Reliable mop-up of surviving pests: a more cost-effective and fail-safe approach to local extirpation

Achieving very low densities of small mammal pests is routinely attained by expensive, large-scale, broadcast poisoning. There are two strategic responses to maintaining these low pest densities (or attempting to eradicate the survivors) – repeat the poisoning at regular intervals, or attempt to detect and mop- up the survivors. Here we report preliminary trials working toward the latter approach for possums (Trichosurus vulpecula) in a large mainland forest in New Zealand, and show that detection-and-mop-up is not only feasible but also possibly the most economically sensible and reliable way of achieving 100% kills. We use a simple bioeconomic model to predict the optimal surveillance (detection) strategy for possum mop-up, based on the characteristics of the detection device, and the few data available on the aggregation patterns of possums at extreme low density.