Local pest control success

Possums: 300 possums were caught in the East Harbour Regional Park's Northern Forest in the past year, the highest number since 2017 and up 120 on last year’s total. Why was that? Well, there are a few likely causes. First, a small new trapline (just 6 traps) was established immediately north of the Point Howard reservoir and caught nearly 50 possumes in the first 9 months. There was also a lot more action on the 13 traps on the eastern edge of the Forest (known as the Kiwi Corridor—hopefully creating a safe route for kiwi from Remutaka Forest Park to the Northern Forest). Here the catch was 50 possums, double that of the previous year. In addition to that, many MIRO trappers are now dual-baiting with fresh apple, which lasts a few days, and a cereal-based lure, which lasts a few weeks.

You may remember that OSPRI (formerly TB-free NZ) undertook ground poisoning to control possums around the perimeter of the Forest in the latter half of 2021, followed by an aerial 1080 drop in the core of the forest in November 2022. Prior to that, they did a monitoring study, which showed a 4.3% tracking rate for possums (roughly equivalent to putting out 100 traps for one night and catching just 4 possums). Generally, we regard anything less than 5% as OK for forest health, but for Bovine TB to die out you need it closer to 2%, hence the further control measures in 2021/22 by OSPRI. As far as we are concerned though, with the tracking rate falling fairly consistently below 5% since 2001, the Forest should be fine in terms of possum browsing. This is a great achievement by all the volunteers who have helped with trapping in the Forest since the 1990s.

Stoats, weasels and feral cats: The last year’s catch and monitoring data are also looking positive for feral cat and mustelid (stoat and weasel) control. No feral cats were caught by the traps we have in the core of the forest (we keep these traps well away from the urban areas), with only 2 caught in the past 6 years. Similarly, the stoat and weasel catch (a total of 11) was the lowest since DOC200 traps were deployed across the whole Forest in 2017. Cat and mustelid populations are difficult to monitor as these animals are wary of monitoring tunnels. Instead, we have continued to use three automatic lure feeders monitored with trail cameras to watch for pests. The sites are always away from main tracks, with the cameras positioned about one metre off the ground and closely focussed on the spot where the lure (usually mayonnaise) drips onto the ground. The latest results are consistent with the previous year in that no feral cats were seen and only one stoat.

Hedgehogs: The story is also positive in terms of hedgehog control, although we don’t understand why. Last year’s catch of 48 is down from 77 the previous year. Similarly, the Greater Wellington monitoring data shows a decrease from 6% tracking in the 2021/22 summer down to zero last summer. We are not aware of any changes that would have caused this decrease, so it will be interesting to see what happens next summer. By comparison, hedgehog tracking in Eastbourne and the Bays, as measured by our regular ERAT monitoring, was at 17% last summer, up from 10% the year before, so bad news for our skinks and lizards.

Finally—rats! As we have explained before, rats are really only controlled in 20% of the Northern Forest through intensive poisoning in the Mainland Island by GWRC, with the aim of keeping tracking below 5%. The boundary of the Mainland Island goes between southern Days Bay, Burdans Gate, the Butterfly Creek picnic area, up Gollans Stream, then back to the southern end of Days Bay. Outside of this area, the rat population is tracking at quite high levels, usually over 25%. Inside the Mainland Island, however, the 5% tracking rate target was almost achieved for over a year, apart from a recent spike up to 21% in May. In our urban area, the rat tracking rate over the same period has averaged less than 6%, which is a good result, thanks to all the home trappers, most of whom are part of the ERAT project.

-Terry Webb, MIRO Chair. Interested in helping MIRO? Email: info@miro.org.nz