Improving the efficacy of aerial poisoning of brushtail possums (Trichosurus vulpecula) through reduced fragmentation of bait

Abstract

In New Zealand, aerial delivery of sodium fluoroacetate (1080) baits to poison introduced brushtail possums (Trichosurus vulpecula) is a key pest management tool. GPS-guided broadcast baiting systems should expose all possums to large amounts of toxic bait, but some possums still survive despite being poisoned. This sub-lethal poisoning might reflect bait fragmentation, so we measured bait size before and after broadcast sowing from helicopter-slung buckets with spinners used to fling bait laterally. Mean bait size was smaller after sowing than before. The amount of fragmentation differed between operations and between buckets. Small fragments were concentrated under the flight path suggesting that they were caused by the spinner. Fragmentation increased with spinner speed. Removing the spinner resulted in less fragmentation. Use of spinners may therefore adversely affect poisoning efficacy by producing sub-lethal bait fragments. Solutions to this bait fragmenation problem include reducing spinner speed, removing the spinner, or development of shatterproof baits. A reduction in the amount of fragmentation is likely to reduce the need to sow large numbers of baits, so our findings could enable major reductions in toxin use.

Keywords:

• aerial poisoning
• bait distribution
• bait fragmentation
• brushtail possum
• Trichosurus vulpecula

Acknowledgements

We thank Caroline Thomson, Morgan Coleman, Roger Carran and Steve Hough for their help in field data collection. Access to study sites was provided from DOC and Timberlands West Coast. David Priest (AHB) provided help with site selection and valuable comments on methodology. We thank Peter Newsome and Guy Forrester for their help with data analysis and statistical interpretation. The research was contracted by the Animal Health Board as part of Project R-10688 (Local elimination: A new strategic approach to large-scale control of small mammal pests), and co-funded by the New Zealand Foundation for Research, Science and Technology (Programme C09X0507Multispecies Pest Control). Comments on the various drafts of the paper by Phil Cowan and by anonymous reviewers helped improve this manuscript.