. Ants became infected as larvae when fed by workers. The subsequent infection shortened their lives by at least 24%. That’s especially bad for tawny crazy ants, because—as the team discovered—their queens only lay eggs from April to November, rather than year-round like most other ant species. By the time egg laying began again in spring, so many workers had died in the infected nests that there weren’t enough to care for the new brood. In the fall, the infected nests lost 75% of their workers in 90 days and were on the brink of collapse.
In the wild, ants move freely between nests, which means infected workers likely spread the pathogen through the entire local population. To test whether that happens, LeBrun and colleagues brought infected ants to a natural area near Austin where the disease was not present. After testing native ant species to make sure they wouldn’t be harmed by the microsporidian, they released the ants into local nests. They also transported 3200 infected worker ants to Estero Llano Grande State Park in southern Texas. There, an infestation of tawny crazy ants had caused the park’s showcase attraction, scorpions, to disappear. Lizards had vanished as well.
In both locations, the pathogen spread throughout the population within 7 months. Within 2 years, the tawny crazy ants were entirely gone, the researchers report today in the Proceedings of the National Academy of Sciences. (Uninfected populations did not decline during this time.) At the park, the scorpions and lizards have returned. LeBrun is waiting to see whether native ant species recover, too.
Biocontrol interventions will typically control a population, but not eliminate it as appears to have happened with the tawny crazy ants. The results are “very suggestive,” says James Wetterer, an ant ecologist at Florida Atlantic University. But he notes that the declines might have occurred by chance, and the number of trials is too low for statistical tests. LeBrun has since taken infected ants to four other sites and says he has seen the disease take hold in two so far. Another three to four tests are planned for this year.
Biocontrol will likely be used to protect ecological reserves. Introducing the pathogen takes a lot of work: Scientists must bring nest fragments from the target site to the lab, for example, so unusual environmental odors don’t disrupt the integration of ants from the same supercolony. Homes and businesses near the intervention sites might benefit, but the problem of tawny crazy ants won’t be solved with a dose of infectious workers. “It not going to be on a shelf at Walmart anytime soon,” LeBrun says.