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Unprecedented ant-ics?

‘Ant mega-colony takes over world’ was the title of a 2009 BBC article about the argentine ant, (Linepithema humile) one of the world’s most troublesome pests. Fifteen years later, and the ant is still proving to be frustrating.

File:Linepithema humile casent0106983 profile 1.jpg - Wikimedia Commons

Argentine ants are what is called a ‘tramp species’ - a species spread inadvertently by humans across the globe, and have been found in every continent except, ironically, Antarctica. Their non-native nature, and the sheer breadth of the world that they have colonised means that they cause trouble wherever they go.

Unlike most ant species, Argentine ant colonies are polygynous - ruled by multiple queens instead of just one. This means they will form a supercolony of numerous nests, which are spatially separate but genetically similar and unaggressive towards each other. This, along with their ability to survive and multiply even if only a small nest (a queen and ten workers) is transported to a new area, has helped establish them across the globe.

In 2009, it was discovered that rather than existing in separate supercolonies, many Argentine ants exist in one large megacolony of supercolonies across the globe. Ants living in Europe, Japan and California share a similar chemical profile of hydrocarbons on their cuticles, and display a lack of aggression to each other, just as shown by separate nests in supercolonies.

Given how large and powerful the supercolonies already are - the one in Europe extends 6000km across the Mediterranean coast, whilst the one in California stretches 900km along the coastline - it is extremely worrying that these are only subsets of a global megacolony. One paper even declares ‘the enormous extent of this population is paralleled only by that of the human society.’

Argentine ants are aggressively invasive: able to displace other ants either violently or by taking over food sources and burrowing their way into every niche they can muster, from cracks to concrete and fence posts. Ecologist Phil Lester once commented: ‘they are sometimes referred to as the ‘Genghis Khan’ of the ant world,’ and they have been listed as one of the world’s 100 most damaging invasive species. They will kill and eat lizards, bird eggs and newly hatched chicks.

Even within the colony, things can get violent, with 90% of nestmate queens being ‘executed’ - either by other queens or by the workers - every spring in order to reduce queen numbers. Although this may seem counterintuitive, reducing the number of queens actually increases egg production as when there are fewer queens, they are more fecund.

The ants also carry deformed wing virus, the primary suspect in millions of honey bee deaths which has been implicated in honey bee decline worldwide. Bees and ants forage on the same plants so disease transfer can happen easily between them. Furthermore, Argentine ants have been observed trespassing inside of beehives, where they will steal honey stores, attack the bees’ larvae and scavenge the dead worker bees. Studies have shown that when Argentine ants are present in hives, rates of deformed wing virus amongst the bees increase.

They can also influence agricultural and horticultural activities, encouraging the growth of sooty moulds which ruin fruit trees and sugar crops. Their displacement of native ants can also limit seed dispersal. Many non-invasive, monogynous ant species are myrmecochorous, meaning that they distribute seeds from one place to another. This is beneficial for the plant - as it gets to spread and reproduce elsewhere - but also beneficial for the ant, as seeds often have fleshy parts full of protein, sugar and lipids which provide nutrition not just for the ants but for the ant queen’s offspring. But when argentine ants take over an area, they ignore the seeds - leaving them untouched and unmoved.

In 2021, a new Argentine ant population was discovered in inland Japan, distributed around the urban Akishino river near Nara city, as well as in paddy and dry fields. The river serves as a major pathway for dispersal for the ant, possibly because in its region of origin, Argentina, it inhabits floodplains. Along with wet areas, the species also prefers areas of high humidity - and these factors impact their survivability.

Argentine ants have been expanding their range in Japan for years, and populations of native ants have consequently shrunk. Scientists studied Japanese urban parks, finding that in parks where Argentine ants were dominant, indigenous ant species abundance remained low, and where Argentine ants increased, indigenous populations of ants like P. punctatus (the Japanese queenless ant, which reproduces asexually and is almost entirely female) decreased. This is why the recent discovery of ants in the Akishino river is so worrying to Japanese entomologists.

Although Argentine ants may seem unkillable, they have been known to shrink and collapse in a similar way to other invasive species, like the yellow crazy ant (Anoplolepis gracilipes). The yellow crazy ant is infamous for massacring crabs on Christmas Island. Christmas Island - a small island south of Indonesia - is home to millions of red crabs (Gecarcoidea natalis), but tens of millions of these have been killed over the years by the yellow crazy ants (first introduced to the island by humans in the early 20th century), who spray them with formic acid, causing blindness and leaving them to starve.

Yellow crazy ants were once present in the Seychelles, but later declined dramatically over time and then suddenly disappeared from several areas, a pattern which has also been observed in Argentine ant colonies in New Zealand. Although scientists are not 100% sure on the mechanism behind this, they hypothesise that it could be because of inbreeding depression or an inability to adapt to new environments.

With the risks that Argentine ants pose to ecosystems, sitting around and waiting for them to die out is probably not the most efficient strategy. But getting rid of Argentine ants has proved demanding. The creatures have evolved spectacularly well to evade any potential means of killing them. They groom each other for parasites, allowing them to eliminate six different types of fungal pathogens with antimicrobial compounds secreted from their glands.

They are also extremely effective at corpse management of their dead, carrying them to ‘refuse’ piles, which also happens to be where they defecate - secreting, as they do so, the compound pygidial, an antifungal which prevents their corpses from becoming infected. This cleanliness is what allows them to persist in areas with high levels of bacteria and fungi like soil and leaf litter.

On Christmas Island, where the yellow crazy ants wreaked havoc on the iconic red crab population, a micro-wasp was deployed to prey on scale insects, (Tachardina aurantiaca) the yellow crazy ants’ food source. Scale insects produce honeydew by sucking the sap out of trees and then secreting it from their anal pore. Yellow crazy ants harvest this honeydew, treating the scale insects as humans treat livestock - even scaring off the scale insects’ predators. The micro-wasp (Tachardiaephagus somervillei) preys upon scale insects whilst leaving other species unharmed. And the technique worked - since the wasp was deployed 5 years ago, the population of red crabs has doubled from 50 million to 100 million.

But finding biological control agents for the Argentine ant has proven even harder, and so scientists have been forced to resort to chemical methods - which of course run their own risks of harming the environment. Most spray insecticides kill very few ant workers directly, and their inefficacy means they have to be used at higher concentrations. This increases the potential damage they pose to surrounding plants and animals.

A lot of research is now concentrated on finding effective solutions to this. One example is the manipulation of horizontal transfer: where chemicals deadly to ants like fipronil are given in small concentrations to specific “donor” ants, who will then return to their colonies and spread it to other recipients. The targeted nature of this method means that only a few nanograms of fipronil are needed to kill hundreds of ants. Another novel method involves placing insecticides in microcapsules, such as small gel ‘beads,’ which swell in response to water. When ants and other pests drink from the beads, they are imbued with slow-acting poison.

This field of research is becoming increasingly important. Climate change is predicted to increase the survivability of Argentine ant populations across the globe, as high temperature and high rainfall decrease the risk of population collapse.

Ants are essential to ecosystems, acting in food webs as predators and prey and turning over soil to aid redistribution of nutrients, but several species like the Argentine ant have become highly invasive as an unintended consequence of human activity. These species pose a threat not just to other ants but to ecosystems as a whole. Humans may have caused this mess but ironically, it is up to us to fix it.

Unprecedented ant-ics?

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