Last week we traveled with students from the University of Maryland to Belize to visit the masters of rainforest recycling, nasute termites. This week we return to the rainforest to meet the most important group of indigenous farmers found in tropical forests in the New World, leafcutter ants.
In addition to humans, leafcutter ants are one of only a few species known to cultivate a crop as a source of food. Night and day members of the worker caste search for nutritious leaves on trees, vines, and shrubs. When scouts find a suitable 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 while smaller workers sometimes hitchhike on leaves and help defend their sisters from marauding predators and parasitic flies. Nearby, large imposing workers, a.k.a soldiers, with massive powerful jaws also defend their sisters and the colony.
With so many hitchhiking helpers, progress back to the colony is sometimes difficult.
Powerful jaws of the soldier can dismember small invertebrate predators and inflict painful bites on larger vertebrates.
As leafcutters remove foliage from a tree, a parade may extend for distances of more than 200 yards as workers carry leafy cargo back to a subterranean nest. Scientists estimate that more than 10% of leaf production in tropical forests is harvested by leafcutter ants.
A leafcutter nest is a marvel of engineering. A single nest 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 vital oxygen seeps in.
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 and this bountiful crop is harvested as the food source for the entire 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. Gongylidia are harvested and fed to the developing larvae and distributed throughout the colony to nourish 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 weight pounds of vegetation annually. When nests are established near orchards or crops, leafcutters can strip trees and leafy vegetables overnight causing significant crop loss. Often, irate farmers destroy colonies of their six-legged counterparts. One humorous account related by Hölldobler and Wilson of a westerner’s attempt to grow a European style vegetable garden in Belize reported that the gardener “… arose one morning and found our garden defoliated: every cabbage leaf was stripped…of the carrots nothing was seen…into a hole in the mound, ants, moving in quickened step, were carrying bits of our cabbage, tops of carrots, the beans-in fact our entire garden was going down that hole.”
Despite their proclivity to be agricultural pests, leafcutter ants play a vital role in recycling plant material and enriching and cultivating tropical soils. For millennia in tropical jungles in the New World, legions of leafcutters have been the consummate farmers in the rainforest.
In addition to leaves, flowers are also carried back to the colony to feed the fungus.
We thank the hearty crew of ‘BSCI 339M, Mayan Culture and the Interface Between Tropical Rainforests and Coral Reefs’ 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.