Leafcutter Ants

Beez Neez now Chy Whella
Big Bear and Pepe Millard
Sun 23 Sep 2012 22:17
Leafcutter Ants




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We do have a bit of a thing for leafcutter ants and always seem to stop and watch these little chaps for ages. Here in Tikal we saw loads of columns of ‘green flags’ making way. There are forty seven species of leaf-chewing ants belonging to the two genera Atta and Acromyrmex. These species of tropical, fungus-growing ants are all endemic to South and Central America, Mexico and parts of the southern USA. As their name suggests these ants "cut and process fresh vegetation, leaves, flowers and grasses to serve as the nutritional substrate for their fungal cultivars."



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The tree that’s being harvested in the distance, Bear at the half way point (nest top far left), chaps on the march


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The nest entrance


Next to humans, leafcutter ants form the largest and most complex animal societies on earth. In a few years, the central mound of their underground nest can grow to more than ninety eight feet across, with smaller, radiating mounds extending out to a radius of two hundred and sixty feet, taking up three hundred and twenty to six thousand five hundred square feet and containing eight million individuals.


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.

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, only 2.5% survive to 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, located within her oral cavity.


Colony hierarchy:

  • 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 than 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. 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 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.

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Ant-fungus mutualism: Their societies are based on an ant-fungus mutualism, and different species of 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 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 only two other groups of insects to use fungus-based agriculture are ambrosia beetles and termites. The fungus cultivated by the adults is used to feed the ant larvae, and the adult ants feed off the leaf sap. The fungus needs the ants to stay alive, and the larvae need the fungus to stay alive.



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Bear examining the impressive waste area


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So neat and so busy


Waste management: 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. The necrotrophic parasite Escovopsis of the fungal cultivar threatens the ants' food source, and is thus a constant danger to the ants. The waste-transporters and waste heap workers are the older, more dispensable leaf-cutter ants, ensuring the healthier and 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.


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 micro-organism lives in the metapleural glands of the ant. Actinobacteria are responsible for producing the majority of the world's antibiotics today.



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The chap at the bottom is cadging a lift – I was laughing too much to keep still



Parasitism: When the ants are out collecting leaves, they are at risk of being attacked by the phorid fly, a parasitic pest which lays eggs into the crevices of the worker ants' heads. Often a minim will sit on the worker ant and ward off any attack. Also, the wrong type of fungus can grow during cultivation. Escovopsis is a highly virulent fungus that has the potential to devastate an ant garden, as it is horizontally transmitted.


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.



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Cut leaves dropped when heavy rain fell and the workers ran for shelter, no use now. We can understand dropping a leaf and running for the nest, we ran for a shandy.



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When it rains here - it really rains.



Atta species are capable of defoliating an entire citrus tree in less than 24 hours.

Deterring the leafcutter ant Acromyrmex lobicornis from 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 thirty days.



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Putting a stick in the way didn’t deter the column for long, scouts went over, some went round, verdict – go round


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