Delivery of toxic bait in clusters: a modified technique for aerial poisoning of small mammal pests

Sowing 1080 baits for vertebrate pest control in clusters, rather than evenly, could potentially reduce toxin use. We developed a new technique for aerial delivery of 1080 baits in clusters and, in a set of four trials, compared its efficacy in controlling pests against conventional aerial broadcast baiting. In an initial trial where non-toxic prefeeding was not used (Molesworth Station, North Canterbury) we confirmed that aerial delivery of bait clusters is technically feasible and operationally practical. The reductions in possum activity indices achieved with cluster sowing (98.4%) were similar to those achieved with broadcasting sowing (97.8%), despite using 60% less 1080 bait (1 kg ha^–1). Comparable efficacy against possums was also recorded in the Landsborough Valley, Westland, where aligned prefeeding and an even lower sowing rate of toxic bait were used (0.25 kg ha^–1). In a third trial (Isolated Hill, Marlborough) the same cluster-sowing approach resulted in large reductions in possum and rat activity indices, but the possum reductions were more spatially variable than with broadcast baiting. At Maruia, Westland, near-total reductions in possum and rat activity were recorded with both broadcast and cluster sowing, even when there was a longer than usual interval (>30 days) between the aligned prefeeding and cluster baiting, and even when a wide (150 m) helicopter-flight-path spacing was used to reduce the cluster sowing rate of toxic bait to just 0.17 kg ha^–1 (92% lower than the broadcast sowing rate used). These trials suggest that cluster baiting at lower-than-usual sowing rates could lower operational costs, and substantially reduce toxin usage, while maintaining high control efficacy against rats and possums in most cases. Reduced use of toxin might go some way to allaying public concerns over 1080 usage. Further operational testing is required to refine aerial cluster baiting and to identify the optimal balance between lowering costs and toxin use yet consistently achieving high control efficacy.