Leaves of the Cecropia are remarkably damage free owing to their Azteca ant bodyguards.
Once again Bug of the Week escapes the icy grip of the mid-Atlantic winter and continues its tropical adventure in Belize. In previous episodes we met beautiful butterflies like the blue morpho and malachite, as well as cryptic lanternflies, hairy tarantulas, motherly treehoppers, and fearsome army ants. This week we explore a remarkable relationship between tropical myrmecophytes, plants that partner with ants, and the feisty ants which serve as bodyguards.
Along the banks of the New River near Orange Walk we encountered a stand of vibrant Cecropia trees with their iconic umbrella shaped leaves. While other trees and shrubs nearby wore telltale scars of encounters with leaf cutter ants and munching caterpillars, leaves of the Cecropia were largely unmolested. When one curious student placed a finger near a small opening on the trunk of a tree, the explanation for the pristine condition of the leaves quickly became clear. A millisecond after the finger contacted the stem; it was attacked by a furious band of ants, one of which delivered a memorable sting. Even after the finger retreated, ants issued forth from the small hole in the tree for several minutes and furiously searched for something to attack.
Feisty ants swarm onto the finger of an inquisitive human and guard the stem of the Cecropia from further incursions.
The secret weapon of the Cecropia, ant bodyguards, is part of a symbiotic deal struck eons ago by Cecropia and ants in the genera Azteca and Pachycondyla. Of these two, Azteca are the most common and widely distributed. The deal works like this. Mated Azteca queens chew a hole in the stem of a Cecropia tree, enter, and initiate a colony in the hollow space within. The queen seals the hole and lays eggs that hatch and develop into sterile workers. Subsequent broods of workers unseal the chamber and issue forth to consume specialized nutritional structures called Müllerian bodies that are produced by the tree. These detachable tidbits of rich carbohydrates serve as a source of energy for the ants. In addition to consuming Müllerian bodies, Azteca ants hunt and kill prey alighting on the plants. Leaves of one species of Cecropia have Velcro-like hairs on their surface that enable the tiny hooked claws of the ants to adhere strongly. With a super powerful toehold, legions of small workers can capture prey thousands of times larger than themselves and collectively dismantle and consume them. Yikes!
The hollow internodes of the Cecropia serve as home and nursery for the Azteca ant colony.
By providing room and board, Cecropia plays the gracious host for Azteca. In return, fearless worker ants provide maniacal protection of the Cecropia from other insect herbivores and nosey vertebrates, including college students. One additional benefit of the bodyguards is their role as vegetation managers. In addition to being devoid of leaf-eating insects, choking vines that ascended other nearby plants were notably missing from the Cecropia. Azteca ants are believed to aggressively remove encroaching vines that might compete with Cecropia for sunlight. All deals should be this good.
References
We thank the fearless crew of BSCI 279M: Tropical Biology in Belize for providing the inspiration for this Bug of the Week.
The wonderful book "The Ants” by Burt Hölldobler and Edward O. Wilson”, and the fascinating articles “Arboreal Ants Use the “Velcro® Principle” to Capture Very Large Prey” by Alain Dejean, Celine Leroy, Bruno Corbara, Olivier Roux, Régis Céréghino, Jérôme Orivel, and Raphaël Boulay, and “The Cecropia-Azteca association in Costa Rica” by John T. Longino were used to prepare this episode.