Effect of the addition of bird repellents to aerially applied 1080 baits on rat and possum abundance

One of the criteria for an effective bird repellent in a pest management context in New Zealand is that possum (Trichosurus vulpecula) and ship rat (Rattus rattus) kills remain high where repellents are used in poison baits. Repellents were used in baits applied within different treatment blocks as part of a large aerial 1080 operation in November 2013 near Haast on the West Coast of the South Island of New Zealand. We compared the proportional reduction in possum and rat population indices between standard aerial 1080 treatment, primary repellent treatment (0.17% wt/wt d-pulegone in prefeed and 1080 baits), and combined repellent treatment (0.17% wt/wt d-pulegone and 0.10% wt/wt anthraquinone in prefeed and 0.17% wt/wt d-pulegone in 1080 baits). All three treatments reduced the post-operational tracking rate of relative abundance for rats compared with the pre-operational rate. The standard treatment (100% proportional reduction in both blocks) was more effective than either repellent treatment, although the small difference between standard and primary (100% and 96% proportional reduction in two blocks) may not be meaningful, given the coarseness of the rat tracking index. The combined repellent treatment was the least effective (78% and 89% proportional reduction in the two blocks), with post-operational tracking indices of 3% ± 2 (standard error) and 8% ± 6. There was no difference in the three-night Bite Mark Index for possums between treatments. The results indicate that both repellent treatments could be used for possum control; however, the combined repellent treatment did not control rats to <5% tracking index, a level considered sufficient to protect native animals from rat predation. The primary repellent treatment reduced both possum and rat population indices satisfactorily, indicating it could be a useful bird repellent candidate if d-pulegone can be stabilised in cereal baits.