Invasive tawny crazy ants are spreading wildly in the southern United States, but these problematic insects seem to have met their match in the form of a highly infectious fungal pathogen. Scientists are now using this naturally occurring fungus to combat crazy ant populations, with surprising success.
New research in Proceedings of the National Academy of Sciences is claiming that the microsporidian pathogen Myrmecomorba nylanderiae is a safe, effective, and natural means to curb the spread of crazy ants. In a University of Texas at Austin press release, Edward LeBrun, the lead author of the study and a researcher with the Texas Invasive Species Research Program at Brackenridge Field Laboratory, said the fungal pathogen holds “a lot of potential for the protection of sensitive habitats with endangered species or areas of high conservation value.”
Native to South America, the tawny crazy ant (Nylanderia fulva) has been spreading through the southern U.S. states of Texas, Mississippi, Florida, and Louisiana for the better part of two decades. The ants are disruptive to populations of insects, spiders, centipedes, crustaceans, and even the odd mammal; the ants are known to blind baby rabbits by oozing an acid into their eyes. They’re an ecological menace, but also a threat to human infrastructure. Crazy ants are infamous for swarming and damaging air conditioning units and other electrical equipment.
Frustratingly, tawny crazy ants don’t respond to conventional insecticides, making them difficult to manage. Write the scientists in their study: “Invasive social insects are among the most damaging of invasive organisms and have proved universally intractable to biological control.”
But it was around eight years ago that LeBrun began to notice something unusual: A surprising number of tawny crazy ants were discovered to have enlarged abdomens swollen with fat. Looking into this further, he and his colleagues identified the cause as spores from a group of fungal pathogens known as microsporidians. This infectious fungus hijacks the fat cells of tawny crazy ants, turning them into veritable spore factories.
The source of this mysterious pathogen is not yet known. The ants might have brought it from South America or contracted it from other insects. Regardless, the scientists began to notice these infections across much of Texas, prompting a multi-year study to investigate further. From 2012 to 2018, the team repeatedly sampled 15 local populations of tawny crazy ant in the state. As they learned, the effects of M. nylanderiae are devastating, to say the least. Every site surveyed suffered dramatic declines in population, and 62% were simply erased.
That’s intense—infectious pathogens aren’t usually this brutal. Normally, infected populations go “through boom-and-bust cycles as the frequency of infection waxes and wanes,” said LeBrun. In terms of an explanation, the declines in local populations “occurred primarily over the winter,” as the scientists wrote in their study. “Perhaps population collapse arises because the lifespan of the infected worker population is insufficient to bridge the gap in winter brood production,” they wrote, adding that the “decline of colony fragments in the laboratory supports this hypothesis.”
Importantly, the fungus seems exclusive to tawny crazy ants and doesn’t seem to affect other ants or arthropods. It’s for that reason that LeBrun and his colleagues are hoping to use the fungus as a targeted biocontrol agent. The team tested this exact thing in 2016 at Estero Llano Grande State Park in Weslaco, Texas, which had been overrun by the ants. “They had a crazy ant infestation, and it was apocalyptic, rivers of ants going up and down every tree,” LeBrun explained in the university release. “I wasn’t really ready to start this as an experimental process, but it’s like, okay, let’s just give it a go.”
For the experiment, the scientists collected crazy ants infected with M. nylanderiae and brought them to Estero. Like ticking time bombs, the infected ants were brought to nesting sites in the state park. Hot dogs were used to lure uninfected local ants, facilitating the merger of the two populations. The results were profound, with the fungal pathogen eventually spreading across the entire crazy ant population in the state park. The number of crazy ants dropped to practically nothing after two years, and native species are now returning to Estero. “In contrast, uninfected populations [of crazy ant] showed no tendency to decline over a similar period,” according to the paper.
The scientists repeated this experiment at an area near Convict Hill in Austin, and with the same results. At both sites, “average disease burden of infected nests increased exponentially throughout the period of pathogen spread and N. fulva population decline,” and crazy ant “populations declined to local extinction following widespread pathogen establishment,” the paper said.
More experiments are planned for later this year at other Texas locations, in what is a very encouraging development. It almost seems too good to be true—a natural biocontrol agent that doesn’t seem to be harmful outside of its effect on crazy ants. Hopefully the results will continue to be positive and that conservationists will finally have an effective tool at their disposal to combat these highly problematic ants.