This video segment from Evolution: "Evolutionary Arms Race," illustrates the coevolution of the leafcutter ant and the fungi on which it feeds. Leafcutters have been "farming" this fungus for millions of years, feeding, fertilizing, weeding, and harvesting it.
Fifty million years before humankind began farming, ancient ants were already in the agriculture business.
Over time, leafcutter ants have evolved a complex system of agriculture in their nests, cultivating bumper crops of fungi that are the ants' sole food source. Foragers cut pieces of leaves from trees and drag them home to their nest, where others chew them into a paste that becomes the fungi's dinner. There are, however, at least two more participants in this relationship. Surprising the scientific community, graduate student Cameron Currie discovered a mold that threatens to kill the fungi, and the antibiotic which the ants produce in order to control it.