Last week Bug of the Week escaped winter’s chill to visit strange grasshoppers and jewel beetles in the Australian wilderness. This week we return to the land down under and travel to the Coral Sea for a visit to Lizard Island, home of arboreal members of the ant clan called weaver ants. While bumbling through the underbrush in search of funnel spiders, I bumped into a small tree bearing several football-sized clusters of glossy leaves. I was surprised and delighted when scores of beautiful green ants issued forth from one leaf-cluster and set to work defending their redoubt by dropping on my arm and attacking.
Fortunately, the furious soldiers lacked stingers and their bite was mildly unpleasant at worst. Their attack left the air laced with the odor of formic acid released from specialized poison glands as a defense. Green tree ants and other weaver ants represent a unique branch of the ant’s evolutionary tree. While other more familiar ants build colonies in soil or decaying wood, weaver ants live the life arboreal and construct clever nests in the canopies of trees. Nest building begins when one or more large workers, known as majors, grasp the edge of a leaf and fold it over or hold it in close proximity to an adjacent leaf. Other majors soon join the effort and in a fascinating display of cooperation they stand shoulder to shoulder, pulling the leaf margins close together. When the gap narrows, the workers stand in place, holding the leaf fast, and await the next step in the nest making process. Other workers gather ant larvae from deep within the colony. These youngsters are approaching pupation, a time when they produce silk, and in their heads are fully functional silk glands. The workers gather several of these larvae and carry them to the construction site where the leaf-grasping workers await their arrival. Using the silk-spinning larvae as living bobbins, workers move the larvae back and forth, weaving silk across the gap and firmly stitching the leaves together. This process is repeated time and again with other nearby leaves until the nest is complete.
Green tree ants are on high alert when nosy humans are nearby.
Green tree ants build multiple nests throughout a tree’s canopy and several trees may be enlisted to house a single colony. In one of these individual nests resides the glorious queen whose task it is to eat meals of protein and carbohydrates brought to her by the workers. These rich nutrients are turned into thousands of eggs. Since workers large and small, young and old, share the same mother, the queen, they are all sisters and the building, care, and defense of the colony truly is a remarkable act of sisterly cooperation. The size of some weaver ant colonies has been estimated to exceed more than 500,000 workers. Like many of their kin, green tree ants are omnivores, consuming other insects they capture and gathering carbohydrates in the form of honeydew excreted by legions of arboreal sucking insects such as scale insects (see “There's a sucker born every day - Tuliptree scale”). The value of weaver ants in pillaging plant-eating insects was recognized nearly 1,800 years ago by citrus growers in China. Ancient writings show that nests of weaver ants were regularly transported and installed in orchards where ravenous workers converted citrus-eating pests into food for the colony and queen. These clever orchardists are credited with one of the earliest records of a practice still in use today called biological control.
Bug of the Week thanks Dr. Shrewsbury for providing the photographs used in this episode and the students and faculty of BSCI 279A, Natural History, Ecology, and Geology of Australia, for providing the inspiration for this story. The spectacular reference “The Ants” by Bert Hölldobler and Edward O. Wilson provided the information used in preparation of this episode. To learn more about green tree ants and other weaver ants, please visit the following web sites: