Leafcutter ant

Colony lifecycle

Reproduction and colony founding

Winged females and males leave their respective nests en masse and engage in a nuptial flight known as the revoada (Portuguese) or vuelo nupcial (Spanish). Each female mates with multiple males to collect the 300 million sperm she needs to set up a colony.

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. 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. Colonies are generally founded by individual queens — haplometrosis. Because colonies with multiple queens over the lifespan of the colony have been found by a large number of investigators – by Weber (1937), Jonkman (1977), Huber (1907), Moser & Lewis (1981), Mariconi & Zamith (1963), Moser (1963), and Walter et al. (1938) — it is believable that some colonies have multiple foundresses — termed pleometrosis. Colony founding by pleometrosis has only been confirmed in Atta texana, by Vinson (1985).

Colony hierarchy

In leafcutter colonies, ants are divided into castes, mostly of different sizes, that perform different functions. Acromyrmex and Atta exhibit a high degree of polymorphism, four castes being present in established colonies — minims, minors, mediae, and majors, also known as soldiers or dinergates. Atta ants are more polymorphic than Acromyrmex, with less difference in size from the smallest to largest types of Acromyrmex.

• Minims are the smallest and thinnest workers, and tend to the growing brood or care for the fungus gardens. Head width is less than 1 mm.
• Minors are slightly larger than minim 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 threats to the foraging lines. Head width is around 1.8–2.2 mm.
• Mediae are the generalized foragers, which cut leaves and bring the leaf fragments back to the nest.
• Majors, the largest worker ants, act as soldiers, defending the nest from intruders, although recent evidence indicates majors also 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 bodies 16 mm long and heads 7 mm wide.

Ant–fungus mutualism

Their societies are based on an ant–fungus mutualism. The only two other groups of insects to use fungus-based agriculture are ambrosia beetles and termites. Different species of ants use different species of fungus, but all of the fungi the ants use are members of the family Lepiotaceae. The ants actively cultivate their fungus, feeding it with freshly cut plant material and keeping it free from pests and molds. This mutualistic 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. The fungus cultivated by the adults is used to feed the ant larvae, and the adult ants feed on leaf sap. The fungus needs the ants and the larvae need the fungus; mutualism is obligatory.

The fungi used by the higher attine ants no longer produce spores. These ants fully domesticated their fungal partner 15 million years ago, a process that took 30 million years to complete. Their fungi produce nutritious and swollen hyphal tips (gongylidia) that grow in bundles called staphylae, which have no other function than to feed the ants. Leucoagaricus gongylophorus is the most commonly documented fungi farmed by higher attine ant species.

Behaviour

Leafcutter ants have very specific roles in taking care of the fungal garden and dumping the refuse. Waste management is a key role for each colony's longevity. The necrotrophic parasitic fungus Escovopsis threatens the ants' food source and thus is a constant danger to the ants. The waste transporters and waste-heap workers are the more dispensable leafcutter ants, ensuring the more precious ants can work. 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, the heap workers organise the waste and constantly shuffle it around to aid decomposition. A. colombica have been observed placing dead ants around the perimeter of the waste heap.

In addition to feeding the fungal garden with foraged food, mainly consisting of leaves, it is protected from Escovopsis by the antibiotic secretions of Actinomycetota (genus Pseudonocardia). This mutualistic micro-organism lives in the metapleural glands of the ant. Most of the antimicrobials known today were originally isolated from actinomycetes, especially from the genus Streptomyces.

Leafcutter ants use chemical communication and stridulation (substrate-borne vibrations) to communicate with each other.

Leafcutter ants prefer disturbed habitats, likely due to higher concentrations of pioneer plant species. These are more attractive food sources because pioneer plants have lower levels of secondary metabolites and higher nutrient concentrations than the shade-tolerant species that will come later.

Parasites

When the ants are out collecting leaves, they are at risk of attack by some species of phorid flies, parasitoids that lay eggs in the crevices of the worker ants' heads. Often, a minim will stand on a worker ant's leaf and ward off any attack.

Also, the wrong type of fungus can grow during cultivation. Escovopsis, a highly virulent fungus, has the potential to devastate an ant garden, as it is horizontally transmitted. Escovopsis was cultured, during colony foundation, in 6.6% of colonies. However, in one- to two-year-old colonies, almost 60% had Escovopsis growing in the fungal garden.

Nevertheless, leafcutter ants have many adaptive mechanisms to recognize and control infections by Escovopsis and other micro-organisms. The most common known behaviors rely on workers reducing the number of fungal spores by grooming, or removing an infected piece of the fungus garden and throwing it away at the waste dump (described as weeding).

Interactions with humans

In some parts of their range, leafcutter ants can be a serious agricultural pest, defoliating crops and damaging roads and farmland with their nest-making activities. For example, some Atta species are capable of defoliating an entire citrus tree in less than 24 hours. A promising approach to deterring attacks of the leafcutter ant Acromyrmex lobicornis on crops has been demonstrated. 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.