Scenario tree risk analysis of zero detections and the eradication of yellow crazy ant (Anoplolepis Gracilipes (Smith)), in New South Wales, Australia

Yellow crazy ant (YCA) Anoplolepis gracilipes (Smith) is ranked among the world's worst invasive species. Following the detection of this ant on Goodwood Island in northern New South Wales, Australia in 2004, an eradication program was initiated. The last detection was made in January 2006 and the declaration of freedom from the pest was made in January 2008, based on the traditional two-year period without a detection. However, although this eradication criteria is widely used, the two year timeframe is an arbitrary period with little or no scientific basis. Here, in addition to describing the eradication, we present a scenario tree analysis of zero detections to predict the level of confidence that the pest would have been detected if it was still present. Following a two year period with no detections, the scenario tree analysis indicated that there was a probability of absence of 0.999 998 under an assumed incursion pressure of one incursion every ten years. After eradication, the scenario tree analysis also indicated that as few as 20 randomly located visual inspections in the high risk area every three months was sufficient to maintain > 0.95% probability of freedom. The analysis was also used to assess the merits of different surveillance techniques.