The Journal of Young Investigators: An Undergraduate, Peer-Reviewed Science Journal

22 September 2005 - A species of Amazon ants poisons plants that compete with its host tree, according to a study published in the 22 September edition of Nature. The study, which is the first evidence of ants using poison to kill plants, explains the enigmatic patches of Amazon rainforest known as "Devil's Gardens".

In the Peruvian Amazon, one of the most diverse places on Earth, Devil's Gardens stand out: they have just one species of tree. The Gardens can be up to 1,300 square meters and comprise hundreds of Duroia hirsuta trees – but no other plants species. Local legend explains this as the work of evil spirits, but researchers have long-suspected that either the trees poison nearby plants, or their large colonies of Myrmelachista schumanni ants do.

Now researchers from Stanford University report that the ants are to blame. When a new plant appears in the Gardens, the ants bite small holes in its leaves and inject them with formic acid. The leaves shrivel and die within a few days.

"The ants are eliminating the plants that are not their host species," says Deborah Gordon, an assistant professor at Stanford and lead author on the paper. "I think what's amazing is how much control the ants exert over their environment."


Between the Devil and a Small Cedar Tree

Gordon and her colleagues tested the two hypotheses – killer ants vs. killer trees - using 10 Devil's Gardens in Peru. In each Garden, they planted two Spanish cedar saplings: one treated with a gooey ant-barrier, the other left alone. They also planted a treated and untreated sapling about 150 feet outside each garden.

The ants immediately attacked the untreated cedar by injecting formic acid into its leaves. Within 5 days, every leaf on the untreated cedar in the Devil's Garden was dead. The cedars outside the Devil's Garden and the cedars treated with the gooey ant-barrier were unaffected.

The Devil in Disguise

But are the ants only protecting their host tree – or any plant they can nest in? To find out, Gordon and her colleagues disguised several cedar saplings as host trees.

The ants nest in hollow stems called "domatia". By attaching foil-wrapped test tubes, partially filled with cotton, to the cedars, the researchers created a tree that might fool the ants. They planted one disguised cedar and one undisguised cedar in the Devil's Garden, and one of each outside as control trees.

The ants destroyed the cedars in the Garden – both disguised and undisguised.

"We still don't know how they recognize their host plant," says Gordon. "We've eliminated one possibility, but we still don't know what they're using. It's possible that there is a chemical cue."

Many species of ants secrete formic acid ("formica" is actually Latin for "ant"); however, they usually use it to kill bacteria and fungi. This is the first time researchers have observed the ants using the acid to kill unwanted plants.

"Formic acid is very widespread in many different kinds of ants, but it's usually used as an alarm pheromone, for communication," says Gordon. "These ants have harnessed the basic circulatory system of all vascular plants, and combined it with some of their basic tools, to produce a lethal injection."

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