Leafcutter Ant (Atta cephalotes): Facts About Fungus Farmers

What does a leafcutter ant look like?

Atta cephalotes is a reddish-brown to rusty ant with long legs, a matte, slightly hairy body and three pairs of short spines on its thorax. The species name means “large-headed”, and the larger workers do have broad, heart-shaped heads packed with the muscles that close their jaws. Like all ants, it has elbowed antennae and a narrow waist between thorax and abdomen.

The most striking feature of a colony is the range of sizes. Leafcutters belong to the genus Atta, which has some of the most size-variable workers of any ant. In one colony you can find tiny workers barely 2 mm long next to soldiers more than 1 cm (0.4 in) long with heads several millimeters wide. The queen is far bigger again, around 2 cm (0.8 in) long, with a large abdomen for producing eggs.

The species was first described by Carl Linnaeus in 1758 as Formica cephalotes and later became the type species of the genus Atta. Its relatives in Atta and the related genus Acromyrmex are also called leafcutter ants; together they make up several dozen species.

Castes: from minims to soldiers

A leafcutter colony divides its work by size. Ant scientists usually recognize four main worker groups, all of them sterile females:

Above the workers are the reproductive castes. Winged queens and males are raised only in mature colonies and leave the nest once a year on a mating flight. The males die soon after mating, while each successful young queen sheds her wings and tries to found a new colony.

This division of labor is so tightly organized that biologists often describe a leafcutter colony as a superorganism: the individual ants act like cells or organs of one larger living unit.

Habitat and range

Atta cephalotes is one of the most widespread leafcutters. It ranges from southern Mexico through Central America and into tropical South America, including the Amazon basin, Colombia, Venezuela, the Guianas, Ecuador, Peru, Bolivia and parts of Brazil. It also lives on Trinidad.

It is mainly a lowland species of humid forest, but it does especially well where the forest has been opened up: along rivers and roads, in clearings, at forest edges, in plantations and in gardens. Young, fast-growing plants that colonize disturbed ground tend to have fewer chemical defenses, which makes them attractive to leafcutters. Other Atta species reach as far north as Texas, Louisiana and Arizona in the United States, and as far south as Argentina. It is one of the classic rainforest animals of the Americas.

Diet: cutting leaves to grow fungus

Leafcutter ants are farmers. They do not digest leaves themselves. Instead, workers cut semicircles from leaves, flowers and other fresh plant material, carry the pieces home and process them into compost for a fungus, Leucoagaricus gongylophorus. The fungus grows only in ant nests and no longer produces spores on its own; it depends entirely on the ants for its survival.

The fungus forms tiny swollen tips called gongylidia, rich in sugars and fats, which the ants harvest. These are the main food of the larvae and the queen. Adult workers also drink plant sap while cutting leaves. A colony can collect leaves from dozens of different plant species, and it learns to stop collecting plants that harm its fungus.

Cutting is done with saw-like jaws that vibrate as the ant slices through a leaf. Foragers follow chemical trails marked with pheromones, and the busiest trails are cleared into smooth “highways” that may run for more than 100 m (330 ft) from the nest. Each forager carries a fragment that weighs many times its own body weight. The full story of how leaves become food is in how do leafcutter ants grow fungus?

Nests and colony life

A mature Atta cephalotes nest is an underground city. On the surface it appears as a broad area of bare, loose soil with many entrance holes, often covering tens to a couple of hundred square meters. Below, tunnels connect hundreds of chambers, some holding fungus gardens, others used for waste. Nests can reach several meters deep, and the ants arrange openings so that air flows through and carries away the carbon dioxide produced by the fungus and the colony.

All of the millions of workers are daughters of one queen, who may lay many thousands of eggs a day at her peak. Waste management is strict: older workers carry spent fungus and dead ants to separate refuse chambers or dumps, and these waste handlers are kept away from the clean garden to reduce the spread of disease.

Leafcutters communicate mainly with chemicals, but they also use sound. By rubbing ridges on the abdomen together (stridulation), a worker produces vibrations that travel through soil and leaves. Workers buried in a collapsed tunnel stridulate, and nestmates dig toward them. For numbers and nest sizes, see how big are leafcutter ant colonies?

Reproduction and life cycle

Like all ants, leafcutters develop by complete metamorphosis: egg, larva, pupa and adult. A colony begins with a mating flight, usually at the start of the rainy season, when thousands of winged queens and males leave their nests together. Each queen mates with several males in the air and stores hundreds of millions of sperm, enough to fertilize eggs for the rest of her life.

Before leaving home, the young queen tucks a small piece of fungus into a pocket inside her mouth. After landing, she bites or breaks off her wings, digs a narrow shaft into the soil and seals herself in a small chamber. There she spits out the fungus, fertilizes it with her own droppings and begins laying eggs. She does not forage at all; she lives on her body reserves, including her now-useless wing muscles, and feeds many of her own eggs to the first larvae.

The first workers emerge after roughly two months and take over the garden and foraging. Most young queens fail: they are eaten by birds, lizards, armadillos and other ants, or their fungus dies. A successful colony grows for several years before producing its own winged queens and males.

Lifespan

The queen is the longest-lived member of the colony. Estimates from wild and laboratory colonies suggest she can live 10–20 years, making leafcutter queens some of the longest-lived insects known. Because no new queen replaces her in Atta, the colony slowly dies out after her death.

Workers live far shorter lives, usually measured in months, and the most dangerous jobs, such as handling waste and foraging far from the nest, tend to be done by older workers that have less life left to lose. Winged males live only a short time once they leave the nest.

Predators, parasites and threats

Leafcutters face many enemies. Army ants, especially Nomamyrmex esenbeckii, raid Atta nests. Armadillos, anteaters, birds and lizards eat workers, and queens on their mating flight are snapped up in large numbers by birds, bats and other predators.

Tiny phorid flies are a constant threat on the trails. They try to lay eggs on workers’ heads, and the larva then develops inside the ant. Minim workers often ride on top of leaf fragments, where they fend off the flies and help clean the leaves. Inside the nest, the biggest danger is a parasitic fungus called Escovopsis that attacks the fungus garden. The ants weed infected pieces from the garden and, in many leafcutter species, carry helpful bacteria on their bodies that produce antibiotic substances against it.

The species has not been assessed by the IUCN, and it is common and widespread. Leafcutters often benefit from forest clearing and roads, which create the disturbed habitats they prefer.

Role in tropical forests

Leafcutter ants are often called the dominant herbivores of the New World tropics. By cutting leaves throughout the year, a large colony prunes the forest around it, opens small gaps in the understory and lets more light reach the ground. Seeds carried into the nest and dropped there may later sprout.

Underground, the ants move huge amounts of soil and bury plant matter, which enriches and aerates the soil. Studies have measured higher carbon dioxide release from soil above Atta nests, produced by the large mass of decomposing plant material inside. For these reasons ecologists call leafcutters ecosystem engineers. The ants, their fungus and their bacteria also form one of the most studied partnerships in biology; only a few other insects, such as some termites and ambrosia beetles, farm fungus in a similar way (see eastern subterranean termite for another social insect with microbial partners).

Leafcutter ants and people

In farms and gardens, leafcutters can strip citrus trees, coffee, cassava and ornamental plants of their leaves within a day or two, so they are regarded as a serious agricultural pest in parts of Latin America. At the same time they are popular in zoos and museums, where glass-walled colonies show the trails and fungus gardens up close.

Winged queens of Atta species are eaten as a traditional delicacy in several countries. In Colombia, roasted queens are sold as hormigas culonas, in southern Mexico as chicatanas, and in parts of Brazil as tanajuras. The genus also gave its name to Princess Atta, an ant character in the animated film A Bug’s Life.

Myths and facts

Myth: Leafcutter ants eat the leaves they carry. Fact: The leaves feed their fungus; the ants eat the fungus.

Myth: Leafcutters sting like fire ants. Fact: Atta workers do not sting. Their defense is biting, and the soldiers’ jaws are strong enough to draw blood.

Myth: Each leaf carrier works alone. Fact: Cutting, carrying, guarding, gardening and waste disposal are split between different sizes and ages of workers, and a leaf fragment may pass through several workers before it reaches the garden.