How Do Leafcutter Ants Grow Fungus? Farming Underground

Step by step: from leaf to fungus garden

The farming process runs like an assembly line, with different jobs done by workers of different sizes, as described in the leafcutter ant profile.

1. Cutting. Medium-sized foragers climb plants and slice out pieces of leaves, flowers or fruit with their vibrating, saw-like jaws.
2. Carrying. They carry the fragments along cleared trails back to the nest, holding them high over their heads. Tiny workers often ride on top to guard against parasitic flies.
3. Cleaning and cutting smaller. Inside, smaller workers clean the fragments and cut them into pieces only a millimeter or two across.
4. Chewing. Even smaller workers chew the pieces into a soft pulp, mixing it with saliva.
5. Fertilizing. The ants add drops of their own fecal fluid, which contains enzymes and nitrogen that help the fungus break down the plant material.
6. Planting. Finally, the smallest workers pluck tufts of fungus from older parts of the garden and plant them on the fresh pulp, where the fungus quickly spreads.

The result is a gray, spongy garden that fills its chamber, crisscrossed by the white threads of the fungus. A big Atta nest contains hundreds of such gardens.

What the ants actually eat

The fungus, Leucoagaricus gongylophorus, belongs to the same family as many lawn and forest mushrooms. In ant nests it produces special swollen tips called gongylidia, gathered in small clusters. These tips are rich in sugars, fats and other nutrients and seem to have no use other than feeding the ants.

Workers pluck the clusters and feed them to the larvae, which depend entirely on the fungus. The queen eats it too. Adult workers get part of their energy from plant sap they drink while cutting leaves, and the rest from the fungus. The partnership is obligatory: the ants cannot live without the fungus, and the domesticated fungus cannot live without the ants. Over millions of years of farming, it has largely lost the ability to reproduce by spores in the wild.

Weeding, waste and disease control in the garden

A warm, damp heap of plant pulp is ideal for molds and bacteria, so the ants work constantly to keep the garden healthy:

• Grooming and weeding. Workers lick the fungus and the leaf fragments, removing spores of unwanted fungi, and pull out pieces of garden that become infected.
• Waste disposal. Spent garden material, which has been used up by the fungus, is carried to separate refuse chambers or outside dumps. Waste is handled by older workers, which are kept away from the clean gardens.
• Antibiotics. In 1999, Cameron Currie and colleagues reported that fungus-growing ants carry bacteria on their bodies that produce substances suppressing Escovopsis, a parasitic fungus that attacks ant gardens. Later studies identified many of these bacteria as Pseudonocardia.
• Choosing safe leaves. Colonies learn to stop collecting plants whose leaves harm their fungus, apparently responding to chemical signals from the fungus itself.

This combination of hygiene, waste handling and natural antibiotics is one reason leafcutter farms can thrive for many years.

How a new garden is started

Every colony’s farm comes from its mother colony. Before her mating flight, a young queen packs a small pellet of fungus into a pocket inside her mouth called the infrabuccal pocket. After mating and landing, she sheds her wings, digs a narrow tunnel and seals herself in a small chamber.

There she spits out the fungus and starts a tiny garden, fertilizing it with her own fecal fluid. She lays eggs on and around it, and she does not leave to forage; she survives on fat reserves and her broken-down wing muscles until the first workers emerge, roughly two months later. Most young queens fail, often because their tiny garden dies. Those that succeed may grow into colonies of millions, as explained in how big are leafcutter ant colonies?

Why farming is such a successful strategy

Most plant leaves are tough and full of defensive chemicals, and few insects can eat many different kinds. By feeding leaves to a fungus, leafcutters let it do the hard work of digestion, and they can use a huge variety of plants. A single colony may harvest from dozens of species.

This has made leafcutter ants among the most important plant-eaters in the American tropics. Their gardens turn large amounts of fresh leaves into fungus, ants and, eventually, soil. Other social insects have found similar solutions: some termites grow fungus on chewed wood and grass, and others rely on microbes in their guts to digest wood (see what do termites eat?).

How old is ant farming?

Fungus-growing ants, the tribe Attini, live only in the Americas. Genetic studies suggest that ants began growing fungus tens of millions of years ago, long before humans started farming about 10,000–12,000 years ago. The true leafcutters, Atta and Acromyrmex, are a younger branch that specialized in fresh leaves.

Simpler fungus-growing ants still farm on dead plant matter, insect droppings and debris, and their fungi can survive outside the nest. In the leafcutters, the fungus has become fully domesticated, a little like crops that can no longer grow without farmers.

Myths and facts

Myth: Leafcutter ants eat leaves. Fact: The leaves are fertilizer and growing medium for their food, the fungus.

Myth: The ants grow mushrooms. Fact: The garden fungus very rarely produces mushrooms. The ants eat the swollen thread tips, not mushroom caps.

Myth: Ants were the only farmers before humans. Fact: Fungus-farming also evolved in some termites and in ambrosia beetles, but leafcutters run the largest and most complex farms of any animal apart from people.