Improving possum capture rates by modifying leghold trap sets
Double-coil spring leghold traps are used to capture possums for pest control, fur recovery and population monitoring. For monitoring, a standardised method (described by the National Pest Control Agencies; NPCA 2015) for setting traps is used. Traps are bedded into the substrate, approximately one hand width from the trunk of a tree, and with the trap dog (latch) nearest the tree. The trap chain is stapled to the tree and a blaze of flour and icing sugar (5:1 mix) is spread on the tree behind the trap to act as a visual and food lure to encourage possums to approach and step on the trap. However, research has shown that on their first visit to a trap set that follows this protocol, only 40–50% of possums are caught.
To increase capture rates of possums, fur trappers have advocated a range of changes to traps and sets, including hazing (placing rocks, sticks or logs beside the trap to act as a fence) to guide possums over the trap, covering traps in case possums visually avoid stepping on them, or using traps that have larger trigger plates for the possums to stand on. Although these modifications might increase capture rates, there are no data to support their use.
In 2013 TBfree New Zealand commissioned Bruce Warburton and Grant Morriss to investigate ways to improve possum capture rates. Early field testing compared the capture rates of standard NPCA trap sets with those with a large trigger plate, double traps, covered traps, and hazed traps. Unfortunately, these trap-set modifications did not improve capture efficiency. Surprisingly, standard sets caught the most possums overall.
Subsequent examination of video footage of possums encountering traps as part of another project indicated that possums do not always directly approach the lure by stepping on or over the trap, but instead often reach around the tree from the side of the trap. Given covered traps did not improve capture rates, reaching around the tree was probably not possums trying to avoid the trap. Hazing was used to try to force possums to approach the lure by stepping on or over the trap, but video footage again showed that some possums still climbed over the hazing material and avoided capture. The results from this trial and examination of video footage indicated that placing lure behind the trap on the trunk of a tree might not be the best solution for maximising capture rates.
With this in mind, Bruce and Grant trialled sprinkling lure in a ring around the trap on the ground so that a possum would encounter a food source before the trap, no matter which direction it approached the trap from, and during feeding had an increased chance of stepping on the trap plate. In addition, white, flat, unbaited chewcards were nailed to both sides of the adjacent tree to act as a visual lure to attract possums to the trap site. An extra card on the back of the tree aimed to pique the interest of possums approaching the trap from the blind side of the tree.
During field tests in one moderate and two low possum density sites in Canterbury more possums were captured with this modified set compared to standard NPCA trap sets. This trap set provided a 30% increased capture rate, which should improve the effectiveness of detection surveys where currently possum bites are frequently recorded on chewcards or wax tags, but follow-up trapping is unsuccessful at capturing possums.
The initial work was funded by OSPRI through their TBfree programme, with later testing funded by the Ministry of Business, Innovation and Employment.
NPCA 2015. Possum population monitoring using the trapcatch, waxtag and chewcard methods. Best practice guideline publication no. A1. Wellington, National Pest Control Agencies.