Declaring Sydney free from red imported fire ants Solenopsis invicta buren – a view from the branches of the scenario tree

Tramps ants are a constant biosecurity threat to Australia. Red Imported Fire Ant Solenopsis invicta was detected in Sydney, New South Wales in 2014, and a successful eradication program was undertaken. Stratified surveillance was conducted as part the program, and allowed for scenario tree analysis to statistically support the claim of freedom. Our analysis demonstrated a mean probability of freedom was 93.1%, 99.5% and > 99.9% after one, two or three surveys for two nests within 2 km of the detection. We demonstrated > 99.9% probability of freedom after three surveys if for a threshold value of five nests were present. Our analysis was the basis of declaring Sydney free from S. invicta. Sydney was declared free 18 months after the first detection of S. invicta.