Parks and reserves

Christmas Island National Park

Yellow Crazy Ants

The exotic invasive yellow crazy ant (Anoplolepis gracilipes), was accidentally introduced to Christmas Island between 1915 and 1934, and became widespread throughout the island.

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.

These ants can form multi-queened super-colonies in which ants occur at very high densities.

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% 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.

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

The impacts of the crazy ant on the conservation values of Christmas Island have been serious. This threat is significantly reduced through an action plan prepared by Park staff and experts in the field. The basic elements of this plan are baiting, education and research.

Poison Baiting

Baiting ants has been a difficult task due to the selective diets of the ants, the potential for impacts on non-target species, such as robbercrabs, red crabs and reptiles, and the ruggedness of the terrain where crazy ant super-colonies have formed.

Parks staff spent much of 1999 and 2000 working to find out the most effective method of baiting ants. The challenge was to find a bait that is attractive to the ants, is slow acting and can be transferred among ants so the complete nests, including egg laying queen ants, are destroyed. Also, the bait had to be harmless to other animals such as birds, reptiles or humans, and have no long term effects.

A bait developed by Bayer Crop Science and Animal Control Technologies consisting of Fipronil in a fish protein base was identified through experiments as meeting these criteria.

Fipronil is very effective against insect pests, and may be used in doses of less than 0.5 grams of active ingredient per hectare. At these low concentrations it is not harmful to reptiles, birds or mammals (including humans). Fipronil does not dissolve in water so there is no danger of it threatening the quality of the island's water supply.

Parks staff undertook large-scale baiting from the beginning of the dry season in 2000.

The Australian Government has spent more than $2.5 million on controlling crazy ants over the past 7 years, $700,000 of which has been funded under the Natural Heritage Trust. A highly successful aerial baiting program was undertaken in 2002 which involved distributing 12 tonnes of a fish-based Fipronil bait over 2400 hectares of rainforest by helicopter. Ant densities within the target supercolonies were reduced by 98%. Ground baiting has continued to try and keep pace with the rate of supercolony development and expansion. A further 1000 hectares of supercolonies have been hand baited since the end of the aerial baiting program, but the yellow crazy ant always seems to be 200-300 hectares ahead of the rate at which Parks Australia staff can bait.

The 2005 island wide crazy ant survey confirmed that supercolonies have been forming at a rate of approximately 400 hectares per year since the aerial baiting operation and are increasing exponentially. The current control rate of 200- 250 hectares per annum is insufficient to control the current rate of spread. Further, due to the lower ant densities in supercolonies compared with pre-aerial baiting densities, non-target impacts are restricting the use of Fipronil in areas where the invertebrate fauna has yet to be decimated by crazy ants. Supercolonies are beginning to reform in many areas that were aerial baited in 2002, most probably from remnant nests in those areas. Supercolonies are also developing from remnant nests left around the edges of previously baited supercolonies, locking Parks Australia into a program of repeat visits to retreat supercolony areas.

A better bait, better distribution method and investigation of complimentary bio control methods are urgently needed to improve the effectiveness of the crazy ant control program.

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.

Research

The Australian Research Council funded a joint research and control project which was undertaken by Parks Australia and the Centre for Analysis and Management of Biological Invasions, Monash University. This research aimed to understand:

Research is important to the control effort and increases the probability of success.

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