Keep an eye out for circular cuts on leaf margins of trees and shrubs in your garden. This is likely the handiwork of marvelous leafcutter bees.
Here: This week we continue our sojourns to warmer climates where in weeks past we met powerful carpenter ants on plants, ants tending tropical caterpillars, ants gathering seeds, tropical wasps, and sap-sucking froghoppers. Let’s start this week’s adventure here in chilly Maryland.
In a tree hollow leaf sections are rolled into a tube and packed with pollen to feed bee babies.
A few years back just before the winter holidays a student arrived in our laboratory with a sturdy hunk of tree trunk that had been split to reveal a rotten cavity riddled with galleries. Within these galleries were several small bundles of carefully rolled leaves. Each bundle looked like a miniature cigar composed of dozens of small circular leaf sections carefully assembled into a cylinder. Each small wrapper contained tiny balls of pollen. These curious creations were the work of one of our most important and interesting native pollinators, the leafcutter bee. Leafcutter bees leave other clues of their presence as they visit our landscapes. While gardening or simply communing with vegetation, you may have seen roses or other trees and shrubs with almost perfectly round circles of tissue removed from their leaves. The artisans behind these creations are leafcutter bees.
These small hairy bees are solitary and do not build large colonies like honeybees or bumblebees. Each female leafcutter bee builds a nursery for her young. During the summer, leafcutters construct nests in voids of trees, hollow branches, or within the pithy stems of plants such as raspberries and roses. Once a suitable nest site has been found, the bee clips small circular sections of leaves and transports them back to the tree or shrub where they are used to line a hollow chamber within the branch or stem. After assembling the leaf disks to form a hollow tube, the leafcutter gathers pollen and nectar from nearby flowers and packs the tube with these nutritious provisions. An egg is then deposited on the pollen cake and the next chamber in the nursery is built with leaf disks and stocked with pollen. This continues until several long tubes line the gallery.
Coneflowers are a favorite source of pollen for leafcutter bees in my garden.
Eggs placed within the rolled leaves hatch into small legless larvae that eat the pollen. After the larvae have completed development, they form pupae from which emerge adult leafcutter bees that spend the winter within the rolled leaves. With the arrival of spring and new supplies of tender leaves, nectar, and pollen, adult bees emerge and begin the work of finding mates and making brood chambers for the next generation of leafcutters. Leafcutter bees are extraordinarily docile and unlikely to sting. As important pollinators of several agricultural crops, including blueberries and alfalfa, and many native trees and shrubs they deserve our care and attention to help preserve their vital ecosystem services. So, with spring just around the corner keep an eye open for small hairy bees collecting pollen in your garden and try to observe their precise handiwork on your curiously clipped leaves.
Powerful jaws of the soldier can dismember small invertebrate predators and inflict painful bites on larger vertebrates.
There: Once again we jump 2000 miles south to the tropical rainforests of Costa Rica to enjoy magnificent leafcutter ants and their handiwork. In addition to humans, leafcutter ants are one of only a few species of animals 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. 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.
Mighty workers clear a dead leaf from the busy jungle trail leading back to the nest.
Leafcutter ants don’t leave much behind when defoliating favored plants.
A leafcutter nest is 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. Irate farmers often 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, leafcutters 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, flower petals and stems will be added to the subterranean garden of the leafcutter ants.
References
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.