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 leafcutter ants, fungus farmers of the rainforest, and fearsome army ants, the ultimate jungle raiders. This week we explore the fascinating relationship between tropical myrmecophytes, plants that partner with ants, and the feisty ants which serve as their bodyguards. While walking along a dusty road, we encountered a brilliant green Acacia tree near a pasture. This remarkable tree was completely unmolested by any type of leaf-munching caterpillar, sucking insect, or large mammal such as horses or cows that grazed nearby. A closer inspection revealed fearsome looking thorns arising from nodes of the branches.
Surely these thorns, locally known as cockspurs or bullhorns, helped explain why large grazing mammals avoided the otherwise delectable looking leaves of the Acacia. A mouthful of thorns would be a painful experience indeed. However, as I fondled the foliage of the Acacia, I was instantly attacked by a furious band of ants that bit and stung my hand with extreme prejudice. Their sting was memorable and my swollen hand throbbed and itched for hours after the encounter. The secret weapon of the Acacia, ant bodyguards, is part of a symbiotic deal struck eons ago by acacias and ants in the genus Pseudomyrmex. The deal works like this. A newly mated Pseudomyrmex queen lands on the bullhorn acacia and locates a large thorn. Either by chewing a new hole of by using an existing one, she enters the hollow thorn and lays eggs. Eggs hatch and develop into sterile workers. Workers, the queen, and subsequent broods subsist on carbohydrate laden nectar produced by specialized glands near the base of leaves.
Small holes in the thorn allow ants to enter and exit.
But ants, like people, cannot live by sugar alone. At their tips, leaves also produce specialized structures called Beltian bodies. These detachable tidbits are rich in nutrients such as proteins and lipids. By providing room and board, acacia plays the generous host for Pseudomyrmex. In return, fearless worker ants provide maniacal protection of the acacia from caterpillars, sap-sucking insects, probably large herbivores, and certainly, nosey entomologists. 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 acacia. In a clever series of studies, Dan Janzen demonstrated that in addition to protecting acacias from herbivores, ants aggressively removed nearby plants that might compete with acacia 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 Insect Societies" by Edward O. Wilson and the fascinating articles “Interaction of the bull's-horn acacia (Acacia cornigera L.) with an ant inhabitant (Pseudomyrmex ferruginea F. Smith) in Eastern Mexico” and “Coevolution of mutualism between ants and acacias in Central America” by Dan Janzen were used as references for this episode.