Media Articles

Chew card versus Infrared camera efficiency

Friday, November 27th, 2020

Chew card versus Infrared camera efficiency when hunting the last few possums – A mainland New Zealand eradication tool use lesson

By Tim Sjoberg, Taranaki Mounga Research and Innovation Lead


In order to successfully indicate whether Predator Free 2050 projects have completely removed possums from specific areas, in 2019 baited chew cards were trialled against Infrared (IR) cameras to quantify detection sensitivity and detection probabilities. The study site was the Kaitake Range, approximately 2400 hectares of coastal podocarp forest, located on the north western section of Egmont National Park. The study measured possum survivorship and tool efficiency after a dual aerial application which occurred May – October 2019.

Stage One detection was undertaken from 13 May – 2 June (13 days post aerial application), while stage two was undertaken between the 4 – 24 November (11 days after stage two aerial operation). In order for these results to be consistent, the IR cameras were already in place. Extra cards and cameras were used in the second stage detection phase as these devices provided increased visibility within a small side block of continuous forest that was identified as having a high possum reinvasion risk.

Stage two chew cards were deployed with peppermint flour blaze to increase their sensitivity to possum detection.

A total of 883 aniseed chew cards (10 x 10 cm sized) were deployed every 100 metres along 85 kilometres of track within the Kaitake Range after stage one of the aerial operation. While 943 peanut butter cards were used along 91 kilometres of track (again spaced every 100 metres, often in the same location) after stage two. During stage two card deployment, flour blaze (5:1 flour/icing sugar with ~25 ml Hansels peppermint essence/6 kg) was used above each card to increase its sensitivity to possum encounter and interaction.

IR cameras (15 LTL Acorn 5210A, 940 nm infrared cameras and 48 Browning Dark Ops Hd Pro X, 940 nm infrared cameras) were deployed in ~42 hectare grid throughout the Kaitake Range. Sixty cameras were used in stage one while 63 were used in stage two. Upon camera activation, a picture was taken and a five minute delay occurred before a further picture could be made. Cameras were attached to tree trunks ~40 centimetres off the ground and faced towards the lure ~ three meters distance away. For monitoring after stage one aerial application chew cards where used as the possum lure while stage two had Zero Invasive Predators (ZIP) Motolure which dispersed a small volume of egg mayonnaise every 24 hours.

Each chew card cost $0.35 plus a single nail, field staff can carry, install and service hundreds per day whereas cameras, rechargeable batteries and SD cards cost ~$500 each and the time to review the footage can be extensive in terms of labour.


Stage one aerial operation: 13 may – 2 June 2019

Of the sixty cameras deployed over the 21-day detection period following stage one operation, fifty cameras recorded continually while ten occurred faults at some stage. Cameras recorded 883 pictures over 1092 camera nights (24 hour time blocks) resulting in 133 possum events. Over the same 21 day period, 18,543 chew card nights recorded 29 possum chews (two chews was recorded at same location). Twelve of these possum chews were within 300 m of the bush boundary and one small area had four cards chewed in a clustering pattern.

Interestingly, chew cards acting as the only camera station lure were not being ‘chewed’ by possums, animals were simply attracted by the visual novelty or walking past not interested in the chew card.

Stage two aerial operation: 4 – 24 November 2019

63 cameras yielded 1950 pictures containing 211 possum events (24 hour presence or absence count) within 1225 camera days. We encountered issues with six of the 63 cameras, mostly human error with incorrect camera setting for time/date stamp records. Of the 943 chew cards deployed over the same time period (19,803 nights between 11 November – 1 December), only 16 possum chews were recorded on the cards.

Table 1 below: Detection probability of camera stations compared to chew cards within the Kaitake Range after stage one and two aerial operations.

Device Number of devices Detection duration (days) Detection probability over 21 days
Stage 1 camera 60 1092 12%
Stage 1 chew cards 883 18,543 0.15%
Stage 2 camera 63 1225 17.2%
Stage 2 chew cards 943 19,803 0.08%

Map of possum records from cameras and chew cards after stage one and stage two aerial operations, Kaitake Range.


So why did chew cards, an industry best practice tool with a proven track record at detecting possums result in such low probability of detection? There are several theories for these results, all of which are complex and not fully understood. The first hypotheses is the inability of an acute toxin to completely remove possums compared to traditional eradication toxins (anticoagulants), while the second highlights the major difference in ‘active’ versus ‘passive’ detection tools.

It is not completely clear why aerial 1080 delivered via an island aerial eradication method failed to remove all or substantially more possums than trial results achieved, although this result mirrors Nugent et al. (2019) study in which their dual aerial 1080 sowing also concluded in not eliminating all possums. We hypothesize that the possums in the Kaitake Range that either survived toxic exposure, or did not consume any toxic pellets were extremely neophobic and subsequently reluctant to bite on baited chew cards. Even when baited with lures not associated with the toxic operations. Had pre-operational monitoring been undertaken with chew cards occurred, we expect that more chew cards would have been chewed by possums. Although Wax tags were not used, we believe the results would have been similar regardless of tool use.

These results also dramatically expose the limitation of ‘active’ detection devices (chew cards requiring an animal to bite, leaving their tooth marks) compared to ‘passive’ devices (simply walking past) within attempts to completely remove possums from mainland New Zealand. The unwillingness of the survivor possums to bite into a chew card is concerning, but also evaluates the probability of detection and value of cameras regardless of the associated cost in time, money and investment in camera instalment skills.

It raises questions regarding all traditional detection or monitoring tools to actual pest numbers. Research led by the Department of Conservation and universities, has now started to focus on developing a standardised camera methodology for measuring pest species abundance, this will provide greater visibility of pest abundance as a repeatable measurement but will differ in design methodology and time duration from eradication attempts.

This trial demonstrates that even under current best practice, operational success is never guaranteed. More research and investment is also required to develop artificial intelligence or software programmes that can reduce the intensive labour need to currently review all camera footage while constant human vigilance is required to ensure that cameras are programmed and installed correctly.


Nugent G., Morriss G., and Warburton B. 2019. Attempting local elimination of possums (and rats) using dual aerial 1080 baiting. New Zealand Journal of Ecology 43:2.



Kaitake Range possum trap network

Sunday, August 16th, 2020

Lessons from our Kaitake Range possum trap network – Why not to use single set removal traps for possum eradication attempts?

Kaitake Range lean detection network.

By Tim Sjoberg, Taranaki Mounga Research and Innovation Lead

In 2016, the New Zealand government announced the ambitious aim of removing rats, possums and stoats from mainland New Zealand by the year 2050 (Predator Free 2050). To achieve this large-scale eradication, aerial tools are needed to remove predators from inaccessible and larger continuous forested landscapes. This, while follow up ground predator control tools to remove remnant animals. For this to be successful, the tools used need to be socially acceptable, cost effective and scalable.

In this summary we explore why single set removal traps will not achieve compete removal of possums from continuous forest in Taranaki and investigate other control tool options and their success to date.

Restore Kaitake is an ongoing pilot programme to completely remove possum from ~ 5000 hectares of land in and surrounding the Kaitake Range. This area includes 2,300 hectares of the Kaitake Range, 2,000 hectares of Kaitake farmland and Oākura town which includes residential properties, a school public reserves and walkways.

This project was initiated by Taranaki Mounga Project (TMP), Department of Conservation (DOC) and the Taranaki Regional Council (TRC) as part of the Taranaki Taku Tūranga – Towards Predator-Free Taranaki, with the overall lead being TRC. As part of this operation, TRC intensely controlled possums from the surrounding ~2,500 hectares of the Kaitake Range with wide range of methods (live capture trapping, removal trapping, bait stations and thermal assisted night predator control etc). While TMP undertook an aerial control 1080 operation to initially knock down possums, a removal trap network was actioned with the aim of removing any surviving possums.

The design of the network started in mid-2018, at this early stage in mainland eradication attempts by Predator Free 2050 (PF2050) we took lessons from past operations and the best available advice from technical advisors. The concept of installing one removal trap every 50 hectares was suggested to be sufficient to expose all possums (and hypothetically remove them) within a low-density possum landscape, as possum home range size increases dramatically after an effective aerial operation. To maximise operational success we decided to trial a high density trapping network. This consisted of 294 single set possum removal traps (143 Sentinal, 121 Possum Master and 30 Timms possum trap), set within a ~12.5 hectare grid covering the Kaitake Range and any surviving animals home range.

Possum master trap.

These three trap types were chosen because of their high capture rates, ease of service, different coloured materials, and previous history with minimal non-target by-catch. Care was taken to locate and install traps in the most suitable “trapping tree” within the constraints of steep country or inaccessible areas. Because of weather delays, some traps were installed and left unbaited up to 6 months before the aerial operations begin.

After the first application of the double (or dual) aerial 1080 operation had occurred and the detection network consisting of cameras (n = 63) and chew cards (n = 900, white corflute impregnated with lure) indicated a sufficient reduction of possums. We baited and set the removal trap network on the Kaitake Range to determine if we could remove the remaining possums without the need for the second stage of the aerial application.

Between the first and second aerial operation, we set all traps starting from seven days after the aerial application and finished 16 days after the application. Traps were baited with Connovation ‘smooth’ lure and flour blaze (5:1 flour and icing sugar lured with ~ 50 ml of Hansells peppermint essence). Both lures are industry standard and good attractants/bait to possums. This trapping network was serviced five times between day 16 to day 70 after the aerial application, with the average service time being 10 1/2  days. Each service consisted of refreshing the trap lure and flour blaze, chew cards and serving the cameras. The above-mentioned lure was used for the first four trap services, the last trap service was baited with a mixture of peppermint, cinnamon, mixed herbs held with a flour, icing sugar and sunflower oil matrix.

From the ~15,876 trap nights, this pilot network removed 18 possums over ~54 nights. We estimated 600 possums left within the Kaitake Range after the double (or dual) aerial 1080 operation was completed. When evaluated to the self-reporting leg hold network (installed since late-2019, using 190 leg hold traps), this network has removed 108 possums from the Kaitake Range over a 54 night periods between 1 May to 23 June 2020 (~10,260 trap nights). It should be noted that this trapping period occurred following New Zealand’s March/April 2020 Level 4 Covid-19 lockdown. During this time no leg hold traps were actively set to capture, meaning trap lure may have increased in novelty through a two month period of no trapping.

Sentinal possum trap.

We hypnotise that the single set removal trap network was ineffective at removing possums compared to the leg hold network for several reasons. Firstly the instalment of traps (unbaited and unset) for long periods (up to six months) before setting live could have allowed the novity of the traps to diminish. A better approach might have been stashing traps near or at track junctions and setting on trees once aerial operation had been undertaken to increase novity for surviving possums. Secondly, the intensive use of chew cards might have caused a distraction to surviving possums as cards where positioned along all trap lines at 100 m intervals. Thirdly, the lures used were not attractive enough given the higher abundance of readily accessible food resources available to surviving possums after stage one aerial operation. And lastly, more animals were captured than found however these were removed by members of the public before staff serviced the trapline.

We place more weight to the first two options as subsequent possums have been caught on the Kaitake Range using the above-mentioned lures, also the small number(s) of traps cleared of animals by members of public would have little impact on the trap networks results (i.e. 18 possums over 15,876 trap nights versus 108 possums caught over smaller trap nights from the leg hold traps).

Ultimately, this trial demonstrates that traditional possum removal traps, even when deployed at high density (i.e. every 12.5 hectares), using three different trap types and baited with industry standard lures, will not completely remove possums from continual forests in Taranaki. We believe more sensitive control tools that require less manual bait refreshment need to be developed in order to achieve PF2050 for possums and although the self-reporting leg hold network is capturing possums at higher frequency than single set removal traps, the labour requirements of install and maintenance needs to be greatly reduced in order for this tool to be scalable.

The Taranaki Mounga Project would like to thank TPFT, DOC, along with our Taranaki Mounga ranges and the Kaitake community for being so accommodating as we carried out this work. These learnings have been shared with key research and predator control groups to help with their large-scale predator possum eradication operations from inaccessible and larger continuous forested landscapes.