Among all the invasive species on Earth, fire ants are among the most formidable.
Solenopsis invicta, the red imported fire ant, has dominated the grasslands of the southern United States since the 1930s, using its deadly venom to keep other ant and insect species at bay.
Recently, however, a powerful rival has encroached upon the fire ant’s American turf. Called crazy ants for their habit of erratically scurrying about, they appear to be particularly demented when encountering fire ants, charging straight into a throng of them in spite of their potent sting.
It turns out there may be a method to their madness. A new study finds that tawny crazy ants are able to survive fire ant venom by smearing their body with acid from their own venom glands. This unique behavior may explain how they’ve been able to displace fire ants from the southern U.S. since being introduced there in the early 2000s.
Ed LeBrun, an ecologist at the University of Texas in Austin, observed the two species compete for bait—such as a grasshopper—that he placed in an area of Texas where their populations overlapped. After fire ants swarmed over the bait, crazy ants would appear, and “they started charging in with what seemed like a willful abandon for their own safety,” said LeBrun.
“I was seeing crazy ants get smeared repeatedly with fire ant venom, and then I’d see them go off to the side and do this odd behavior where they would tuck their abdomen underneath their body and touch the tip of their abdomen to their legs and proceed to groom, running their legs over all the parts of their body they could reach,” LeBrun said. After this behavior, the crazy ants would return to the fray.
LeBrun wondered if the tawny crazy ants were somehow using their strange behavior to detoxify fire ant venom. “We got them to the lab and did the experiments and found that, yep, not only are they doing it but it’s incredibly effective,” he said. “That was definitely exciting.” LeBrun reported his results in a study published February 13 in the journal Science.
“It’s an incredibly neat study,” said Micky Eubanks, an entomologist at Texas A&M University who was not involved in the research, who noted that it was the first he’d heard of this kind of detoxifying behavior in an ant species. “When the ants are fighting, there are all kinds of rapid movements, and it’s just very perceptive on Ed’s part that he noticed this behavior,” he said. (Related video: “Fire Ants Swarm.”)
Detoxifying Their Bodies
When studying the behavior in the lab, LeBrun noticed that crazy ants rose up on their hind and middle legs and then smeared a droplet of fluid from an orifice on their abdomen called an acidopore, first onto their legs and then onto the rest of their body. To test the importance of the fluid, LeBrun used nail polish to seal the acidopore of some of the crazy ants he placed in a vial along with fire ants. Nearly all the ants that had unsealed acidopores and engaged in normal detoxification behavior survived the fire ants’ stings, compared with fewer than half of those with sealed acidopores.
To figure out what crazy ants were secreting to protect themselves, LeBrun applied solutions of fire ant venom and crazy ant gland secretions to another species of Argentine ant that does not have the ability to detoxify fire ant venom. All the ants treated with crazy ant venom gland secretions survived, suggesting that the protective compounds came from the venom gland.
Crazy ant venom is known to consist primarily of concentrated formic acid, so that was the likely candidate for detoxifying fire ant venom. Sure enough, when LeBrun exposed the Argentine ants to fire ant venom and then treated them with a formic acid solution, almost all of them survived. Most of the ants not treated with formic acid died. (Related: “Ants Use Acid to Make ‘Garden’ in Amazon, Study Says.”)
While it is not yet clear how formic acid detoxifies fire ant venom, LeBrun thinks that it might alter enzymes in the venom, preventing the toxins from getting through the ant’s tough exoskeleton. He plans to test this theory in follow-up studies and also investigate how unique the crazy ant’s detox behavior really is.
“It seems to be unique, but it could potentially be pretty widespread,” said Texas A&M’s Eubanks. “A lot of ants produce formic acid, and so would potentially have the ability to do this physiologically.”
Tawny crazy ants’ defensive adaptation to fire ants may have evolved in South America, where the native ranges of both ant species overlap, said LeBrun. He found that crazy ants used this behavior far more frequently when confronted with fire ants than when facing other ant species.
“This kind of interaction is a signal of evolution,” he said. “You don’t get these kinds of specialized behaviors without adaptation.” (Related: “How Do Ants Get Their Magnetic Compasses?“)
A Bigger Nuisance
“[The study’s] results suggest that fire ants are not going to limit the spreading of crazy ants,” and this has ecological implications for the southern U.S., LeBrun said.
Tawny crazy ants usually spread only about 200 meters (656 feet) a year, unless humans inadvertently help. “The reason they’re popping up all over Texas and the southeastern U.S. is that people move them around all the time, in potted plants, recreational vehicles, cardboard boxes they leave outside,” LeBrun said.
When LeBrun compared areas in the southern U.S. with a high density of fire ant and crazy ant populations, he found that the areas with crazy ants had fewer species of arthropods—organisms that include spiders and centipedes. “Crazy ants were having a larger impact on the food web than fire ants [were], and that’s troubling,” he said. According to LeBrun, there have been reports of crazy ants destroying beehives, and more studies will be required to figure out their impact on agriculture.
Crazy ants are also an annoyance for humans, even though they don’t sting. “If you’re a homeowner, they’re a much bigger nuisance typically, because they come in your house in really large numbers, whereas fire ants tend to stay in your yard,” LeBrun said. They invade by the thousands, and can cause electrical boxes and electronic equipment to short circuit, he said.
“Generally, people would like to have their fire ants back,” LeBrun said.