For 60 years, imported red fire ants have terrorized people, livestock, and other ants as they spread across the southeastern United States, aggressively stinging those who got in their way and proving nearly invincible. But in Texas, these invasive pests have met their match—there, the tawny crazy ant is taking over. New research has revealed this newcomer's secret weapon: a built-in antidote to the fire ant's deadly venom.

Like the fire ant, the tawny crazy ant hails from South America. Pest controller Tom Rasberry first spotted it in the United States in Houston, Texas, in 2002, and entomologists soon determined the same species was also invading Florida. They dubbed it “crazy” because of the ant’s erratic movements. Like the fire ant, the tawny crazy ant builds nests with lots of queens, so "it reaches really astronomical densities in the environments it invades," says Edward LeBrun, an ecologist at the University of Texas, Austin.

Unlike the fire ant, however, which stays outdoors, the tawny crazy ant is a "tramp" ant willing to take up residence almost anywhere—under logs, inside walls, and in circuit boxes, where they become so numerous they can short out electronic and electrical equipment. "From a societal perspective, it's a real nuisance," LeBrun says. Outdoors, tawny crazy ants decimate the populations of other invertebrates and have even been known to take over fire ant nests.

LeBrun wondered how these upstart invaders were able to beat the fire ants, whose sting emits a toxin more potent than the insecticide DDT. To study how the two species duke it out, he ventured into the field and placed a dead cricket at the boundary between fire ant and tawny crazy ant territories. He noticed that after a fire ant swipes its opponent with its venom-filled stinger, the tawny crazy ant walks off, brings the tip of its abdomen up to its head, and squirts a drop of liquid onto its mouthparts. Then the ant runs its legs across its mouth and uses them to spread the liquid along its body.

Suspecting an antidote to the fire ant venom, LeBrun tested the compounds secreted by the two glands at the tip of the tawny crazy ant’s abdomen. He took each compound and wiped it on a similar-sized ant that was subjected to the fire ant's sting. When it came to neutralizing the fire ant venom, the key compound appeared to be the formic acid in the tawny crazy ant’s own less powerful venom, which it usually sprays out of a pore during battle, LeBrun and his colleagues report online today in Science. Synthetic formic acid also has the same protective effect.

Fire ant venom contains not just toxic compounds but also proteins that help get those compounds inside cells. LeBrun suspects the formic acid destroys these proteins, so the toxins can't reach their targets. A few other animals are known to counter the effects of the toxins of their competitors, predators, or prey, but they do so by preventing the toxin from binding to their target molecules, not by chemically breaking down the poison, says Ashlee Rowe, an evolutionary neurobiologist at Michigan State University in East Lansing who was not involved in the work. "I think this example is unique."

LeBrun suspects that this detoxification behavior evolved in the tawny crazy ant in South America, where the two ants have been in close contact for a long time. Jules Silverman, an entomologist at North Carolina State University in Raleigh, who was not involved in the work, isn’t so sure. "It's possible that this same mechanism that these researchers observe in the U.S. occurs in the native range, but maybe not. Maybe it's new."

One study did show, however, that while the fire ant usually dominates its native landscapes, it can still be laid low by the tawny crazy ant, suggesting that their detoxification defense is used there as well. "It would be fascinating to go back to South America and watch these interactions more carefully," says Michael Kaspari, an entomologist at the University of Oklahoma, Norman. Meanwhile, it looks like "this 60-year dynasty of the fire ants [in the United States] is coming to a close, and it's coming to a close in a fairly unusual way."