Attempting local elimination of possums (and rats) using dual aerial 1080 baiting

New Zealand aims to eradicate possums (Trichosurus vulpecula) and ship rats (Rattus rattus) nationally by 2050. This aim will require more effective tactics for locally eliminating these pests. Therefore, we explored whether possums and rats could be eliminated from large areas using pre-feeding and two applications of sodium fluoroacetate (1080) bait spaced a few months apart. The specific key question we investigated was whether any bait aversion learned by possums or rats that survived the first application of 1080 cereal bait would be overcome by a second application. A two-stage field trial was undertaken in 2016 and 2017, using 100-ha forested study blocks near New Creek, Westland. In the first stage, eight blocks were aerially baited with 1080 after being aerially pre-fed twice, once, or not at all. The relative abundance of rats fell to zero immediately after the baiting in all four blocks that were pre-fed, but lesser reductions in the four blocks not pre-fed. The survivors in the non-pre-fed blocks appeared to be universally and strongly bait averse. In the second stage, the non-pre-fed blocks were then aerially pre-fed twice and 1080-baited (by hand laying) with a bait type that differed slightly in appearance and smell from the original. Based on trail camera visitation rates and chew card interference, the relative abundance of rats fell to zero or close to it in the second group of four blocks, whereas possum activity indices increased markedly after an initial one week dip. The increase in possum activity was attributed to the peanut bait used in the chew cards attracting possums to monitoring sites. We conclude that dual application of 1080 using a similar bait in a single operation has the potential to locally eliminate rats from large areas, and that with further development involving use of two very different bait types and/or different pre-feeding sequencing, could potentially achieve the same for possums.