There are several species of leaf-cutter ant. The best known, Atta sexdens, is an animal of the tropical rainforest. This ant is mainly found in Central and South America (Costa Rica, Argentina, Paraguay, and Brazil) but it also reported in Texas, Louisiana and occasionally as far north as New Jersey. They are remarkable ants because they are very large, they live in particularly well-structured societies and, most peculiar of all, they farm some of their food.

Leaf-cutter ants ensure their food supply through a mutually-beneficial association with a type of fungus. The ants grow these fungi in gardens created from crescent-shaped pieces cut from living leaves. The ants hold the pieces of leaf above their heads as they scurry back to their nests. This habit earns the leaf-cutters their other common name: parasol ants.

The ants do not eat the leaves because they cannot digest cellulose¹. Instead, they use the leaves to farm a special sort of fungus. This fungus digests the cellulose in the leaves and uses it to fuel its own growth. The leaf-cutter ants then feed on the fungus. The ants and the fungus have never been found to exist separately from each other in nature and they each gain something from the relationship; the ants get food from the fungus and the fungus gets a place to live. The fungus also benefits from a guaranteed food supply in the form of leaves and ant droppings and it receives protection from predators, parasites and competition from other types of fungi. In return, the ants gain a reliable source of food and a supply of necessary nutrients that they cannot obtain from other food sources. This mutual arrangement is an example of symbiosis².

Leaf-cutter ants can only cultivate their fungus gardens by gathering a very large amount of leaves and this means that they must build enormous nests. The nests cause problems for agriculture but leaf-cutter ants are important to the ecological systems in which they live because by breaking down plant material they enrich the soil and encourage new plant growth.

Description

Leaf-cutter ants are relatively large ants. They are a rusty red or brown colour and have a spiny body and long legs. There are three main castes within a nest: the queen, worker and soldier. Only the queens have wings and these ants are also known as 'reproductives' or 'swarmers'. Although most of the ants in the nest are female, only the queens produce eggs. Queens are very large - usually well over 50mm long.

The worker ants range in size from 13mm to 27mm in length and specialise in particular tasks useful to the colony. Three of the most important tasks are leaf carrier, leaf preparer and garbage collector.

Leaf carriers cut the crescent-shaped leaf sections from the source plants with their scissor-like jaws and carry the foliage to the nests. They can carry leaves and parts of leaves that weigh many times their body weight. They forage at distances of up to 250 metres away from the nest. They may do this to avoid killing the plants around their nest or perhaps to ensure a varied diet with a lower risk of toxicity. The ants leave an invisible pheromone scent on their trails so that they can find their way home.

As leaves are brought into the nest, the leaves are handed on to the leaf preparers who lick and chew the edges of each leaf, combining it with droplets of fecal liquid, to form a pulp ready to be added to the football-sized fungus gardens. Sometimes the leaf preparers carry out this task while the leaf carrier is still bringing the leaf back to the nest. The leaf preparers are smaller than the leaf carriers so that they can ride on their backs.

The garbage collectors remove any leaf parts that are not suitable for use within the fungus garden. They move unwanted material to special dump chambers within the nest. They also drag used garden material and any ants that die within the nest into these chambers.

Soldier ants are female and are between 25mm and 50mm in length. They defend the nest from intruders. The jaws of a soldier are so strong that they would be capable of cutting leather. The few males in the nest are much smaller. They are rusty brown. Their only function is to wait around the nest until the queen leaves the nest for mating. This usually happens in the night during the months of April or May.

Lifestyle

Leaf-cutter ants are eusocial³ insects that live in large colonies in underground nests.

Nests are usually sited in areas of the rainforest floor that have high moisture such as drainage channels and dry streambeds. The nests are very large and consist of as many as 2,000 football-sized chambers. The soil displaced from digging all these chambers can weigh as much as 80,000kg. The ant nests can extend up to six metres underground and will have many dozens of entrances.

The diet of leaf-cutter ants can sometimes include seeds, fruit and cereals but the hyphae⁴ of the cultivated fungus are the major source of nutrition for the adults and the only source of nutrition for the young. Although the fungus is capable of producing fruiting bodies similar to mushrooms under laboratory conditions, the ants never allow the mushrooms to appear in the nest. Instead, the ants eat the mycellium. The fungus feeds on the leaf pulp provided by the ants and uses the nutrients to grow. The workers transplant bits of fungus tissue from older parts of the garden to the new leaf pulp and the new material quickly becomes covered with a cotton-like mass of fungal mycellium. It is these threads, the hyphae, that the ants eat. The workers continuously tend their gardens, eating any foreign hyphae that try to invade the chamber and adding fecal secretions. These secretions contain chemicals that help the ants eliminate unwanted species of fungi. In the absence of ants other types of fungi quickly colonise the fungus garden.

Eventually a garden can no longer support the growth of the fungus. The ants build a new garden in a different chamber, or the old garden is recycled. The old leaf material is transported to the surface and dumped several metres away from the nest.

Love and War

The enemies of leaf-cutter ants are small birds, insects, frogs, ant bears, anteaters and, of course, humans. One of the leaf-cutter ant's most serious threats is from a small fly related to the coffin fly. These flies lay their eggs on the backs of the worker ants' necks. When the larvae hatch they eat away the brains of the ants. To protect themselves from these flies the worker ants carry smaller worker ants called 'minima' that act as bodyguards.

Matings of leaf-cutter ants take place in areas called leks. A queen will mate with from three to eight males. She can store up to 250 million sperm for more than ten years, in her spermatheca⁵ although queens in captivity have been known to live and remain fertile for up to 27 years. When a queen has been fertilised, she will fly some distance from the mating site – perhaps more than 10km - to form a new colony. She discards her wings and then digs 30mm straight down into the soil where she creates a small chamber, lays a small number of eggs and starts a new fungus garden by regurgitating some of the fungus she took from her old nest. The queen will continue to look after the garden of fungus as well as the eggs until enough workers hatch to take over those duties. The growth of the colony may be slow at first but within two or three years may contain between five and eight million ants. Once the queen has established a nest she continues to produce eggs for the duration of the nest's existence of ten to 16 years.

Nasty Habits and Redeeming Features

Leaf-cutter ants cause major problems for agriculture and are therefore considered by many to be pests. They cause millions of dollars' worth of damage each year to economically important crops such as citrus, cocoa, coffee, maize, and cotton and they compete with cattle for grasses. In addition, cattle sometimes fall into old collapsed nests.

Furthermore, leaf-cutter ants eat stored dried foodstuffs such as cereal grains, flour, dried beans, and cattle meal and their colonies undermine the foundations of buildings causing severe structural damage. Leaf-cutter ants also occasionally invade homes looking for cereals to eat but they never nest indoors. However, they can sometimes be found crawling on buildings in large numbers, attracted by the lights, but they cause no damage.

The only recommended method for removing a leaf-cutter ant nest is to pour a diluted residual insecticide into it. However, the nest may be deep so care should be taken to avoid polluting water supplies.

On the other hand, leaf-cutter ants help maintain the health of the environment; the by-products from the leaves, fungi and ant wastes fertilise the soil. Also leaf-cutter ants are a useful source of protein for humans; they are eaten in parts of Mexico. It is also alleged that the Native Americans used the jaws of leaf-cutter soldier ants as sutures to hold together the edges of a wound.

The well-ordered form of society evolved by leaf-cutter ants is of great scientific interest. Each worker carries out its specific task for the benefit of the whole colony and the studying of the leaf-cutter ants' social behaviour has helped us learn about how such systems have evolved.