Improving baiting for rabbit control and minimising risks to non-target animals
Plane distributing bait. Image - James Smith.
Declining efficacy of the rabbit haemorrhagic disease virus has seen a renewed need for broad-scale conventional rabbit control, including aerial baiting. This has prompted research into improvements to the cost-efficacy of aerial baiting for rabbit control, and reassessment of the non-target risks associated with this control method. Apart from their economic impacts, rabbits also help support predator populations. Improved rabbit control could therefore help with initiatives to make New Zealand predator free.
Dave Latham is working to refine aerial application of 1080 carrot bait to keep rabbit populations low. Field trials completed in Otago (winter 2011 and 2012) showed reduced per-hectare amounts of bait sown in strips can achieve effective rabbit control, with cost savings of about 40% over current ‘total cover’ baiting practices. Further trials to investigate bait sown in strips are underway and will be completed in 2014.
The non-target animals present in habitats where baits with 1080 or pindone are used for rabbit control can differ substantially from those present where the same toxins are used to control possums and rodents. Also, toxin concentrations are lower in the chopped carrot or pellet baits applied for rabbits. With these differences in mind, Penny Fisher reviewed the non-target risks of baiting for rabbit control. This research revealed a significant lack of toxicological and field data about pindone relative to 1080, which created higher uncertainty around its estimated risks to non-target species. Both 1080 and pindone were estimated to present a high risk of primary poisoning to some non-target mammals and birds that might eat bait laid for rabbit control, and also medium–high risks to mammals and birds that might scavenge the carcasses of poisoned rabbits.
Both projects identified carrot bait quality, i.e. uniformity of bait size and toxin concentration, as a critical factor in the success of rabbit baiting operations. Current manufacturing and distribution practices still produce large numbers of small carrot fragments (chaff) with higher concentrations of toxin relative to larger baits. Such bait pieces may be sub-lethal to rabbits yet lethal to small non-target animals, so poorly prepared carrot bait will not only decrease the efficacy of rabbit control, but also increase the risk of unwanted non-target mortality.
Additional field-based information is needed to improve managers’ understanding of non-target risk in rabbit baiting, and their ability to mitigate unacceptable risks. Key information needs are estimates of:
- annual usage of pindone and relative use of carrot, pellet and oat baits for rabbit control to identify areas where the primary risk to non-target animals is likely to be highest
- quality of 1080 and pindone carrot bait (size distribution, chaff content and toxic concentration) as an aid to improving best-practice bait preparation
- rates at which toxic baits are removed after application, what animals remove bait, and the degradation/detoxification rates of uneaten bait
- availability of rabbit carcasses to scavengers following 1080 or pindone baiting, and of the rates at which carcasses of poisoned rabbits degrade and detoxify under various environmental conditions
Dave’s project was funded by the Ministry for Primary Industries’ Sustainable Farming Fund (Grant no. 12/058) and was conducted in collaboration with Otago Regional Council.
Penny’s review was undertaken for the Marlborough District Council under an Envirolink Advice Grant (MLDC82).
Penny Fisher & Dave Latham