After years of effort, the invasive African big-headed ant (Pheidole megacephala) has successfully been eradicated from Lord Howe Island, an island off the coast of Australia, helping to protect this World Heritage site and its unique diversity of plants and animals.
The big-headed devi-ant
The African big-headed ant is considered to be one of the world’s 100 worst invasive species, causing serious ecological and economic problems. They can nest in walls and have been reported to chew through telephone cables and electrical wires. They are also a significant agricultural pest for many crops, as they have a symbiotic relationship with sap-sucking insects which cause substantial crop damage; the ants protect the sap-sucking insects from predators and parasites, whilst feeding on the honeydew that they produce. African big-headed ants are also highly aggressive and have severe impacts on biodiversity; they can displace native invertebrates and have been known to kill and eat hatchlings birds and baby sea turtles, and injure nesting seabirds.
African big headed-ants have such a large impact partly because of their ability to form vast, high-density supercolonies. Each supercolony consists of a series of interconnected nests which have multiple queens and which act together as a cooperative unit. Working as a cooperative unit means there is a lack of fighting between ants from the same supercolony, which is thought to be important in enabling these ants to live at such high densities. Within a supercolony, individual nests can be packed as densely as one per square metre. Supercolonies grow when one of the queens, along with a number of workers, leaves the nest to create a new one a short walk away. Over time, a supercolony can expand over a large area; one supercolony was recorded to have expanded over 49 km.
Mapping and eradication
It was the way in which African big-headed ant supercolonies spread which allowed ecologists to plan a strategy for their eradication from Lord Howe. Because supercolonies spread by expanding into areas close to their nests, they require assistance, mainly from unwitting humans, to spread long distances. This means it is relatively easy to predict where African big-headed ants will occur: in areas close to other nests, or in areas where they can be accidentally transported to by people. This is critical information for knowing where to set baits to eradicate them. On Lord Howe Island, the baiting programme used a product which contains a very low dose of insecticide that has an extremely low toxicity to terrestrial vertebrates and which breaks down to a harmless chemical when exposed to light. As African big-headed ants kill most other invertebrates with which they come into contact, there were very few other invertebrate species present that might have been negatively affected by the bait. Once all known colonies were baited and exterminated, almost 200,000 lures were deployed across the island, the equivalent of 13 per square meter. When no African big-headed ants came to the lures, it allowed ecologists to confirm the eradication of this species from Lord Howe.
Commonwealth Scientific and Industrial Research Organisation (CSIRO) researcher and exotic ant specialist, Dr Ben Hoffmann, who was part of the team responsible for the eradication, said the program was one of the most significant invasive ant eradications in the world. However, whilst the African big-headed ant has been eradicated from Lord Howe, this species, as well as other invasive ant species, are still widespread. Dr. Hoffman, however, is optimistic that the eradication technique developed for Lord Howe could be used to stop these invasive ants on other islands around the world.