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Yellow Crazy Ants on Christmas Island


The yellow crazy ant Anoplolepis gracilipes was accidentally introduced to Christmas Island more than 70 years ago, but did not present any undue environmental concern until the mid 1990s. Around this time something (perhaps some environmental factor, or secondary introductory of a new genotype) triggered the formation of "supercolonies" by this highly invasive species.

In most ant species there is a single queen in each nest, and worker ants from neighbouring nests display aggression towards each other, even though they are the same species. These factors tend to keep ant density at relatively low levels.

However crazy ants have the ability to form multi-queened "supercolonies", where rather than fight each other, the offspring of different queens cooperate to form infestations. There can be several thousand ants per square metre of forest floor in these supercolonies, but at any one time there are just as many ants foraging in the treetops above.


The first supercolony on Christmas Island was detected in 1989, but supercolonies really became widespread from about 1995 onwards. At present, crazy ants infest more than 2000 ha of rainforest on the island. These supercolonies can be huge; one has been mapped at over 700 ha.

Crazy ant supercolonies are wreaking havoc on the natural environment across the island. At supercolony densities crazy ants have the ability to overwhelm and kill the red land crabs, robber crabs, endemic reptiles, and a host of native invertebrates. There is also grave concern for the island's native birds - crazy ants forage mostly in the canopies of large forest trees, so nesting land and sea-birds are at risk.

It is estimated that since 1995, crazy ants have killed 10-20 million red crabs, which is 20-25% of the entire population. This has been a catastrophic loss, and the deletion of red crabs from large tracts of forest on the island is immediately obvious. In the absence of crabs, 

seedlings are now germinating en masse in the forest understory, forever altering the structure of the rainforest in those places.

The up-side to this horror story is that a toxic bait has been identified which is extremely effective at controlling crazy ants at very low doses. The active ingredient in the bait is fipronil, and just 0.6 grams of fipronil per hectare (delivered in a fish meal attractant) is sufficient to achieve a 99% reduction in worker ant density in just a few days.

These two images contrast the forest floor in places with and without crazy ants
Invaded forest; no red crabs, lots of crazy ants   Intact rainforest; lots of red crabs, no crazy ants
Dead crabs

The biggest problem now is distributing the bait over the island's rugged and often inaccessible terrain. In September 2002 a helicopter will arrive on the island to spread bait over all remaining supercolonies, and if all goes according to plan, the crazy ant problem on Christmas Island should be under control by the end of 2002.