Edward LeBrun arrived at the battlefield to watch two invading armies clash. But the ecologist from the University of Texas’s Brackenridge Field Laboratory had to peer at the ground to see the action because the soldiers were ants. On one side, a species of fire ant called imported fire ants—named for their burning sting—tried to hold the Texas prairie where they’d settled. Their challengers, tawny crazy ants—named for their fast, erratic movements—were gaining ground.
To draw the competitors together, LeBrun set out a dead cricket as bait. The fire ants, whose venom kills other ants, found the food first.
“When you’ve got an item covered with fire ants,” says LeBrun, “you don’t expect to see other ant species coming up and challenging the fire ants for that bait.” But this time, LeBrun witnessed something strange. Tawny crazy ants raced to the scene and attacked—and they weren’t dying.
By studying the insects in his lab, LeBrun discovered that tawny crazy ants have a chemical defense against fire ants that can shift the battle in their favor. As tawny crazy ants slowly make their way across the southern U.S., they’re causing big problems not only for their ant rivals but for people and other creatures as well.
Both of the dueling ant species originally came from South America. In the 1930s, imported fire ants may have hitched a ride to the U.S. in soil used for ships’ ballast—and that meant trouble for native species. “Imported fire ants are very aggressive,” says David Oi, an entomologist at the U.S. Department of Agriculture. “And they have big populations, so oftentimes they’ll outcompete other ants for resources.”
One reason for the imported fire ants’ success is a potent venom that’s painful to humans and deadly to small animals. As the invasive species spread through the Southern states, it pushed out native fire ants, and its deadly venom reduced populations of ground-nesting birds and reptiles.
But by 2002, a competing invader species was on the rise in the U.S. with its own chemical weapon. Tawny crazy ants don’t sting, but they spray formic acid from an opening called an acidopore on the tip of their abdomen. “They use it like a nozzle from an aerosol can,” says LeBrun. As the newcomers moved in, they used their deadly acid against other ant species—even imported fire ants.
As LeBrun watched the brawl over the cricket bait, a tawny crazy ant curled its abdomen under its body and sprayed formic acid at an imported fire ant. The fire ant arched its body, lifting its stinger into the air, and smeared venom on the crazy ant. “That is usually a fatal event for the victim,” says LeBrun.
But instead of dying, the tawny crazy ant ran off to the side, stood on its hind legs, and curled its abdomen. It took formic acid from its acidopore and wiped the acid all over itself, as if it were using it as an antidote for the venom. Then the ant ran back into battle. “I was intrigued to see if they were able to counteract the fire ant venom somehow,” says LeBrun. So he conducted an experiment to find out.
In the lab, he sealed the acidopores of some tawny crazy ants with nail polish so they couldn’t secrete formic acid but left the acidopores of others unsealed. Then he put the tawny crazy ants in vials with imported fire ants. After the crazy ants got smeared with fire ant venom, he separated the ants and watched. About half of the tawny crazy ants with sealed acidopores died, while 98 percent of those who secreted their antidote survived.
Researchers aren’t yet sure how the antidote works. It may prevent fire ant venom from entering the crazy ants’ cells, so the poison can’t deliver its usual toxic effects. Another possibility arises from the fact that chemical reactions occur easily between acids (like formic acid) and bases (like the fire ants’ venom). Maybe a chemical reaction detoxifies the venom.
ON THE MARCH
However they’re doing it, tawny crazy ants have the upper hand—and that doesn’t seem to be a good thing. “In some areas, you have huge populations,” says Oi. “It looks like the ground is moving.”
The invaders are decimating the native arthropods they prey on, such as insects and spiders. Since some of these arthropods control insect pests and provide food for birds and reptiles, their decline can trigger additional problems. All of the species in the ecosystem are interconnected.
People in infested areas aren’t happy either. Imported fire ants live in mounds outside, but tawny crazy ants invade houses, crawling over surfaces and short- circuiting electrical devices with their bodies. “Even though they don’t sting, they’re a lot more annoying,” says LeBrun.
No one knows if the tawny crazy ant boom will continue. LeBrun points out that imported fire ants used to swarm in greater numbers than they do now, so it’s difficult to predict what may happen to the new invaders over time. Meanwhile, the fierce battle for the Southern states continues underfoot.