Targeted Mop up and Robust Response Tools Can Achieve and Maintain Possum Freedom on the Mainland

SIMPLE SUMMARY: To achieve the goal of Predator Free 2050, new pest control methods must be found and existing ones refined. We achieved the first >11,000-hectare possum elimination (all possums removed, and re-invaders subsequently mopped up) on mainland New Zealand at an unfenced site, largely protected from possum reinvasion by mountains and rivers. This was within a project to eliminate rats, possums and stoats. We spread a biodegradable poison called sodium fluoroacetate twice by helicopter and used existing ground-based methods (leghold traps, cage traps and a dog) paired with new technology (trail cameras and remote reporting of traps instead of manual checking) to maximise response efficiency. Possums were eliminated in 11 months from 11,642 hectares at a cost of NZD 1.6 million (1080 costs for all 3 species combined and ground-based costs for possums only). New possums entered the site at a rate of four per year and were detected and caught before a breeding population could establish. This proof of concept can now be repeated to confirm its success elsewhere, refine methods, and reduce costs. In the future, if possums are the sole target, we will likely spread 1080 once only and use effective mop-up tools to finish the job. ABSTRACT: Unfenced sites on mainland New Zealand have long been considered impossible to defend from reinvasion by possums, and are thus unsuitable for eradication. In July 2019, we began eliminating possums from 11,642 ha (including approximately 8700 ha of suitable possum habitat) in South Westland, using alpine rivers and high alpine ranges to minimise reinvasion. Two aerial 1080 (sodium fluoroacetate) applications, each with two pre-feeds, were used. Here, we detail the effort to mop up existing possums and subsequent invaders in the 13 months following the aerial operation. Possums were detected and caught using a motion-activated camera network, traps equipped with automated reporting and a possum search dog. The last probable survivor was eliminated on 29 June 2020, 11 months after the initial removal operation. Subsequently, possums entered the site at a rate of 4 per year. These were detected and removed using the same methods. The initial elimination cost NZD 163.75/ha and ongoing detection and response NZD 15.70/ha annually. We compare costs with possum eradications on islands and ongoing suppression on the mainland.