Yellow crazy ants
ABC Radio National's Science Show
Christmas Island's environment
Christmas Island is a place of natural beauty, where a unique ecosystem of plants and animals has developed over millions of years.
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The exotic invasive yellow crazy ant (Anoplolepis gracilipes), was accidentally introduced to Christmas Island between 1915 and 1934. They are thought to have come with produce from either Malaysia or Singapore. They didn't appear in the records until about 1930. The ants have no natural predators on Christmas Island and thrive on the habitat and sources of food available. They have a high reproductive rate and can form multi-queened super-colonies in which ants occur at very high densities.
Crazy ants are recognised by their pale yellow body colour, unusually long legs and antennae. The name "crazy ant" was derived from their frantic movements and frequent changes in direction, especially when disturbed.
A single super-colony was discovered on a high terrace above the Grotto in 1989. This colony remained isolated and eventually declined. Super-colonies were again found from 1995 to 1997. Subsequent surveys indicated at least 10 separate infestations, ranging from several hectares to at least one square kilometre, and distributed throughout the island.
At the height of their population growth, the super-colonies affected some 2500 hectares of the island, or 25 per cent of the total forest area. Once a super-colony is established, it can expand rapidly, in some cases doubling in size in 12 months. To put this in context, the edge of a super-colony can expand at around three metres per day or around one kilometre per year.
Effects on Christmas Island ecology
The crazy ant has a significant destructive impact on the island's ecosystem, killing and displacing crabs on the forest floor. The super-colonies also devastate crab numbers migrating to the coast. This has seen a rapid depletion of land crab numbers which are vital to Christmas Island's biodiversity. They are a keystone species in the forest ecology by digging burrows, turning over the soil, and fertilising it with their droppings.
Seedlings that were previously eaten by crabs started to grow, and as a result, changed the structure of the forest. Weeds also spread into the rainforest because there are no crabs to control them. One of the most noticeable changes in the forest is the increased numbers of stinging tree Dendrocnide peltata, which now flourish along many of the walking tracks and other areas that people frequently visit around the island.
Robber crabs, red crabs, and blue crabs are completely wiped out from infested areas. Populations of other ground and canopy dwelling animals, such as reptiles and other leaf litter fauna have also decreased.
During crab migrations many crabs move through areas infested with ants and are killed. Studies show the ant has displaced an estimated 15-20 million crabs by occupying their burrows, killing and eating resident crabs, and using their burrows as nest sites.
Although crazy ants do not bite or sting, they spray formic acid as a defence mechanism and to subdue their prey. In areas of high ant density, the movement of a land crab disturbs the ants and as a result the ants instinctively spray formic acid as a form of defence. The high levels of formic acid at ground level eventually overwhelms the crabs, and they are usually blinded then eventually killed. As the dead crabs decay, a bonus source of protein becomes available to the ants.
Ants in general require two main types of food: carbohydrate to provide energy for the foraging workers, and protein to enable the queens to produce eggs. Crazy ants get much of their food requirements from scale insects. Scale insects are serious plant pests that feed on sap of trees and release honeydew, a sugary liquid. Ants eat honeydew, and in return protect the scale from their enemies and spread them among trees. This relationship is called a mutualism.
The honeydew not eaten by ants drips onto the trees and encourages the growth of sooty mould over the leaves and stems giving the plants an ugly, black appearance, and reducing the health and vigour of the plant.
In summary, crazy ants kill the fauna, but encourage scale insects. Increased densities of scale insects causes forests trees to dieback, creating light gaps in the forest canopy. Light gaps and removal of crabs encourages seedling growth and weed invasion into the forest.
Control of yellow crazy ants
Over recent decades yellow crazy ants have emerged as one of the worst 100 invasive species in the world. Communities from Hawaii to Madagascar are trying to eradicate them, but no one has found a permanent solution yet.
Christmas Island is a focal point for this international control effort. It is the only place where the ants are known to have formed supercolonies - groups with up to 300 queens that can contain millions of ants. These supercolonies spread further and cause more damage than single colonies, and they pose the single greatest known threat to the island's biodiversity.
Staff from Christmas Island National Park have worked constantly in recent years to keep ant numbers in check. With help from the Christmas Island Crazy Ant Scientific Advisory Panel and support from the Australian Government they are holding ground.
Baiting
To reduce the impacts of crazy ants on red crabs and the island's ecosystems the national park carried out a major aerial baiting program in 2009, to follow up the first aerial baiting conducted in 2002. The first step was conducting an extensive island-wide survey to work out exactly where the supercolonies were. For several months staff traversed the entire island surveying over 900 sites. The result was a map of crazy ant supercolonies and red crab burrow densities together with other biodiversity data.
In September 2009, a helicopter was used to precisely bait crazy ant supercolonies, which covered 784 hectares of the island. A very low concentration of Fipronil bait (a tenth of 1 per cent) was used to control the ants. Monthly monitoring of these baited supercolony sites shows that crazy ant densities were reduced by 99 per cent.
Park staff placed a high emphasis on minimising any non-target impacts of baiting. Food lures were dropped from a helicopter to attract robber crabs away from areas that were about to be baited. This technique, combined with the low concentration Fipronil bait, proved to be highly successful with extremely low numbers of robber crabs and no red crabs known to be killed by the baiting.
Research: the Crazy Ant Scientific Advisory Panel (CASAP)
The role of the Crazy Ant Scientific Advisory Panel is to provide the Australian Government with scientific and technical advice for the management of crazy ants on Christmas Island.
The seven member panel contains experts and internationally recognised research scientists with high level experience in the fields of ecology and invasive ant management. Some of the members have spent many years working and researching crazy ant issues on Christmas Island.
Biocontrol options
One of the options enabled by the Government's recent $4 million investment is research into the possible use of biological methods of crazy ant control.
Scientists from Monash and LaTrobe Universities think crazy ant super-colonies may depend on the honey dew produced by the tiny scale insects that coat the island's trees. If so, it may be possible to manage crazy ants by controlling scale insects.
This is being studied through a four-year research collaboration between the Director of National Parks and Monash and LaTrobe Universities. It involves three key components:
- Investigating the role that honey-dew producing scale insects have in sustaining crazy ants.
- Identifying if there are natural enemies to control scale insects.
- Identify the best strategies to implement a biological control program if appropriate natural enemies are found.
The research project is based on the best available scientific evidence and advice (including from CASAP) and it will not involve the release of any biological control agents.
Any future release of a suitable biological control agent would only be considered if it was determined that its release would have no impact on humans or the island's native plant and animal species.
Find out more
- Crazy ant baiting and the effects of Fipronil
- Final report | Monitoring of the 2009 aerial baiting of yellow crazy ants (Anoplolepis gracilipes) on non-target invertebrate fauna on Christmas Island 2011
Education
Parks staff are keeping both the local Christmas Island and broader community informed as plans evolve and action takes place on this extremely important issue. An information brochure about crazy ants has been produced in the three main island languages and updates on the research and efforts are made in local papers and on news section of this website.
