Wood ants combine tree resin with formic acid to protect their nests from fungi and bacteria.

Simon Williams/Minden Pictures

‘Chemist’ ants brew antibiotic cocktail to protect their colony

Ants have all sorts of jobs we normally think of as human, from architect to farmer to insect-in-chief. Now, scientists are adding one more occupation to that list: chemist. A new study shows that wood ants protect their colonies from disease by crafting a potent antibiotic “cocktail” made of tree resin and poison from their own bodies. The finding, one of the most sophisticated examples of animal pharmacology, could explain how some ants evade epidemics.

Like humans, wood ants (Formica paralugubris) live in dense groups, with colonies numbering in the hundreds of thousands. That should make them prime targets for widespread disease, especially because their nests are warm, humid, and full of dead insects to be used as food. Most ant species manage to avoid epidemics by grooming each other and obsessively cleaning their colonies, and wood ants take the added precaution of collecting antimicrobial tree resin to bring back to their nests. But Michel Chapuisat, an evolutionary biologist at the University of Lausanne in Switzerland, suspected this species might be hiding an even more sophisticated secret for staying healthy.

To investigate, Chapuisat and his colleagues first measured how well wood ant–exposed resin warded off a deadly fungus—which infects ants and spreads through spores grown in their bodies—compared to tree resin alone. In petri dishes covered with the fungus (Metarhizium brunneum), resin stored with the ants for 2 weeks resulted in a 50% larger fungus-free area, the team reports this month in Ecology and Evolution. Stones and twigs, both common in nests, didn’t get any antifungal boost from being around the ants. That was an indication that something special was happening between wood ants and resin.  

Next, the researchers used liquid chromatography, a technique for analyzing chemical mixtures, to spot any substances left behind by the wood ants. One compound they found was formic acid: a caustic substance produced by several ant species to fight off threats, subdue prey, and clean their offspring. When the scientists dipped tree resin in the acid, they found the resulting mixture did a better job of warding off fungus than resin alone, or glass dipped in formic acid. That was enough for the researchers to confirm the ants are mixing the two substances—one found, one created—to keep their nests healthy. “They exploit the tree and then they combine it with their own poison,” Chapuisat explains.

“I thought it was a really interesting and exciting paper,” says Michael Singer, an evolutionary ecologist at Wesleyan University in Middletown, Connecticut. Plenty of animals defend themselves with substances they find or create, but Singer says this new substance shows unique “synergistic effects,” meaning that the mixture of resin and formic acid is more than just the sum of its parts. The only other example of an animal brewing up a combination like this—which Singer calls “defensive mixology”—is humans and our drug cocktails.

But rather than springing suddenly from the mind of a brilliant human inventor, this mixture is the result of a long evolutionary tug-of-war. “Ants have been coevolving with their pathogens for 50 million years,” and perhaps longer, says Christopher Pull, an evolutionary biologist at the Institute of Science and Technology Austria in Klosterneuberg. And for now, the insects seem to be keeping their microbes at bay. With issues like antibiotic resistance making human drugs less effective, Pull says the time-tested strategies of ants may be worth a closer look. “Maybe they’ve come up with viable solutions to these problems we’re just now encountering.”