Local elimination – a necessary stepping stone to dreams of national freedom from vertebrate pests
Aerial delivery of toxic bait. Image - Graham Nugent.
The vision of a pest- or predator-free New Zealand has focused attention on the key differences between eradicating pests from islands and eradicating them from large parts of mainland New Zealand. Eradication on the mainland has so far only been achieved in small areas and then only by using fences to prevent reinvasion – one of the obligatory rules of eradication. To achieve eradication over large areas of New Zealand without using fences will require either scaling up operations to cover an entire island in one operation or finding a way to progressively ‘rollback’ pest populations. Scaling up is a plausible option for eradicating pests from Stewart Island, but currently would seem to be an unaffordable option for controlling pests across all of the North or South Islands. That leaves some form of progressive rollback as the only option, with prevention of reinvasion without fences being the key new tool or tactic needed.
Graham Nugent, Bruce Warburton, Dave Morgan, Peter Sweetapple, Grant Morriss and others have been exploring the concept of ‘local elimination’ of pests since 2006. Their research has had three main aims: (1) to find ways of achieving near zero density within infested areas, (2) to develop tools for cheaply detecting survivors and/or invaders, and (3) to find affordable ways of reducing reinvasion.
Graham’s team focused first on improving the aerial delivery of toxic bait used for ‘initial knockdown’. Aerial baiting is currently by far the most affordable approach for the control of small mammals in areas on mainland New Zealand that are difficult to traverse on foot, and has been widely used for eradication of rodents from islands. Trials in 2006 and 2007 using aerial 1080 baiting against possums, rats, and mice showed that the number of non-toxic prefeeds was ultimately more important in increasing the kill than either sowing rate or sowing pattern, especially for rats. More importantly, these and other trials identified that because a few of the baits sown were not lethal to large possums, sowing rates had to be set at levels that permitted possums multiple encounters with bait. That insight led to development of new strip and cluster sowing strategies for aerial baiting. In the few operational trials completed using these strategies, control efficacy has not been as consistent as that achieved using current ‘best practice’ (the culmination of decades of research and development). However, in the best result to date, near total reductions in possum, rat, and mouse populations were achieved using sowing rates of 167g/ha (i.e. 95% lower than normal). This achievement indicates that there is potential for substantial reductions in the cost and amount of 1080 used per operation, which would make it more feasible to reduce populations over very large areas.
The second aim, cheaply detecting survivors or invaders, was progressed by the development of a low-cost high-sensitivity detection device and strategy-of-use that could be deployed over entire control areas and used to either map where survivors were (so they could be targeted) or confirm the area as being pest free. Peter Sweetapple and Graham successfully developed chewcards for this purpose, i.e. small sheets of core-flute plastic (with a peanut-butter-based attractant) that pests bite and leave identifiable evidence of their presence. The cards require only two visits by observers, and can detect multiple species. Chewcards are now being used widely, particularly as part of surveillance undertaken by the Animal Health Board to ‘prove’ areas controlled for possums have very low possum populations and consequently likely freedom from TB.
The third aim, affordably reducing invasion, was attempted for possums in an area near Lake McKerrow, South Westland, using several lines of lethal long-life baits (a gel containing cholecalciferol) placed in parallel along the boundary of an area (i.e. ‘perimeter control’) within which the possum population was heavily reduced. Chewcard monitoring and trapping within the ‘cleared area’ showed a rapid build-up of possum numbers, indicating low effectiveness of such perimeter control. The reasons for the failure are unclear – one possibility is that dispersing possums arriving in a new area could be wary of unfamiliar objects such as baits.
Overall, the ‘Local Elimination’ programme and related research indicate that rollback eradication of pests will be difficult – not so much because of difficulties in achieving zero density locally at an affordable cost or in identifying where animals remain, but more because of the difficulties of preventing reinvasion without using fences. However, other strategies for perimeter control may become feasible when working at landscape scales – most notably the use of buffers tens of kilometres wide in which control is frequently repeated. Such buffers could be successively expanded as the areas inside them are progressively cleared.
This work was funded by the Ministry of Business, Innovation and Employment and the Animal Health Board.