Pest Monitoring

Catch records provide some information about the effects of trapping on pest numbers. Tracking tunnels are used for additional monitoring. Mustelids and rats are attracted to enter small tunnels, so these are provided in the form of folded plastic sheets. A white card with a central zone of wet ink is placed in the base of the tunnel so that any animal passing through leaves its characteristic foot prints. A dab of peanut butter is placed inside each end of the tunnel for further attraction.

tracking tunnel and card

Tracking tunnels are set out permanently in randomly positioned lines. Periodically, blank tracking cards are laid out in the tunnels for one night and then retrieved. The percentage of cards marked with foot prints then provides an index of the population of that species.


Card showing rat tracks, after one night in a tracking tunnel before pest control:

Rat prints on tracking card

Forty tunnels (four lines of 10) are set out in each of the Plateau area and the Te Tawa Stream catchment, the areas in which intensive rat trapping started in December 2005 and January 2008 respectively. No mustelid tracks have been recorded in any of the tunnels, but useful results have been obtained for rats, as shown below:


Tracking Tunnel Results - Plateau Area:

Tracking Tunnel Results - Plateau Area

On the Plateau area, the percentage of tunnels tracked plunged dramatically from 100% in April 2005 before the start of trapping, to zero in January 2007. Trapping was halted in this area from March 2007 in anticipation of Department of Conservation poison control for possums, which was also expected to remove rats. The poison operation was delayed until October, and by September 2007 the rat population had increased in this area to a tracking rate of 8%. After poisoning November-December 2007 and the resumption of trapping in January 2008, the tracking rate fell again to zero in May 2008.

In the Te Tawa Stream area, the tracking rate reduced from 75% in September 2007 prior to the poison operation, to zero in January 2008 after the operation. Trapping started for the first time in this area in January 2008.

These results indicate that both trapping and poison have been effective in controlling rat populations, but that continual control is required because of re-invasion. In particular, the target tracking rate of less than 5% has been achieved.

The Trust has the view that trapping is less expensive than poison in the long term, and the problem of poisoning of non-target species is also avoided. It remains to be seen whether a population of trap shy rats will develop, requiring alternation with poison control methods at some time in the future. Poison shyness has been observed in other pest control projects.

This page last updated: 21 June 2008

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