Predator Free: Stoat control on a landscape scale

The development of a ready-made bait containing PAPP will be an important step forward in protecting native birds, lizards, and invertebrates in Aotearoa New Zealand.

For many of New Zealand’s threatened birds, stoats are ‘public enemy number one’. Originally introduced to control rabbits, stoats failed in that, and now take a huge toll on a wide range of native animals instead. They are highly efficient predators, found in all habitat types from the coast to the snow-line, and when food is abundant they increase in numbers rapidly. Stoats are often difficult to control, and are one of the species targeted for nationwide eradication by the Predator Free 2050 programme.

Stoats can also move large distances surprisingly quickly, so to be successful, control programmes must occur on a wide-enough scale to reduce reinvasion from surrounding areas. At present however, kill-trapping is the most commonly used control technique, and this is labour intensive (and therefore expensive), and the areas that can be protected are limited.

We currently have no aerially-applied control method to target stoats directly, particularly in habitats such as high-altitude forests, alpine areas, and beech forest. What is needed is a ready-made toxic bait that could be deployed either in bait stations or aerially for landscape-scale control. The PF2050 ‘Tools to Market’ programme is funding the development of such a bait.

This is not as straightforward as it may sound—while stoats do eat a wide range of prey, they prefer to kill it themselves, so developing a bait they will eat in the wild has been a challenge. Trials were initially undertaken with captive stoats at Lincoln University and although various combinations of offal and meat were tested, it came down to stoats favouring chicken sausages! The toxin that is being incorporated into the sausages is para-aminopropiophenone, or PAPP. This is a relatively new toxin that has a number of advantages. While carnivores such as stoats are particularly sensitive to it, other mammals (including humans) and birds are less so. It also has a low secondary poisoning risk, and there is an effective antidote. PAPP is also a humane toxin, which acts by reducing oxygen to the brain, causing stoats to become lethargic, sleepy, and unconscious prior to death.

Following development in the laboratory, the baits needed to be tested in the field to see whether they are effective at reducing stoat density. DOC, the Mammalian Corrections Unit and Connovation Ltd are currently undertaking a field trial in the Borland Valley, Fiordland. Trail cameras have been used to monitor bait stations containing the PAPP baits, and the results look very promising, with good up-take of baits by stoats in the wild. A second trial is planned for autumn to confirm this result, and then we will begin the process of registering the bait for use in bait stations.

While ground-based methods such as bait stations are essential and can be very effective, they have limited use in remote and difficult terrain. The next step in the development of the PAPP bait will therefore be to investigate its suitability for aerial delivery, which would allow stoat control in those hard-to-access areas. It would, for example, be a very useful tool to protect the takahē population in the Murchison Mountains. Aerial methods are also cheaper than ground-based methods, and cost becomes especially important at a landscape scale. It should be noted however, that the use of aerial PAPP baits would not replace the use of aerial 1080 baits; this is because possums and rodents, which are important targets in 1080 operations, are not as sensitive to PAPP as stoats.

The development of a ready-made bait containing PAPP will be an important step forward in protecting native birds, lizards, and invertebrates in Aotearoa New Zealand. It will complement the current trapping effort, and provide those involved in endangered species protection with a new, targeted tool to control stoats, potentially over wider areas and in difficult terrain.