Possum Control Research Articles

Encapsulated sodium nitrite as a new toxicant for possum control in New Zealand

Encapsulated sodium nitrite as a new toxicant for possum control in New Zealand

Landscape-scale analysis of nutritional traits of New Zealand tree foliage using near-infrared spectroscopy

Near-infrared reflectance spectroscopy (NIRS) was used to predict the nutritional quality of tree foliage from a New Zealand forest inhabited by the introduced common brushtail possum (Trichosurus vulpecula) as part of a study to understand the relationship between possum browsing and the nutritional quality of foliage. The traits of interest were the foliar concentrations of total nitrogen, in vitro dry matter digestibility, and available nitrogen (with and without incubation with polyethylene glycol to indicate the effect of tannins). A total of 1399 samples from five plant species were studied, with 650 included in a calibration set and 50 in a separate validation set. We evaluated the performance of classic (GLOBAL) and alternative (LOCAL) calibration techniques. Both were successful, although a GLOBAL equation incorporating all calibration samples was used for the final predictions. Modelling of the spectra enabled the nutritional quality of the foliage samples to be estimated with coefficients of determination (r2) of 0.83–0.99 using modified partial least-squares regression. Nutritional quality varied both within and between five tree species for which possums show varied preferences. The predictive models developed in this study facilitate landscape-scale ecological investigations into the role of plant nutrients on the behaviour and demography of introduced herbivores. This provides new opportunities to test theories on invasiveness and to prioritize landscapes for conservation and possum control.

Place invaders: Identity, place attachment and possum control in the South Island West Coast of New Zealand

The pesticide 1080 (sodium fluoroacetate) is used in New Zealand as the primary means of controlling the Australian brushtail possum, a problem pest. This study identified place and its role in identity as an important mediator of the perspectives of those that either strongly supported or opposed its use. Individuals' differing constructions of place influenced the way they perceived ‘others’ as a threat to their place and their views on control of its management and protection. This study may contribute to a new understanding of the importance of place in this and other similar natural resource disputes.

Governance of national parks at the crossroads: New Zealand's silent reform

New Zealand’s national parks are major attractions for tourism and recreation, while hosting other commercial activities considered compatible with that primary role, like grazing, commercial filming and renewable electricity production. Commercial activities can only be carried out according to the terms of legal documents referred to as ‘concessions’ (typically, permits, licences and leases). There are currently 14 national parks, all managed by the Department of Conservation (DOC). Most of the country’s native birds, reptiles, frogs, bats and plants are unique in the world, but highly vulnerable to introduced predators and human activities. DOC has argued frequently that its conservation activities are ‘heavily weighted towards the trapping and poisoning of … introduced animals’. However, ‘less than 25% of conservation land receives interventions on key threats, with around 8% receiving possum, rat and stoat control’.

Quantifying short-term foraging movements in a marsupial pest to improve targeted lethal control and disease surveillance.

In New Zealand, the introduced marsupial brushtail possum (Trichosurus vulpecula) is a pest species subject to control measures, primarily to limit its ability to transmit bovine tuberculosis (TB) to livestock and for conservation protection. To better define parameters for targeted possum control and TB surveillance, we here applied a novel approach to analyzing GPS data obtained from 44 possums fitted with radio-tracking collars, producing estimates of the animals’ short-term nocturnal foraging patterns based on 1-, 3- or 5-nights’ contiguous data. Studies were conducted within two semi-arid montane regions of New Zealand’s South Island High Country: these regions support low-density possum populations (<2 possums/ha) in which the animals’ home ranges are on average larger than in high-density populations in forested habitat. Possum foraging range width (FRW) estimates increased with increasing monitoring periods, from 150-200m based on a single night’s movement data to 300-400m based on 5 nights’ data. The largest average FRW estimates were recorded in winter and spring, and the smallest in summer. The results suggest that traps or poison-bait stations (for lethal control) or monitoring devices (for TB surveillance), set for > 3 consecutive nights at 150m interval spacings, would likely place >95% of the possums in this type of habitat at risk of encountering these devices, year-round. Modelling control efficacy against operational expenditure, based on these estimations, identified the relative cost-effectiveness of various strategies that could be applied to a typical aerial poisoning operation, to reduce the ongoing TB vectorial risk that possums pose in the High Country regions. These habitat-specific findings are likely to be more relevant than the conventional pest control and monitoring methodologies developed for possums in their more typical forested habitat.

Epidemiology and control of Mycobacterium bovis infection in brushtail possums (Trichosurus vulpecula), the primary wildlife host of bovine tuberculosis in New Zealand

The introduced Australian brushtail possum (Trichosurus vulpecula) is a maintenance host for bovine tuberculosis (TB) in New Zealand and plays a central role in the TB problem in this country. The TB-possum problem emerged in the late 1960s, and intensive lethal control of possums is now used to reduce densities to low levels over 8 million ha of the country. This review summarises what is currently known about the pathogenesis and epidemiology of TB in possums, and how the disease responds to possum control. TB in possums is a highly lethal disease, with most possums likely to die within 6 months of becoming infected. The mechanisms of transmission between possums remain unclear, but appear to require some form of close contact or proximity. At large geographic scales, TB prevalence in possum populations is usually low (1–5%), but local prevalence can sometimes reach 60%. Intensive, systematic and uniform population control has been highly effective in breaking the TB cycle in possum populations, and where that control has been sustained for many years the prevalence of TB is now zero or near zero. Although some uncertainties remain, local eradication of TB from possums appears to be straightforward, given that TB managers now have the ability to reduce possum numbers to near zero levels and to maintain them at those levels for extended periods where required. We conclude that, although far from complete, the current understanding of TB-possum epidemiology, and the current management strategies and tactics, are sufficient to achieve local, regional, and even national disease eradication from possums in New Zealand.

Bird-repellent effects on bait efficacy for control of invasive mammal pests.

Repellents to reduce crop damage from birds and mammals have been investigated extensively, but their efficacy in reducing risk to non-target birds in aerial poisoning operations for control of mammal pests is less known. We assessed the impact on bait acceptability, palatability and kill efficacy for captive wild rats (Rattus rattus L.) and possums (Trichosurus vulpecula Kerr) of adding bird repellents (anthraquinone and d-pulegone) to baits used for their control in food choice trials. For possums, anthraquinone at 0.25% reduced acceptability and palatability but not the efficacy of poison baits, whereas d-pulegone at 0.17% had no significant effects. Rats showed little response to d-pulegone, but developed a marked aversion to prefeed baits containing anthraquinone at both 0.1 and 0.25%, such that almost no exposed rats ate poison baits and mortality was reduced significantly. The aversion induced by anthraquinone was generalised to the bait, as anthraquinone-exposed rats did not eat bait with only d-pulegone. Anthraquinone is not suitable for inclusion in bait for rat control at the concentrations tested, and also presents some risk to efficacy for possum control. D-pulegone would be suitable for inclusion in bait for possums and rats, but problems related to its volatility in bait manufacture and storage would need to be overcome. Further studies should focus on an alternative secondary repellent, or on establishing the maximum anthraquinone concentration that does not reduce efficacy for rats and testing whether or not that concentration is sufficient to repel native birds from baits reliably.

Assessing the Effectiveness of Tuberculosis Management in Brushtail Possums (Trichosurus vulpecula), through Indirect Surveillance of Mycobacterium bovis Infection Using Released Sentinel Pigs.

In New Zealand, wild pigs acquire Mycobacterium bovis infection by scavenging tuberculous carrion, primarily carcasses of the main disease maintenance host, the brushtail possum (Trichosurus vulpecula). We investigated the utility of captive-reared, purpose-released pigs as sentinels for tuberculosis (TB) following lethal possum control and subsequent population recovery. Within 2-3 years of possum control by intensive poisoning, TB prevalence and the incidence rate of M. bovis infection in released sentinel pigs were lower than in an adjacent area where possums had not been poisoned. Unexpectedly, TB did not decline to near zero levels among pigs in the poisoned area, a fact which reflected an unanticipated rapid increase in the apparent abundance of possums. Monitoring infection levels among resident wild pigs confirmed that TB prevalence, while reduced due to possum control, persisted in the poisoned area at >20% among pigs born 2-3 years after poisoning, while remaining >60% among resident wild pigs in the nonpoisoned area. When fitted with radio-tracking devices, purpose-released pigs provided precise spatial TB surveillance information and facilitated effective killing of wild pigs when employed as “Judas” animals to help locate residents. Sentinel pigs offer value for monitoring disease trends in New Zealand, as TB levels in possums decline nationally due to large-scale possum control.

Goodnature Automatic Traps for Vertebrate Pest Control: Field Trials Using New Kill Traps Targeting Animal Pests in New Zealand

Author(s): Peters, Darren H.; Schumacher, Karen; Schumacher, Robert J.; Baigent, Dan W. | Abstract: Goodnature Ltd. humane self-resetting kill traps are being developed in New Zealand for use in animal pest control to protect nature conservation values. Two models are commercially available: the Goodnature Ltd. A12™ trap for common brushtail possums and the Goodnature Ltd. A24™ trap for rats and stoats. These species are significant animal pests in New Zealand, contributing to the decline of biodiversity and extinction of native species. Large amounts of conservation effort and resources are targeted at their control and elimination. Current best-practice trapping techniques include the use of single-action possum and rat traps. The A12 and A24 traps have the advantage over the existing single-action traps in that they self-reset. They are powered by compressed carbon dioxide and humanely kill individual animals by striking their skulls, producing instant irreversible unconsciousness. Dead animals clear the trap by falling away; the trap automatically resets, up to 12 times for the A12 and up to 24 times for the A24. This study presents the results of three preliminary field trials that evaluate the effectiveness of these traps as tools in the control of possums and rats. Using 0.75 A12 traps per ha in a 256-ha block at Pouiatoa Conservation Area, we achieved a Residual Trap Catch Index (RTCI) for possums of 5.7%, down from 15%, after 15 months. This RTCI was comparable with the level of control previously achieved using hand-laid cyanide baits, and close to the desired conservation target of 5%. At Ohane, we deployed 290 A12 traps over 476 ha (0.6 traps per ha) and achieved a reduction in RTCI from 22.6% to 6.8%. Field trials for the A24 trap were conducted in a 46-ha block at Rimutaka Forest Park with 1.72 traps per ha. The minimum number of rats caught per trap night declined sharply from 0.15 from the first trapping period to 0.01 after the second trapping period, and remained below 0.03 for the entire 4-month trap trial. The New Zealand Department of Conservation is presently conducting several large-scale operational field trials for both traps.

The Effect of Density-Dependence on Foraging Dominance between Two Pest Species

Author(s): Ross, James G.; Sam, Shona | Abstract: 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 effect of aerial application of 1080 cereal baits on radio-tagged South Island fernbirds (Bowdleria punctata punctata)

In New Zealand, the toxin sodium fluoroacetate (compound 1080) is used to reduce numbers of introduced brushtail possums (Trichosurus vulpecula) to enhance forest condition and reduce bovine tuberculosis (Tb) infection in livestock. However, there is concern that the use of toxins may cause mortality in non-target species. We investigated the effect of possum control using aerially applied cereal baits containing 1080 on 36 radio-tagged South Island fernbirds (Bowdleria punctata punctata) in winter 2010. During monitoring, five fernbirds dropped their transmitters, one was killed by a predator and three died of 1080 poisoning (a mortality rate from 1080 poisoning of 9.4%, 95% CI = 2.4–22.6%). This study suggests that acute impacts of aerial 1080 operations on fernbird numbers are small and the observed impact could be outweighed by improved productivity and survival resulting from the reduction of impacts caused by introduced mammalian predators that die from 1080 poisoning.

The effect of colour on bait consumption of kea (Nestor notabilis): implications for deterring birds from toxic baits

The kea (Nestor notabilis), an endemic parrot, is at risk of primary poisoning during 1080 possum control operations. This project aimed at providing information on the effect of colour on bait attractiveness for kea to ultimately increase the safety of toxic bait. A feeding trial with captive kea investigated their colour preferences by offering cake dyed in six different colours simultaneously over six consecutive days. On average, the order of preference was yellow>red>brown>mid-blue>dark-blue>green; all birds encountered and consistently consumed less green bait than all the other colours throughout the trial. These results suggest that green is the most promising colour to deter captive kea from feeding in a multi-choice situation. However, due to kea's opportunistic feeding behaviour and inquisitive nature this might not be the case if green is the only option, suggesting that a stronger deterrent than colour may be necessary to deter free-ranging birds from consuming toxic bait.

Spatial prediction of brushtail possum (Trichosurus vulpecula) distribution using a combination of remotely sensed and field-observed environmental data

Context In New Zealand, the introduced brushtail possum, Trichosurus vulpecula, is a reservoir of bovine tuberculosis and as such poses a major threat to the livestock industry. Aerial 1080 poisoning is an important tool for possum control but is expensive, creating an ongoing need for ever more cost-effective ways of using this technique. Aims To develop geographic information system (GIS) models to better predict spatial variation in the distribution of unmanaged possum populations, to facilitate better targeting of control activities. Methods Relative abundance of possums and their distribution among habitat types were surveyed in a dry high-country area of the northern South Island. Two GIS-based models were developed to predict the relative abundance of possums on trap lines. The first model used remotely sensed (digital) environmental data; the second complemented the remotely sensed data with fine-scale habitat and topographic data collected on the ground. Key results Digital environmental factors and habitat features proved to be key predictors of relative possum abundance. In both GIS models, height above valley floor, presence of forest cover and mean annual temperature were the strongest predictors. Conclusions Predictive maps (projections) of relative possum abundance produced from these models can provide useful decision-support tools for pest-control managers, by enabling possum control to be targeted spatially. Implications Spatially targeted pest control could allow effective control activities for invasive species or disease vectors to be applied at a lower cost for the same benefit.

Dietary changes in response to population reduction in the possum Trichosurus vulpecula in New Zealand

Context Efforts to protect or restore degraded plant communities by population control of invasive herbivores frequently fail to achieve their goals. Aims We seek to quantify changes in diet of an introduced herbivore, the brushtail possum (Trichosurus vulpecula), following population control, and determine how these may contribute to variable responses in plant condition. Methods Stomach contents of possums from five areas of indigenous forest in northern New Zealand were analysed to measure diet before and after the application of possum control. Key results The contribution of fruit, and foliage of some early successional forest species, to total possum diet increased up to 27-fold following possum population control. This was accompanied by declines in consumption of the main pre-control possum foods (foliage from common canopy trees). Dietary changes were a combination of an immediate response to control (1 year) and a strengthening of these initial changes with increasing time since control. Conclusions Possums in the study areas changed diet following population control, from a diet dominated by foliage of common canopy tree species to one dominated by fruits, and foliage of uncommon early successional plants. Pest control instantaneously increased the per capita availability of all foods, and probably permitted absolute increases in some foods through plant recovery, enabling possums to substitute scarce, high-preference foods for abundant but less preferred canopy foliage. Implications Following control of a pest herbivore, dietary changes reduce benefits for the most vulnerable preferred plant foods, but enhance benefits for less favoured plants. Intense pest control can permit some recovery of highly preferred foods, despite increased per capita consumption of these foods by survivors of control.

When deer must die: large uncertainty surrounds changes in deer abundance achieved by helicopter- and ground-based hunting in New Zealand forests

Context When environmental, economic and/or social effects of wildlife are considered undesirable and need to be reduced, managers require knowledge of the effectiveness of candidate control techniques, particularly the relationship between control effort and change in abundance. Aims We evaluated the effects of control on the abundances of introduced red deer (Cervus elaphus scoticus) and sika deer (Cervus nippon) at three New Zealand forest sites (two North Island, one South Island) in an 8-year adaptive-management experiment. Methods We identified paired areas of 3600 ha at each site that were as similar as possible in geology, physical environments and forest composition and applied deer control (helicopter- and/or ground-based hunting) to a randomly selected member of each pair. The abundances of deer were monitored in each treatment and non-treatment area for up to 7 years by using faecal pellet counts on 50 randomly located transects. Key results The difference between deer abundances in the treatment and non-treatment areas was significantly negative at one site, significantly positive at one site and indistinguishable at the other site. Faecal pellet abundances declined with increasing helicopter-based hunting effort but did not change with increasing ground-based hunting effort. There was evidence that aerially sown 1080 baits used for possum control in two treatment areas reduced deer abundances. Conclusions The substantial uncertainty surrounding the relationships between deer control effort and changes in deer abundance means that managers cannot assume that the environmental, economic and/or social problems caused by deer will be alleviated with the quantum of control effort applied in the present study. Implications Reducing the abundances of deer in forests may require substantially more control effort than is currently believed.

Importance and mitigation of the risk of spillback transmission of Mycobacterium bovis infection for eradication of bovine tuberculosis from wildlife in New Zealand

Introduced brushtail possums (Trichosurus vulpecula) are wildlife maintenance hosts for Mycobacterium bovis in New Zealand, often living sympatrically with other potential hosts, including wild red deer (Cervus elaphus scoticus). Population control of possums has been predicted to eradicate tuberculosis (TB) from New Zealand wildlife ; however, there is concern that long-lived M. bovis-infected deer could represent a ‘ spillback’ risk for TB re-establishment (particularly when possum populations recover after cessation of intensive control). We constructed a time-, age- and sex-structured, deer/TB population generic model and simulated the outcomes of deer control on this potential spillback risk. Maintaining intensive possum control on a 5-year cycle, the predicted spillback risk period after TB eradication from possums is ~7 years, while the probability of TB re-establishing in possums over that period is ~6%. Additional targeted control of deer would reduce the risk period and probability of spillback; however, even with high population reductions (up to 80%) only modest decreases in risk and risk period would be achieved. We conclude that possum control alone remains the best strategy for achieving TB eradication from New Zealand habitats in which possums and wild deer are the main M. bovis hosts.

Impacts of an invasive herbivore on indigenous forests

Summary Invasive herbivores can have large negative impacts on natural ecosystems. Management of invasive populations often requires frequent, broadscale, expensive control, which must be justified by demonstrating progress in achieving conservation objectives. This study evaluates benefits of regular extensive control of an invasive herbivore and develops an alternative strategy based on damage thresholds. We carried out replicated experimental management of brushtail possums, Trichosurus vulpecula, in three areas of native forest in New Zealand. Each area included a site that had extensive possum control for 10 years, prior to and during the 5‐year study, and a paired site with no control. We measured indices of possum browse on c.2400 possum ‘preferred’ and c.1200 ‘non‐preferred’ trees, and an index of possum abundance, at the beginning and end of the experiment. Extensive control was effective in reducing possum browse on preferred tree species. Reductions in browse led to increased foliage cover and decreased probability of tree mortality. The probability of browse on an individual tree decreased with increasing amounts of possum‐preferred foliage on nearby trees but increased with tree size and with increasing levels of browse on nearby trees. At one site where possum control ceased prematurely, foliage cover decreased, reducing benefits from earlier control. Synthesis and applications. Our study provides evidence that sustained, extensive control of invasive herbivores can result in significant conservation benefits to susceptible tree species, and that both impacts and benefits can be measured using data typically collected in herbivore impact studies. Furthermore, it shows how local factors such as forest composition can influence the impact of herbivory, how this can be included in large‐scale assessments of the benefits of pest control and how site‐ and species‐specific damage thresholds can be derived for improving pest management.

Low-dose cholecalciferol bait for possum and rodent control

A new low-concentration formulation of Feracol® paste containing 0.4% concentration cholecalciferol was tested for efficacy against possums in cage trials, and against possums and rodents in a single unreplicated field trial. A new low-concentration 0.4% cholecalciferol cereal pellet was also tested in the field against possums and rodents. 20 wild-caught possums, held in individual cages, were presented with 15 g of 0.4% cholecalciferol paste. Possum body weight, amount of paste eaten and days until death were recorded. The field trial occurred on Banks Peninsula, Canterbury, from September to November 2009, using a treatment site of c. 100 ha and a non-treatment site of c. 200 ha. The treatment site was divided into two even blocks, one for paste and one for pellets. Pre- and post-monitoring were carried out using leg-hold traps for possums, and tracking tunnels for rodents. Of the 20 caged possums, a kill rate of 89.5% with an average time to death of 7.0 days±0.41 SEM was achieved, similar to earlier work on cholecalciferol using a higher concentration. In the field trial pellet treatment block, possum numbers were reduced by 94±11%, mice numbers by 100±0% and rat numbers by 89±12%. In the paste treatment block, possum numbers were reduced by 93±16%, mice numbers by 75±29% and rat numbers by 100±0%. Under the current study conditions, the effectiveness of the new Feracol® paste and pellets containing 0.4% concentration cholecalciferol was comparable to the currently registered 0.8% concentration cholecalciferol Feracol® paste formula, and was effective in controlling multiple pest species while reducing the poisoning risk to non-target species. Replication of this field trial using the new bait matrices is necessary to increase confidence under similar and differing conditions, to ensure that the efficacy seen here is not arbitrary.

Serving two masters: Reconciling economic and biodiversity outcomes of brushtail possum (Trichosurus vulpecula) fur harvest in an indigenous New Zealand forest

Introduced brushtail possums (Trichosurus vulpecula) cause significant damage to biodiversity values in New Zealand native forests, but their fur and skins also represent an economic resource, particularly to small, predominantly rural Māori (indigenous people of New Zealand) communities. We used harvester interviews, trapping data and a combination of economic and spatial population models to ask whether the seemingly contradictory goals of economically-sustainable harvest and biodiversity protection can both be accommodated in native podocarp-dominated forests in the North Island of New Zealand. Harvesters expected trap-catch indices (TCIs) of 50–70% when initiating a trap-line, stopped trapping when pull-out trap-catch index (POTCI) was 26% (range 20–30%), and left trapped areas for at least 1year before returning. The effective trapping distance around a trap-line was 200m and the decline in possum captures on a line over 24 harvest-days declined exponentially from an initial 63% on day 2 to under 8% from day 21 on.The optimum strategy for sustaining an income of NZ$30,000 from harvest required access to 8–10 lines of 80 traps per year, ceasing trapping at 25% POTCI and leaving possums to recover for 3years, but this is unlikely to achieve conservation gains. Harvesters could trap to the levels required to achieve these outcomes if subsidized by management agencies that, in return, would benefit from possum control to the same degree, but at lower cost, than current standard ground-control methods.

Minimising number killed in long-term vertebrate pest management programmes, and associated economic incentives

AbstractManagement of invasive vertebrate species often requires the use of lethal control tools such as toxins, traps, or shooting. However, because these pest species are sentient and have the capacity to suffer, the application of such tools raises concerns about welfare impacts. To address such concerns, research, policy and regulation have focused most often on the welfare impacts (humaneness) of the tools at the individual animal level (ie the ‘quality’ of the impact) with no attempt to assess welfare at the population level (ie the ‘quantity’ of the impact). Because control programmes often target large numbers of animals, we suggest that when the welfare costs of pest control operations and strategies are being evaluated, the numbers of individuals involved should be considered in addition to the intensity and duration of individual suffering. We explore this concept using a modelling framework and three New Zealand case studies (brushtail possums [Trichosurus vulpecula], ship rats[Rattus rattus], and Bennett's wallabies[Macropus rufogriseus])to assess the extent to which typical control strategies used by land managers influence the numbers of animals killed. We test whether a predicted relationship between numbers killed and position on the population growth curve holds across these scenarios, and identify whether it would be economically viable for end-users to adopt more welfare-friendly control strategies (ie those that kill fewer individuals to achieve the required management outcomes) for these pest species, or whether some form of incentive would be required. Computer modelling showed that for four simulated brushtail possum control strategies, the number of animals killed on a 1,000-ha area over 30 years ranged from approximately 13,000 to 26,000. Similarly, for two ship rat control strategies, numbers killed over a 20-year period were 977 for an aerial strategy versus 1,517 for a ground-based strategy. For both species, the strategies that killed fewest animals generally also cost the least. For Bennett's wallabies, because farmers only carry out control for production benefits, the control strategy they are most likely to select would result in the highest number of wallabies killed. To reduce the number of wallabies killed while allowing farmers to achieve some production benefits, farmers would need to receive some additional financial benefit. The concept of welfare incentive then raises questions such as ‘what willingness is there to pay for increased welfare’ and ‘to what extent can reducing control costs substitute for incentive payments in reducing numbers killed?’