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Leafcutter ants are social insects found in warmer regions of the Americas. These unique ants have evolved an advanced agricultural system based on ant-fungus mutualism. They feed on special structures called gongylidia produced by a specialized fungus that grows only in the underground chambers of the ants' nest.

Different species of leafcutter ants use different species of fungus, but all of the fungi the ants use are members of the Lepiotaceae family. The ants actively cultivate their fungus, feeding it with freshly-cut plant material and maintaining it free from pests and molds. This mutualist relationship is further augmented by another symbiotic partner, a bacterium that grows on the ants and secretes chemicals- essentially the ants use portable antimicrobials. Leaf cutter ants are sensitive enough to adapt to the fungi's reaction to different plant material, apparently detecting chemical signals from the fungus. If a particular type of leaf is toxic to the fungus the colony will no longer collect it.

Atta colombica workers transporting leaves

The only two other groups of insects that have evolved fungus-based agriculture are ambrosia beetles and termites.

Leafcutter ants comprise two genera — Atta and Acromyrmex — with a total of 41 species^[1] (15 in Atta and 26 in Acromyrmex), some of which are major agricultural pests.

The Acromyrmex and Atta ants have much in common anatomically; however, the two can be identified by their external differences. Atta ants have 3 pairs of spines and a smooth exoskeleton on the upper surface of the thorax while Acromyrmex have 4 pairs and a rough exoskeleton.^[2] A mature leafcutter colony can contain more than 8 million ants, mostly sterile female workers. They are divided into castes, based mostly on size, that perform different functions. Acromyrmex and Atta exhibit a high degree of biological polymorphism, four castes being present in established colonies - minims, minors, mediae and majors. Majors are also known as soldiers or dinergates. Atta ants are more polymorphic than Acromyrmex, meaning that there is comparatively less difference in size from the smallest to largest types of Acromymex.

Reproduction and colony founding

Winged females and males leave their respective nests en masse and engage in a nuptial flight known as the revoada. Each female mates with multiple males to collect the 300 million sperm she needs to set up a colony.^[3] Once on the ground the female loses her wings and searches for a suitable underground lair in which to found her colony. The success rate of these young queens is very low and only 2.5% will go on to establish a long-lived colony. In order to start her own fungus garden, the queen stores bits of the parental fungus garden mycelium in her infrabuccal pocket, which is located within her oral cavity.^[4]

Colony Hierarchy

Leaf cutter ant Atta cephalotes

• Minims are the smallest workers, and tend to the growing brood or care for the fungus gardens. Head width is less than 1 mm.

• Minors are slightly larger minima workers and are present in large numbers in and around foraging columns. These ants are the first line of defense and continuously patrol the surrounding terrain and vigorously attack any enemies that threaten the foraging lines. Headwidth are around 1.8-2.2 mm

• Mediae are the generalized foragers, who cut leaves and bring the leaf fragments back to the nest.

• Majors are the largest worker ants and act as soldiers, defending the nest from intruders, although there is recent evidence that majors participate in other activities, such as clearing the main foraging trails of large debris and carrying bulky items back to the nest. The largest soldiers (Atta laevigata) may have total body lengths up to 16 mm and head widths of 7 mm.

Parasitism

When the ants are out collecting leaves, they are at risk of being attacked by the phorid fly, a parasitic pest which lay eggs into the crevices of the worker ants head. Often a minim will sit on the worker ant and ward off any attack.^[5] The necrotrophic parasite Escovopsis of the fungal cultivar threatens the ants food source. Escovopsis is highly virulent and has the potential to devastate an ant garden. Escovopsis is horizontally transmitted. Currie et al. (1999) found that Escovopsis was cultured, during colony foundation, in 6.6% of colonies. However, in 1 to 2 year old colonies almost 60% had Escovopsis growing in the fungal garden. ^[6][7]

Interactions with humans

In some parts of their range, leafcutter ants can be quite a nuisance to humans, defoliating crops and damaging roads and farmland with their nest-making activities.^[3] For example, some Atta species are capable of defoliating an entire citrus tree in less than 24 hours. In Central America, leafcutter ants are referred to as "Wee Wee" ants, though not based on their size. They are one of the largest ants in Central America. Deterring the leafcutter ant Acromyrmex lobicornis defoliating crops has been found simpler than first expected. Collecting the refuse from the nest and placing it over seedlings or around crops resulted in a deterrent effect over a period of 30 days. ^[8]

Antibiotics produced by mutualistic Actinobacteria

In addition to feeding the fungal garden with foraged food, mainly consisting of leaves, it is protected from Escovopsis by the antibiotic secretions of Actinobacteria(genus Pseudonocardia). This mutualistic microorganism lives in the metapleural glands of the ant. ^[9] Actinobacteria are responsible for producing the majority of the worlds antibiotics today.

Waste management

Interestingly, leaf-cutter ants have very specific roles when it comes to taking care of the fungal garden, and dumping the refuse. Waste management is a key role for each colony's longevity. Escovopsis is of course a constant danger to the ants. The waste-transporters and waste heap workers are the older more dispensable leaf-cutter ants, ensuring that the healthier younger leaf-cutter ants can work on the fungal garden. The Atta colombica species, unusually for the Attine tribe, have an external waste heap. Waste-transporters take the waste, which consists of used substrate and discarded fungus, to the waste heap. Once dropped off at the refuse dump, heap-workers organise the waste and constantly shuffle it around to aid decomposition. A compelling observation of Atta colombica was that the dead ants were placed around the perimeter of the waste heap. ^[10][11]

Atta colombica, queen with larvae and workers on substrate	Workers of Atta colombica at work	Two leafcutter ants	Collecting leaves.
Leafcutter ants in Costa Rica.

See also

• Atta sexdens
• Lepiotaceae

References