The current possum defence toolset within the Remove and Protect virtual barrier is limited to two devices: live-capture leghold traps and Trapinator kill traps.

The leghold trap is by far our most effective tool for invading possums, intercepting approximately 60% of those that attempt to pass through each defence line.

We currently use leghold traps in the first and third lines of the ‘virtual barrier’, and along the first part of the ridgeline track which runs the length of the peninsula, at 10 metre intervals along each line. Traps are raised on platforms a minimum of 1.3 m off the ground to prevent the accidental by-catch of ground birds such as weka.

A leghold trap platform with visual lure on Defence Line 1 (D1) at Bottle Rock field development site

A leghold trap platform with visual lure on Defence Line 1 (D1) at Bottle Rock field development site

When ZIP began operations at Bottle Rock in late 2014, the leghold traps were lured using a 90x90mm white corflute chew card nailed to the tree approx. 500mm above the trap platform. This was baited with aniseed possum lure and refreshed every 6 weeks.   

However, when we observed catch data over time, it became clear that refreshing the lures did little to improve trap effectiveness, and catch rates did not reduce between services.

Defence Line 3 (D3) leghold trap triggers over time. The left-hand axis represents the trap number and distance in metres along the line. The vertical dot/dash line represents the date on which the food lure was refreshed. Note that possum interaction and catch rates appear insensitive to the freshness of the foodlure (i.e. capture rates do not increase immediately after refreshing the lure).

Defence Line 3 (D3) leghold trap triggers over time. The left-hand axis represents the trap number and distance in metres along the line.

The vertical dot/dash line represents the date on which the food lure was refreshed. Note that possum interaction and catch rates appear insensitive to the freshness of the foodlure (i.e. capture rates do not increase immediately after refreshing the lure).

We have come to believe that the visual ‘flag’ created by the white corflute square is acting as the attractant, and is itself sufficient to cause possums to investigate traps. This could be particularly true for invading animals moving through a new environment, who may be driven by curiosity to investigate any ‘novel’ visual cues that they encounter.

Other studies also support the theory that possums have a relatively poor sense of smell but are strongly curious, and therefore may respond more strongly to visual lures.

Trials by Morgan et al (1995) indicated that possums’ sense of smell does not extend beyond 2 metres, and that visual cues (i.e. white flour) may be more important in luring possums in the wild. The possum eradication on Kapiti Island also supports the theory that curiosity plays a large part in capturing possums, with raised sets claimed as the “unusual object” often investigated within a possum’s territory (Sherley, G. 1992).

This visual luring result at Bottle Rock is a particularly significant finding for the Remove and Protect model as it reduces the servicing cost of these traps by removing the need to replace food lured chew cards every 6 weeks.

ZIP is developing an automated system to enable the remote monitoring of leghold traps, which has the potential to dramatically reduce the labour involved with these devices. This development, along with the move to a visual lure, could allow traps to be left in the field for a matter of months, requiring servicing only when they have caught an animal or to replace the visual lure.

We are proposing to test the relative effectiveness of visual lures more rigorously with a trial this summer.

One way in which we have begun to observe that scent might play a part in luring possums to traps is through what we have termed the ‘funnel’ effect, whereby possums appear to follow the ground trails of other possums directly to certain traps.

More on this later...

References:

Morgan, D.R., J. Innes, C.M. Frampton & A.D. Woodhouse (1995) Responses of captive and wild possums to lures used in poison baiting, New Zealand Journal of Zoology, 22:2, 123-129.

Sherley, G.H (1992) Eradication of brushtail possums (Trichosurus vulpecula) on Kapiti Island, New Zealand: Techniques and Methods, Science and Research Series #46, Department of Conservation, Wellington.