Crazy AntsCrazy Ants

Crazy Ants

The exotic invasive yellow crazy ant (Anoplolepis gracilipes) arrived on Christmas Island more than 70 years ago, and is now widespread throughout rainforest (Orchard et al. 2002). The ants can form multi-queened 'super-colonies', in which they occur at very high densities. This has apparently been a relatively recent phenomenon; with the first supercolony discovered in 1989, with further dramatic increases probably beginning around the mid-1990s.

At supercolony densities, the ant is having a devastating impact on the island's ecosystems. Red crabs, robber crabs, blue crabs and most other ground dwelling animals such as reptiles, have been and are continuing to be severely impacted, sometimes to the point of local extinction in heavily infested areas. By eliminating local populations of red crabs, the ants are also having a marked effect on forest composition and structure, and litter dynamics in infested areas. The feeding activities of the ants and their mutualistic scale insects can stress large trees to the point of death, and appear to be causing canopy dieback in areas of supercolony formation (Orchard et al. 2002). In addition groundings of birds in crazy ant supercolonies would most probably result in the death of the bird. Although research has shown that crazy ants have little impact on canopy insects, it is possible that the ants may reduce insect populations. In the recent Risk Watch List compiled for the Christmas Island National Park and Conservancy, the crazy ant invasion was rated as an Extreme Risk to biodiversity and conservation values, with catastrophic consequences of failure to implement effective control measures.

PAN field workers and assistants conducted an island-wide ant survey during May-August, 2001. The survey was designed by CAMBI (Centre for the Analysis and Management of Biological Invasions, Monash University). Of 972 sites surveyed; 741 surveyed points fell in natural forest. Crazy ants were recorded at 46.7% of sites in natural forest. Super-colony densities were recorded at 22.7% of sites in natural forest. Using these survey points as a representative sample of the forest, it was estimated that 2,379 ha of the estimated 10,492 ha of forest on the island was infested with crazy ants at supercolony densities (Orchard et al. 2002). Crazy ants are not evenly distributed throughout natural forest but are more commonly found on terrace forests and less commonly found in deep-soil tall-closed forest, which make up most of the natural forests on the island plateau. Deep-soil tall-closed forest made up 31% of census sites in natural forest but only 6.5% of supercolony records (Orchard et al. 2002).

In September 2002 an aerial baiting program was undertaken, and all known supercolonies treated with insecticide. Results so far indicate the program was successful in controlling supercolonies over 2500ha of Christmas Island. Crazy ants are still present in low densities, however, further high densities of crazy ants may establish in the terrace forests without warning. PAN staff will continue to monitor any new supercolony formation and treat by hand baiting over the next few years.