How Does the Ants’ Garden Grow?
By Mariana Relós
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
Worker ants tend this garden of white fungus.
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.
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.
A queen ant, which is giant
In this warm, moist mush, the ants grow a white fungus—a
compared to a worker, eats
kind of mushroom. The ants eat the fungus just as we eat
large amounts of fungus to
the food we grow.
produce her eggs.
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.
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
Dr. Cameron Currie digs for
the fungus, the ants make the leaf-mush and keep it warm
ants
in the tropical rain forest.
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?