Devil’s gardens randomly appear in the Amazonian rainforest. They “consist almost entirely of a single species, Duroia hirsuta,” wrote Frederickson and her colleagues. 

The team found “that the ant, Myrmelachista schumanni, which nests in D. hirsuta stems, creates devil’s gardens by poisoning all plants except its hosts with acid. By killing other plants, M. schumanni provides its colonies with abundant nest sites—a long-lasting benefit, as colonies can live for 800 years.”

Most tropical rainforests are densely populated with a remarkable diversity of trees, vines, shrubs and wildflowers. But devil’s gardens usually consist of a single plant, D. hirsuta, which happens to be the preferred habitat of the devil’s garden ant, M. schumanni. 

“By killing other plants, the insects create a space for young D. hirsuta saplings to grow, thereby allowing the ant colony to expand as it occupies new nesting sites in the saplings,” Frederickson said.

She and her colleagues conducted a series of experiments at the Madre Selva Biological Station in the Amazonian rainforest of Loreto, Peru. The group studied 10 devil’s gardens, ranging in size from one to 328 D. hirsuta plants. 

The researchers planted two saplings of a common Amazonian tree, Spanish cedar inside each garden near the base of a D. hirsuta tree actively patrolled by worker ants. The scientists put a sticky insect barrier over one cedar sapling to exclude ants. 

The researchers also planted two additional saplings—one treated, one untreated—about 150 feet outside of each garden but within the rainforest.

Worker ants promptly attacked the untreated saplings, the researchers said, injecting a poison called formic acid into the leaves, which began to die within 24 hours. 

“Most of the leaves on these saplings were lost within five days, and the proportion lost was significantly higher than on ant-excluded saplings,” the authors write. On the other hand, the protected cedars fared well, whether inside or outside devil’s gardens.

Chemical analysis revealed that the only compound produced by the ants’ poison glands is formic acid, a toxin that is common in many ant species and, in fact, got its name from formica, which is Latin for ant. 

“Treatment of leaves with formic acid induced leaf necrosis [death] on all the plants we tested,” the authors write. “To our knowledge this is the first record of an ant using formic acid as a herbicide—although it is known to have bactericidal and fungicidal properties.”

The ants employ a very effective system of lethal injection, notes Gordon, associate professor of biological sciences at Stanford. “The system harnesses two fundamental tools: formic acid, which many ant species use for other purposes, and the basic circulatory system of all vascular plants,” which contain channels for moving fluids inside them, she said. 

A census of the rainforest from 2002 to 2004 revealed that devil’s gardens grew by 0.7 percent per year, the researchers said. 

“Using this growth rate, we estimate that the largest devil’s garden in our plot, with 351 plants, is 807 years old,” the authors conclude. They estimate that a typical garden is tended by a single ant colony with as many as 3 million workers and 15,000 queens, adding that the presence of multiple queens “undoubtedly contributes to colony longevity.”

The plants also benefit by spreading and eliminating the competition, said co-author Michael J. Greene, of the University of Colorado at Denver.

A devil’s garden begins when a M. schumanni queen colonizes a single D. hirsuta tree, the authors wrote. 

“Over time, D. hirsuta saplings grow within the vegetation-free area created by the ants, and the ant colony expands to occupy them.”

Frederickson is conducting new studies to determine which chemical cues the ants use to discriminate between host plants and other species. She also has begun searching for devil’s gardens in other parts of the western Amazon to see how widespread the herbicide phenomenon is.

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