Weaver ants are a formidable presence in trees across the tropics. However, their kingdoms (more accurately “queendoms”) in the canopies are not built in isolation. Nature often presents itself in an intricate display of all things biotic and abiotic woven together. This is especially true in the case of weaver ants. When a colony of up to half a million ants share their space with nearly every other form of life that lives in the tropics, staring at a tree can promise more action and drama than a television show.
Ants in my plants
Among all the relationships that the weaver ant (Oecophylla spp.) shares with its environment, the relationship with its host tree is foundational. The leaves of this tree will be engineered into grand structures that become the home of a queen, thousands of workers, and young larvae. Its trunk and branches will act as a vast network of transport lines for the colony. The flora and fauna on the tree will contribute to feeding the hunger of an entire colony. In exchange, the ants defend these trees with an unwavering sense of territoriality.
When I got close enough to a weaver ant colony, I was left wondering if I unknowingly said or did something to offend the entire colony. The workers that spotted the intruder, immediately signalled this to their colleagues using a mix of movements and chemical releases. Within moments, there were hundreds of ants with their abdomens raised and mandibles wide open ready to defend their territory. I took solace in knowing that a gecko that visited the colony shortly after received the same treatment.
This deep-rooted sense of territoriality combined with sheer numbers seemingly induces the same “I wonder if I did something to offend them” feeling in herbivores (or pests, depending on whose side you’re on) that feed on these host plants. So much is their effect on these herbivores that records as early the third century AD have documented their use as biological pest control in citrus orchards. Through the centuries their effectiveness in preventing these crop-eating herbivores has spread to a wide list of crops like mango, cocoa, coffee, and cashew nut among others.
In their aggression towards nearly all visitors to their territory, they have been known to repel more than just herbivores that harm the host. Their aggression has been shown to repel some of the host’s natural pollinators as well. Interestingly though, despite their effect on lowering the number of visits by pollinators, host trees typically still go on to successfully bear fruit.
The dietary needs of these tree-dwelling omnivores are met with more than just the rewards of a hunt or scavenge. Like all great empires, weaver ant colonies run systematised farming operations. Their choice of produce is a sweet honeydew released by homopterans (sap-sucking insects) on their host tree or plant. These homopterans sit patiently on various parts of young shoots of trees or plants and feed on their sap. As they get their fill, they release a drop of carbohydrate-rich honeydew.
For a colony of ants with thousands of workers that are practically running a marathon every day, this acts as a vital source of energy. In the time that I’ve spent around these micro-civilisations, I’ve seen more workers tending honeydew farms than actively out on a hunt or carrying back scavenged remains.
These farms also serve as a gentle reminder of two signature traits of the weaver ant that make it such a successful coloniser of the canopies. First, on finding a spot where homopterans are seen aggregating, they weave a protective structure. This effectively turns these sites into dedicated farms or dairies that produce honeydew for the colony. These structures protect the homopterans from environmental factors like rain. Second, in addition to structural protection, these farms are sufficiently well-staffed with worker ants. Each ant is seemingly well-trained in the art of making intruders feel unwelcome. Workers position themselves to protect their homopterans and tend to them when needed.
This relationship between weaver ants and honeydew producing homopterans has contributed immensely to the energy needs of growing colonies and active workers. In turn, this could very well result in improved health of these trees and plants through reduced instances of herbivory.
No Love Lost for Neighbours
A single organisation of weaver ants can colonise an area as large as 1,500 square metres. Building an empire of this magnitude requires resources to fuel the colony as they scout, hunt, gather, and weave. When two colonies expand and begin to overlap in their foraging areas, workers will encounter individuals from the other colony. Ambitions collide. Between the first encounter and subsequent run-ins with ants from the other colony, each ant seems to deduce that these outsiders belong to a different colony that is uncomfortably close. Through this differentiation, they are effectively forming a concept of self, a potential threat to their collective and complete strangers.
With the looming threat of competition, weaver ants use this distinction to show more aggression to their own kind from neighbouring colonies than to unidentifiable strangers or weaver ants from distant colonies. In much the same way that my neighbours might fondly describe our relationship, a weaver ant colony and its neighbouring colonies seem to share what ethologists (scientists who study animal behaviour) like to call the “nasty neighbour” effect. The phenomenon suggests that certain territorial animals act strongly towards individuals of their own species — likely driven by competition for resources. This inherently amplified aggression towards their neighbours allows them to defend crucial foraging areas that could prove instrumental to the growth and/or sustenance of a colony. To this end, interactions between neighbours are gruesome to say the least.
From a distance, the scene of neighbouring weaver ants engaged in combat may look like an animated geometric art installation of miniscule dots and lines. Get closer and you will see a scene of uninhibited gore. On detecting a single intruder in their territory, a resident weaver ant will quickly secrete an alarm pheromone and proceed to attack the intruder. If a larger number of intruders are detected, the resident will quickly lay a trail of chemicals to the nearest leaf nest; drawing a path for nestmates to return to the imminent battlefield. At the scene of the battle, the ants turn tools to weapons as leaf-cutting mandibles are used with the singular aim of separating one body part of the enemy from another. The deceased opponents are taken back to the nest as spoils of war.
Chance Encounters of the Bird Kind
On a dusty path to the seashore on Havelock Island in the Andamans, melodious whistles and special effects sounds from the Star Wars movies filled the air as a pair of racket-tailed drongos began to compose a tune. Their cacophony and aerial ballet broke the stillness of the canopies and urged me to break the stillness on the ground by running frantically to fetch my camera. After clipping on a large lens to capture the two birds, I noticed something that had me running back just as frantically to find a macro lens. On the ground was a clump of leaves held together by silk and studded with slow-moving emeralds. The drongos had attacked a weaver ant nest and torn it clean off its anchorage in the canopy.
Weaver ants readily express their disdain at the presence of any intruder in the colony by biting and spraying them with formic acid. This treatment is extended to potential predators with the intensity one would expect from an animal fighting to preserve self and colony. The drongos, however, had quite literally caught a nest of weaver ants with their “pants down”. The nest was filled with recently mated queens all wearing their enlarged emerald green abdomens alongside many males.
As the nest lay on the ground, the drongos took turns sweeping down to snack on the nutrient-rich queens-to-be. Their special effects renditions managed to attract quite a few human spectators that may have gotten a bit too close to the scene of the action. Displeased by the company of their newly gathered fanfare, the drongos made an early exit. However, the worst was yet to come for the queens that escaped the acrobatic dining talents of the drongos.
Just as the newly mated queens and males recovered from the shock of the aerial assault, a massacre ensued on the ground. Worker ants from a nearby colony entered the scene and seemed quite keen on rearranging the newly mated queens. Once a female has mated, workers from any colony (including her own) will perceive her as a potential threat to their colony.
30-million-year-old learning curve
From their use as a biological control agent in China to their use by indigenous tribes across India and Australia as an ingredient in traditional medicines, humans have been closely associated with weaver ants. Aside from their direct contributions to societies living in harmony with nature, they generously teach anybody willing to learn from them. Their silk has shown promise as a hollow nano-fibre with potential medical applications, that range from nano-fibre-based drug delivery systems to tissue reengineering. Their systems of operation have inspired parallel computing formulas.
Humans are not the only ones that are aware of the benefits of copying a weaver ant’s methods. Through their 30 million years of weaving through the tropics, they’ve inspired entire ecosystems around them. So much so that many different species of spiders have evolved to mimic them for benefits ranging from protection to predation. In our next story in this series, we will take a closer look at the curious lives of the weaver ant’s eight-legged impersonators.
“How all things live and work, and ever blending, weave one vast whole from being’s ample range!” – Faust, Part One by Goethe