Bug of the Week January 26,2009

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Once again Bug of the Week stowed away with some adventurous students on a remarkable trip to the tropical rainforest of Belize. Last week we crossed paths with the rapacious army ants in “Jungle Raiders”. This week we visit the most important group of herbivores found in tropical forests in the New World, leafcutter ants. In Belize leafcutter ants are known as Wiwis. In contrast to army ants who are the consummate hunters of the forest floor, leafcutters are the farmers in the rainforest. In addition to humans, leafcutter ants are one of but a few species known to cultivate a crop for food. Night and day members of the worker caste of leafcutters search for nutritious leaves of trees and shrubs. When scouts find a good food source, they direct other workers to the bounty by releasing trail-marking chemicals called pheromones. With powerful jaws, workers clip small sections of leaves and carry them to the ground where they join a rambunctious procession of nest mates. In this parade intermediate sized workers busily transport leaf sections and smaller workers sometimes hitchhike on leaves and help defend their sisters from marauding predators and parasitic flies. Nearby, large imposing workers called soldiers also defend their sisters and the colony with powerful jaws. Last week we observed the ability of an army ant solider to suture a wound by allowing the ant to clasp the skin with tenacious skewer-like jaws. In addition to pinching skin the army ant’s jaws are well adapted to latching onto prey and transporting it back to the colony. We repeated the “wound suturing” experiment with a large leafcutter soldier with a somewhat different and surprising result. Jaws of leafcutters are uniquely adapted to severing leaf tissue as a carving knife severs meat. When applied to a finger, serrate jaws of the leafcutter soldier neatly sliced the skin opening a bloody wound rather than closing one, thereby initiating a hasty search for an army ant solider to use as a suture. As leafcutters remove foliage from a tree, a parade of ants may extend for distances of more than 200 yards as workers carry leafy cargo back to a subterranean nest. A leafcutter nest is a marvelous structure that may contain several million ants and occupy 600 square meters of forest floor. Sophisticated ventilation systems cool the bustling nest and allow carbon dioxide to escape while drawing in oxygen. Once inside the nest, leaves are delivered to other workers that take leaf sections and clip them into ever smaller fragments. These fragments are carefully inserted into a garden of living fungus maintained by the ants. Leaves serve as a substrate for fungal growth harvested as the source of food for the ant colony. The fungus garden is meticulously tended by workers. Destructive alien fungi are detected and removed. Secretions produced by the queen and workers facilitate the growth of the cultivated fungus. Fungal strands produce specialized structures called gongylidia that are harvested by workers. Gongylidia are fed to the developing larvae and distributed throughout the colony to feed workers and the queen. Due to their agrarian life style, leafcutter ants are also commonly called fungus growing ants. To support their enormous colonies, leafcutters remove vast amounts of vegetation each day. It is estimated that large colonies may remove more than 500 dry matter pounds of vegetation annually. When nests are established near orchards or crops, leafcutters can strip trees and vegetables overnight. Often, their nests are destroyed by farmers.

We thank Payal Patel for providing the inspiration for this Bug of the Week. The wonderful book "The Ants" by Bert Hölldobler and Edward O. Wilson was used as a reference. For more information on leafcutter ants, please visit the following web sites.

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