The abundance of nitrogen in fungus gardens grown by tropical ants is due to nitrogen-fixing bacteria living symbiotically with the insects. This discovery of the essential element’s source solves a conundrum in insect chemical ecology (Science 2009, 326, 1120). The smallest farmers around, leaf-cutter ants play an essential role in nutrient cycling of tropical ecosystems by harvesting hundreds of kilograms of leaves off the forest floor per year and using them as fodder to grow fungus, which the ants use as food. Researchers trying to balance nutrient cycles have long been stumped by the high levels of nitrogen in the fungus gardens, compared with the low amount of nitrogen available from the leaves. Now, researchers led by Cameron R. Currie of the University of Wisconsin, Madison, report that the extra nitrogen comes from the atmosphere, injected by nitrogen-fixing bacteria living in the fungus gardens. Although bacterial nitrogen fixation has long been known to occur in the nodules of legumes, symbiotic relationships between nitrogen-fixing bacteria and insects are rare, with termites being one of the few well-studied examples.