Sections of the rainforest made-up almost entirely of the tree species Duroia hirsuta, are called "Devil's gardens." Local legend holds that they were produced by an evil forest spirit, report Megan E. Frederickson of Stanford University and her colleagues. But the results of a four-year field study reveal that ants that make their nests in D. hirsuta are the driving force between the homogeneous plots of vegetation. The team introduced saplings of another common Amazonia tree to "Devil's gardens," using a barrier to protect some of them from the ants. Those that were unprotected, however, were quickly attacked by worker ants, which injected poisonous formic acid into the leaves. Trees began to show signs of withering within 24 hours and most of the leaves were lost after five days.
Because the trees protected by barriers still thrived, the researchers ruled out a second theory of Devil's garden formation known as allelopathy, which would require D. hirsuta to produce a chemical that inhibits the growth of other plant species. The ants' ability to pave the way for greater D. hirsuta growth helps them survive by providing more nests sites. Indeed, the researchers calculate that the Devil's gardens that they studied initially formed more than 800 years ago. Although many ant species produce formic acid, the authors note that, "to our knowledge, this is the first record of an ant using formic acid as a herbicide."