Brushtail possums (Trichosurus vulpecula) dominate dama wallabies (Notamacropus eugenii) and Bennett’s wallabies (Notamacropus rufogriseus) at bait feeders: implications for invasive species management

Introduction

Dama (or tammar) wallaby (Notamacropus eugenii) and Bennett’s (or red-necked) wallaby (N. rufogriseus) are invasive pests on the New Zealand mainland. Dama wallabies were liberated in the Rotorua district in 1912 and again in 1940 and have since expanded in abundance and geographic range throughout that region (Latham et al. 2019). Bennett’s wallabies were liberated near Waimate, South Canterbury in 1874; their initial spread was limited but low-density populations have since emerged over a large area of Canterbury and parts of Otago (Latham et al. 2019). Brushtail possums (Trichosurus vulpecula, hereafter ‘possums’) were repeatedly introduced from Australia in the mid to late 1800s and are now widespread across the New Zealand mainland (Cowan and Glen 2021), including throughout most habitats occupied by wallabies.

Both wallaby species are primarily grazers, supplementing their diet with browse when in scrub and forest habitats. Dama wallabies are a potential pasture pest and damage new production forestry plantings, although their largest impacts are on indigenous vegetation (Latham and Warburton 2021). In 2016, their annual impact on agriculture and the environment was estimated as being as high as NZ$4 million (Latham et al. 2016). Bennett’s wallabies have been recognised as a significant pest of pasture, crops and production forestry since the 1940s (Warburton 1986). In 2016, their annual impact was estimated as being as high as NZ$24 million (Latham et al. 2016).

Coordinated efforts to control dama wallabies in the Rotorua district using aerial 1080 operations began in 1962, with various methods and intensities of control applied in subsequent decades. Regional pest management plans for the dama population, which overlaps the boundaries of three administrative regions, have variously aimed for containment or elimination of the species (Latham and Warburton 2021) but have been unsuccessful in preventing spread.

Government-subsidised hunting to suppress the Bennett’s wallaby population was first implemented in 1947. In late 1989, government subsidies ended and thereafter the full cost of control has been met by landowners (Latham and Warburton 2021). A regional pest management plan aimed at preventing further geographic spread of Bennett’s wallabies has been unsuccessful, in part because culling was insufficient but also because of deliberate relocations of wallabies (Latham et al. 2019).

In July 2020, the Tipu Mātoro National Wallaby Eradication Programme – a partnership between central and regional government, landowners, and other stakeholders – was initiated. The Programme’s stated goal is to push mainland wallaby populations back into containment areas designated in regional pest management plans, as a first step towards the longer term goal of eradicating wallabies from New Zealand (MPI 2022).

In addition to contracted and private hunting, deployment of toxic 1080-impregnated cereal baits from aircraft has proven an effective method for large-area wallaby pest control (Latham and Warburton 2021). There are; however, significant limitations to where aerial 1080 can be deployed because of potential risk to non-target species and other social concerns. Consequently, there have been ongoing efforts to develop effective ground-based wallaby baiting methods, including the development of alternative toxins (e.g. Shapiro et al. 2011), foliage baiting techniques (Warburton 1990) and the use of bait feeder stations (Williams 1997).

Bait feeders deploying cereal baits are an attractive alternative to aerially-sown or hand-laid distribution of baits because they facilitate removal of uneaten toxic bait at the end of an operation, thereby eliminating ongoing risk to livestock and other non-target species. Some feeder designs also provide weather protection for the cereal bait. Bait feeders deploying an initial pulse of non-toxic pre-feed followed by a toxic baiting phase are a well-established control method in New Zealand for possums (Warburton and Thomson 2002) but are of unproven efficacy for wallaby control (Williams 1997).

The observations reported here arose from a field trial funded by the Tipu Mātoro National Wallaby Eradication Programme with the aim of developing more effective bait feeder designs to assist wallaby control efforts. During set-up of the trial, we were advised by local field staff (D. Williams, Bay of Plenty Regional Council, pers. comm.) that possum interference with the feeders was likely. Consequently, it became an additional aim of the study to investigate whether possum activity at the feeders might reduce the efficacy of the feeders for wallaby control. Here, we report and discuss only our wallaby–possum behavioural findings; our feeder-design results will be reported elsewhere.

Methods

Bait feeder deployment

In early summer (October – December) 2022, we recorded wallaby and possum visits to four designs of bait feeders stocked with non-toxic bait and to a pile of cereal pellets on the ground, which served as an experimental control. Two of the feeder designs consisted of plastic containers mounted on stakes, each containing 1 kg of cereal pellets (e.g. Fig. 1). The other two designs consisted of 8 × 18 g Ferafeed ‘strikers’ (https://www.connovation.co.nz/products/ferafeed-striker-18g-213-paste-backing-card) stapled to wooden boards (one laid on the ground and the other mounted on a stake). In this report we focus on behavioural interactions between wallabies and possums seen at these various baited sites. We refer to all sites as ‘feeders’ but emphasise that this term encompasses all five treatments, including cereal pellets on the ground.

Fig. 1.

Example of a bait feeder deployed for dama wallabies at Titoki; the feeder contains 1 kg of non-toxic cereal pellets and is being monitored by a pair of infrared trail cameras.

At both field sites, replicates of these five treatments were deployed at 200 m intervals along a transect that ran through habitat with abundant recent wallaby signs (i.e. runs, pads and faecal pellets). At Titoki, six replicates of each treatment were observed for 10 weeks. At Blue Cliffs, where deployment of stations was delayed by livestock operations, eight replicates of the treatments were observed for 4 weeks. At weekly intervals, any missing bait was replaced.

During the field study we received an anecdotal report that wallabies tended to avoid possum carcases lying close to feeders (R. Harland, Greater Wellington Regional Council, pers. comm.). To investigate this possibility, for an additional 2 weeks at Blue Cliffs, a single possum carcase (freshly kill-trapped by a local pest control contractor) was placed beside three to four replicates of each treatment for either 1 week (n = 12 feeders) or 2 weeks (n = 8). Notes were kept on the rate of carcase decomposition. The field trial at Titoki was near completion when this extension to the study was approved, so we did not have an opportunity to deploy carcases at that site.

In winter (July – September) 2023 we ran a second field trial to evaluate the effectiveness of a pulsed non-toxic/toxic baiting strategy aimed at reducing possum and wallaby numbers at each treatment. Feeders were deployed in the same general areas as the summer trial but at locations where wallabies would not have encountered the previous feeders. At Titoki, the new transect was in similar habitat as before. At Blue Cliffs, a change in livestock grazing meant that the new transect was located in high-elevation tussock grasslands distant from possums’ preferred habitat. Feeders were monitored for 8 weeks at both sites. At Titoki, 30 feeders were pre-fed with non-toxic bait in Weeks 1, 2, 5 and 7, and with toxic bait (Pindone plus either Feratox or Cyanara50 assigned randomly) in Weeks 3, 4, 6 and 8. At Blue Cliffs, the toxic bait was deployed in Weeks 2, 6, 7 and 8.

Results

Over 10 weeks in October – December 2022, we recorded 1228 visits by dama wallabies at the 30 Titoki feeders, with multiple wallabies present during 613 visits (49.9%). We also recorded 4476 possum visits to the feeders, for an average visitation rate of 12.1 wallabies and 22.1 possums per feeder per week. Over 4 weeks in November 2022 at Blue Cliffs, we recorded 2660 visits by Bennett’s wallabies at the 40 feeders, with multiple wallabies present during 604 visits (22.3%). We also recorded 2668 possum visits, for an average visitation rate of 26.1 wallabies and 16.7 possums per feeder per week. We also recorded several hundred visits by European hedgehogs (Erinaceus europaeus) at both sites and by ship rats (Rattus rattus) at Titoki, and infrequent visits by various other birds and introduced mammals; all of these animals were essentially ignored by the wallabies and possums.

In early summer, possum activity at the feeders was highest from 20:00 to 05:00 h at Titoki and 21:00 to 05:00 h at Blue Cliffs (Fig. 2a, b). The 1-h difference between sites consistent with sunset being about 40 min earlier at Titoki in the north (at c. 20:20 h) relative to Blue Cliffs in the South Island (at c. 21:00 h). This timing of brushtail possum activity matches past reports that possums typically become active about 30 min after sunset both in their native Australia (e.g. MacLennan 1984) and in New Zealand (e.g. Ward 1978).

Fig. 2.

Wallaby and brushtail possum diurnal activity at bait feeders: (a) at 30 stations monitored for 10 weeks at Titoki (Rotorua district, October – December 2022); (b) at 40 stations monitored for 4 weeks at Blue Cliffs (South Canterbury; November 2022). Shaded blocks indicate the hours between sunset and sunrise in November at the two sites.

Dama wallabies’ activity at feeders occurred almost entirely between sunset and sunrise, coinciding with possums’ active period (Fig. 2a). Bennett’s wallabies visited throughout the night but were also often seen at feeders in daylight hours when possums were inactive, particularly in the late afternoon (Fig. 2b). Both wallaby species’ activity levels peaked near sunset, declined as possums become active, and then increased again when possum activity dropped off as dawn approached.

Despite >10,000 visits by wallabies and possums recorded across all treatments, it was uncommon for wallabies and possums to be seen together at a feeder. During our 2022 trial, possums were seen during 134 of 1228 visits by dama wallaby and during 22 of 2660 visits by Bennett’s wallaby (10.9% and 0.8% of wallaby visits, respectively; Fisher Exact test, P < 0.0001). In winter 2023, possums were seen during a further 95 of 1826 dama wallaby visits but were never seen during 1530 Bennett’s wallaby visits (5.2% and 0% of wallaby visits, respectively).

Outcomes of the 251 encounters recorded between wallabies and possums are summarised in Fig. 3. When encounters occurred, possums tended to be first at the feeder (55% of encounters at Titoki, 68% of encounters at Blue Cliffs; Fig. 3a, b). At both sites, the most frequent outcome of the encounter was that the possum excluded the wallaby from the feeder (72% of encounters at Titoki and 95% of encounters at Blue Cliffs; Fig. 3a, b). Exclusion could result simply from the presence of the possum; aggressive behaviours by the possum were exhibited in only around half of encounters that resulted in wallaby exclusion (48% of encounters at Titoki, 55% of encounters at Blue Cliffs; Fig. 3a, b). Possums’ aggressive behaviours included vocalisations, bipedal threat displays, pouncing and chasing (e.g. Fig. 4a–c). During the remaining encounters, the possum either ignored the wallaby or simply exhibited increased vigilance – the wallaby retreated from the feeder nevertheless (e.g. Fig. 4d). We recorded only three encounters, all at Blue Cliffs, where a wallaby responded aggressively towards the possum. All three efforts were unsuccessful, each ending with the wallaby retreating from the feeder.

Fig. 3.

Outcomes of encounters between wallabies and brushtail possums at bait feeders at: (a) Titoki, Rotorua district; (b) Blue Cliffs, South Canterbury. Numbers, which are pooled for 2022 and 2023, represent tallies of the total number of encounters in each category. Numbers are low at Blue Cliffs in part because the 2023 transects at that site were distant from possum habitat.

Fig. 4.

Examples of encounters at bait feeders between brushtail possums and: (a, b) dama wallabies; (b, c) Bennett’s wallabies. Aggressive behaviours by possums included bipedal threat displays (a-frame3, b-frame2, c-frame3) and pounces (b-frame 3). Wallabies often retreated from the feeder as soon as they detected an approaching possum (visible by its eye-shine in the upper-right of d-frame4).

The majority of the excluded wallabies moved away from the feeder site (116 of 165 at Titoki; 15 of 21 at Blue Cliffs); the remainder waited near the feeder until the possum departed and then began feeding at it (49 of 165 at Titoki; 6 of 21 at Blue Cliffs). Regardless of which species arrived first, wallabies were never successful in excluding possums from a feeder, although occasionally both species departed (6% of encounters at Titoki, 5% of encounters at Blue Cliffs; Fig. 3a, b), usually because the possum chased after the retreating wallaby.

At Titoki, we recorded 50 examples (22% of encounters) of possums and dama wallabies both accessing bait during the same visit. These possums and dama wallabies never fed side by side from the feeder itself; concurrent feeding only occurred when one or both animals were feeding from the ground on bait spilled from the feeder. When dama wallabies investigated feeders in the absence of possums, 32% of their visits resulted in feeding – this was a significantly higher rate than the 22% seen feeding when possums were present (Fisher Exact test, P < 0.0001).

No concurrent feeding by Bennett’s wallabies and possums was observed at the feeders, whereas 54% of Bennett’s wallabies investigating feeders in the absence of possums fed at them (again a highly significant difference; Fisher Exact test, P = 0.004).

At Titoki in 2022, where 1.8 times more possums than wallabies visited the feeders (average visitation rate of 12.1 wallabies and 22.1 possums per feeder per week), 100% of bait was removed from every feeder during every week during the 2022 trial; we suspect mostly by the possums. Wallabies visiting these depleted feeders consequently had no opportunity to consume bait until the feeders were replenished the following week. At Blue Cliffs in 2022, where 1.6 times more wallabies than possums visited (average visitation rate of 26.1 wallabies and 16.7 possums per feeder per week), the raised feeders were rarely emptied although bait on the ground was typically consumed within 2 or 3 days, mostly by the possums.

Wallaby response to possum carcases

During the Spring 2022 Blue Cliffs trial, placing a possum carcase close to a feeder initially deterred wallabies from interacting with that feeder (Fig. 5), with their visitation rate falling by 86% during the first week the carcase was in place (Fig. 6; P < 0.01). The ‘missing’ wallabies presumably came close enough to detect the carcase but not close enough to be photographed.

Fig. 5.

Progressive decay of a brushtail possum carcase at a feeder: (a) on placement day; (b) after 1 week; (c) after 2 weeks. In the first week all baits were removed by possums, which were not deterred by the carcase. Wallabies resumed feeding on baits during the second week.

Fig. 6.

The effect of brushtail possum carcase placement on the average number of weekly visits of wallabies with bait stations. (Vertical lines show means ± s.e.m.; n = 20 sites before placement and for Week 1 of placement; n = 8 for Week 2 of placement. **, significant difference (P < 0.01) compared to mean weekly visits when no carcase was present; NS, no significant difference (P ≥ 0.05).

This deterrent effect was relatively short-lived. The carcase placement trial was undertaken during warm summer weather and by the second week of placement all carcases had either decomposed (Fig. 5c) or had been dragged away by feral pigs or feral cats. The wallaby visitation rate during the second week was only 38% lower than the pre-carcase average, which was no longer a statistically significant reduction (Fig. 6; P = 0.52).

Discussion

Our field study, which was primarily aimed at developing more effective designs of bait feeder for wallaby pest control, also provided an opportunity to investigate whether possums’ behavioural interference reduces the efficacy of bait feeders targeting wallabies. Here we present our findings on this latter objective. We acknowledge that had our focus only been on wallaby–possum interactions, we would have adopted a more straightforward study design than is reported here.

Two weeks of pre-trial possum control were undertaken at Titoki in an attempt to reduce possum interference; nevertheless, thousands of possum visits were recorded both at Titoki and at Blue Cliffs. Similarly, in 2023, pulses of toxic bait were deployed in the feeders with the expectation that this might reduce possum interference as well as wallaby numbers – but again we recorded many possum visits to the Titoki feeders. In contrast to the possums’ unimodal activity pattern, both wallaby species exhibited bimodal activity with peaks near dawn and dusk (Fig. 2a, b). Nevertheless, the times of day during which most wallaby and possum feeder visits occurred overlapped extensively, although wallaby activity declined as possums became more active and then rebounded once possums stopped visiting the feeders. The extent to which the wallabies’ bimodal activity pattern might imply proximate avoidance of possums at our sites – rather than simply an intrinsic diurnal pattern of the species – is unclear, as dama wallabies and particularly Bennett’s wallabies are described in some studies as being crepuscular (e.g. Havlin et al. 2018; Dickman and Calver 2023; Latham et al. 2023). However, we note that on Kawau Island (off the north-eastern coast of the North Island), where possum numbers were low, dama wallaby exhibited a unimodal activity pattern that peaked around midnight, with no indication of bimodality (Lentle et al. 2005). Relative to dama wallaby, a much higher proportion of Bennett’s wallaby activity was during daylight hours, during which time they were able to forage without encountering possums.

We observed that possums were consistently able to dominate both wallaby species, despite the Bennett’s wallabies’ size advantage. Based on studies reviewed by Cowan and Glen (2021), the body weight of adult possums at our sites probably averaged c. 2.6 kg. Adult wallabies at both our sites were significantly larger than this. The weight of female and male dama wallabies on Rotorua farmland averages 4.3 kg and 5.6 kg, respectively, with some males exceeding 10 kg (Williams 1986, in Latham and Warburton 2021). The weight of female and male Bennett’s wallabies in South Canterbury averages 11 kg and 13 kg, respectively, with some males exceeding 20 kg (Wodziki and Flux 1967, in Latham and Warburton 2021). Despite their size advantage, we recorded no instances of either wallaby species exhibiting behavioural dominance over possums. This finding is consistent with a previous unpublished report from New Zealand (Williams 1997), and with a report from Tasmania (Statham et al. 2010), that brushtail possums were aggressive to wallabies. We are not aware of any reason having been proposed for why this happens.

Both wallaby species approached and used bait feeders cautiously, maintaining high levels of vigilance throughout their visits. There was evidence to suggest that Bennett’s wallabies were particularly wary of possums. First, many more encounters were recorded between dama wallabies and possums than between Bennett’s wallabies and possums (229 and 22, respectively), although this is explained in part by possums visiting feeders at a higher rate at Titoki than at Blue Cliffs, and also by the greater daytime activity of Bennett’s wallabies. Second, when possums approached, Bennett’s wallabies always (7 of 7 encounters) moved away from the feeder before the possum exhibited aggression, whereas half of the dama wallabies (51 of 102 encounters) did not leave until the possum exhibited aggression. Third, we recorded 50 encounters where a dama wallaby continued feeding on bait at a feeder site while a possum was present; this outcome was never seen for Bennett’s wallabies.

When a possum was already feeding at a station, both wallaby species often waited at a distance and did not attempt to use the feeder until the possum left of its own accord. Given the limited field of view of the cameras, this ‘waiting’ response will have been underestimated for both species; we speculate that this helps to explain why so few encounters between wallabies and possums were recorded. Statham et al. (2010) and Williams (1997) both suggested that the aggressive behaviour of possums towards wallabies that they observed appeared to be reducing wallabies’ use of bait feeders.

Bennett’s wallabies were also clearly averse to the presence of possum carcases, at least while the carcases were relatively fresh. We did not have an opportunity to conduct an equivalent carcase trial at Titoki, so whether dama wallabies would respond in a similar way requires further study.