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The brushtail possum, a marsupial native to Australia, was widely introduced in New Zealand (NZ) to develop a fur industry. Before the settlement of humans in NZ, there were no terrestrial mammals; therefore, the local species evolved without mammalian predators. This resulted in native species populations declining at alarming rates and many possibly facing extinction on the NZ mainland, especially large-bodied endemic birds. In response to this problem, private investors (supported by the NZ government) developed an initiative to eradicate key mammal predators (possums, rats, and stoats) on the NZ mainland by 2050 (PFNZ2050). As a result, control efforts have significantly expanded over the past decade, and there are now 17 PFNZ2050 landscape projects covering 757,000 ha. Research has looked at combining audio, visual and social lures to improve control efforts for possums. Early results indicate that a combination of lures increases both encounter and interaction rates around control devices. In particular, the combination of audio and visual lures was consistently the top performer in both captive and small-scale field trials. Large-scale field trials are currently underway to confirm these initial results, investigating the effectiveness of combination lures paired with multi-kill AT220 possum traps. Additionally, the lures are being assessed at different times of the year and with varying possum densities. With the drive to PFNZ2050, pest mammal monitoring has markedly increased as researchers attempt to evaluate competing management control strategies. Traditionally, this has relied on single-use plastic monitoring devices such as chewcards and tracking tunnels. An investigation of pest-animal interaction with chewcards indicates that approximately 12% of plastic deployed ends up as microplastic pollution. Another NZ government initiative seeks to ban all single-use plastics, and research is currently investigating the efficacy of non-plastic (biodegradable) alternatives. This research also includes a cost-effectiveness analysis and the reliability of identifying animal bite marks on both plastic and non-plastic chewcards.

Possum numbers have been significantly reduced in many regions of New Zealand. However, research has indicated some unexpected consequences of possum control. At some sites, rat numbers have more than doubled 2 years after possum control. What this suggests is that the removal of a direct competitor has enabled a rapid increase in rat numbers relative to slower possum recovery. This has serious implications, as high rat numbers could inhibit ongoing possum ground control. To investigate this, an experimental trial was run where we tested the following research hypotheses by manipulating the rat density: i) Null hypothesis (H0) ‒ rat density has no influence on possum foraging behaviour around bait stations, ii) Alternative hypothesis (H1) ‒ rat density indirectly influences possum behaviour by removing all bait before possums can access it, and/or iii) Alternative hypothesis (H2) ‒ rat density directly influences possum behaviour by physically excluding them from bait stations. The experimental site was divided into 2 parts, a treatment block and a control (non-treatment) block. Rat control was undertaken using Victor® kill traps and 96 rats were removed over 6 nights (density estimated at 4.6 rats/ha). To quantify the effect of rat density on possum foraging behaviour, non-toxic bait stations were stapled to a tree every 10 m along 18 monitoring lines (n = 50). Possum behaviour was then monitored using both modified tracking cards and IR camera traps. Prior to trapping, 92% of all the baits were removed by rats on the first night. Following trapping, this reduced down to 8% in the treatment block. Rats turned up earlier than possums at the majority of the monitored sites (~1 hour before sunset). Before trapping, baits were only available 33% of the time when a possum visited a bait site (n = 15), and no baits were removed. After trapping, baits were available 100% of the time and 2 baits were removed by possums. These results support H1 and suggest that the smaller-bodied competitor can dominate food resources in NZ forests. This has the potential to make possum ground control more difficult when using control techniques that do not target rodents.

The Australian brushtail possum is a pest in New Zealand, as it has caused the decline of a number of native bird species and has become the greatest barrier to the eradication of bovine tuberculosis (bTB) from livestock. As such, possum populations are reduced (i.e., controlled) at a cost of millions of dollars every year. Yet, control is still timely, costly, and not always 100% effective. Behavioral changes in possums have the potential to substantially affect the ability to control possum populations. For example, it is currently unknown whether possums change their behavior, such as den-site use, following control. How possums use dens is an important consideration in controlling bTB, as dens are a potential infection reservoir for both possums and livestock. Knowledge of any changes in den-site use may allow the design of more targeted and efficient control operations that identify potential increases in transmission risk. To investigate changes in den use due to control, possums were fitted with VHF devices, and were tracked to their den sites before and after a control operation that killed approximately 54% of the adult population. The total number of dens they used and the number of times they changed dens were recorded. Density reduction resulted in an increase in the number of times possums changed dens. However, den use was more strongly influenced by the sex of an individual, with males changing their dens more often than females. Density reduction did not appear to result in a change in the number of dens used. Instead, this behaviour was also largely driven by the sex of an individual, with males using more dens than females. This potential increase in transmission risk does not mean that control strategies should not be undertaken, but that managers should keep in mind the likely responses of possums to control and adapt their strategies accordingly. For example, control measures may need to be targeted towards those individuals that use more dens, such as males. This research also further highlights the need for efficient control that reduces populations to very low densities, to negate this potential increase in transmission risk.