How Does the Ants’ Garden Grow?

By Mariana Relós

	Worker ants tend this garden of white fungus.

To raise a crop of fungus, ants use weed-killing bacteria.
If you could look into a nest of leaf-cutter ants, here’s what you might see.

A warm, humid pile of ants surrounded by darkness. Millions of worker ants zip this way and that over a mixture of leaf mush and something that looks like dirty cotton.

One of the workers rushes to a corner of the chamber. She walks back and forth, mixing the pile of mush with the pincer-like mandibles of her mouth. (All of the workers are female.)

She rubs her underside against the mush. A few more workers join in and imitate her. As quickly as they started, they stop and move to other parts of the chamber.

A queen ant, which is giant compared to a worker, eats large amounts of fungus to produce her eggs.

What were they doing? They were tending their garden. The first worker had noticed a destructive fungus, and the group treated it with their own weed killer, made by bacteria that grow on the ants’ undersides.

Insect Farmers
These are no ordinary ants. They are often called gardener ants or leaf-cutting ants. They stand out because they are some of the few creatures that grow their own food. Only humans, wood-boring beetles, and some kinds of termites are also known to be farmers.

Gardener ants live in many tropical areas, where they cut up enormous numbers of plant leaves. They take the pieces into the nest, chew them into a mush, and deposit the mush in chambers inside the nest.

In this warm, moist mush, the ants grow a white fungus—a kind of mushroom. The ants eat the fungus just as we eat the food we grow.

Like human gardeners, ants have to tend their garden constantly. Scientists have found that if the ants are removed from their nest, other fungi, which are always there, will grow like weeds and destroy the white fungus.

When the ants tend their garden, only their crop of white fungus grows—and the ant colony prospers. Some colonies support eight million ants. That’s a lot of ants. All together, this many ants would weigh as much as a cow.

What’s Their Secret?
Scientists wondered how the ants control their garden pests. This mystery went unsolved for a long time.

An ant expert named Dr. Cameron Currie and his co-workers at the University of Toronto have solved the mystery. They have discovered a pest-control system that is used by many different species of gardener ants.

In this magnified picture, the line points to a white mass of bacteria growing just below an ant's head.   When the bacteria are magnified even more, they look like a tangle of string.

While Dr. Currie was working in the rain forest of Panama, he noticed a whitish-gray powder under the forelegs or below the head of tiny gardener ants. Other scientists had seen the powder before. They supposed it was something made by the ants’ bodies.

Dr. Currie wondered what the powder might be. He looked at it under a microscope. To his surprise, the “powder” was made up of live bacteria.

Next, Dr. Currie and his team of scientists asked, “What kind of bacteria are these?” They discovered that the bacteria growing on the ants belonged to a group called Streptomyces (strep-toh-MY-seez).

This finding was important. Scientists know that many Streptomyces bacteria make chemicals called antibiotics, which can kill some kinds of fungi.

	Dr. Cameron Currie digs for ants in the tropical rain forest.

Was it possible that the ants were using antibiotics from the bacteria to control their garden pests? To answer this question, the scientists mixed antibiotics from these bacteria with several types of fungi from the ants’ nest. This experiment showed which kinds of fungi the chemicals could kill.

The antibiotics did not harm the fungus that was grown by the ants. But it did kill the “weed” fungi. The bacteria were helping the ants take care of their garden by killing only the garden pests and not the ants’ food.

Partners
Each of the three species (ants, white fungus, and bacteria) gives something that one of the other species needs to live.

The ants take care of both the fungus and the bacteria. For the fungus, the ants make the leaf-mush and keep it warm and humid. They also kill off other fungi with chemicals made by the bacteria. For the bacteria, the ants provide a home for growth.

In return, the fungus provides food for the ants, and the bacteria make the antibiotics that kill “weed” fungi.

The surprising partnership of ants, fungi, and bacteria has survived in tropical rain forests for a long time, probably for millions of years. What other amazing things will scientists discover in those forests?