Possum Control Research Articles

The admixed brushtail possum genome reveals invasion history in New Zealand and novel imprinted genes

Combining genome assembly with population and functional genomics can provide valuable insights to development and evolution, as well as tools for species management. Here, we present a chromosome-level genome assembly of the common brushtail possum (Trichosurus vulpecula), a model marsupial threatened in parts of their native range in Australia, but also a major introduced pest in New Zealand. Functional genomics reveals post-natal activation of chemosensory and metabolic genes, reflecting unique adaptations to altricial birth and delayed weaning, a hallmark of marsupial development. Nuclear and mitochondrial analyses trace New Zealand possums to distinct Australian subspecies, which have subsequently hybridised. This admixture allowed phasing of parental alleles genome-wide, ultimately revealing at least four genes with imprinted, parent-specific expression not yet detected in other species (MLH1, EPM2AIP1, UBP1 and GPX7). We find that reprogramming of possum germline imprints, and the wider epigenome, is similar to eutherian mammals except onset occurs after birth. Together, this work is useful for genetic-based control and conservation of possums, and contributes to understanding of the evolution of novel mammalian epigenetic traits.

Predicting People’s Motivation to Engage in Urban Possum Control

Initiatives such as education, incentives, and regulations are used to change people’s behaviour and thereby achieve policy objectives. Understanding and predicting the willingness of people to change their behaviour in response to an initiative is critical in assessing its likely effectiveness. We present a framework proposed by Kaine et al. (2010) for understanding and predicting the motivation of people to change their behaviour in response to a policy initiative. The framework draws on the marketing concept of ‘involvement’, a measure of motivation. Through application to a predator control case study, we show how the framework may be used to predict people’s responses to a policy initiative and how these predictions might help agencies develop strategies to promote behaviour change.

The effects of beech masts and 1080 pest control on South Island robins (Petroica australis)

We assessed the effect of aerial 1080 control of possums (Trichosurus vulpecula), ship rats (Rattus rattus) and stoats (Mustela erminea) on the survival and nest success of South Island robins (Petroica australis) at Tennyson Inlet, Marlborough Sounds, from 2012–2017. Cereal baits containing 1080 were applied in 2013 when rat and stoat numbers were low, and again in 2014 after a beech mast when rat numbers were high. Survival rates of 134 banded adult South Island (SI) robins and 209 SI robin nests were monitored.

Increasing the capture rates of brushtail possums in Victor #1 leg-hold traps

ABSTRACT Leg-hold or foot-hold traps have been used in New Zealand for trapping brushtail possums (Trichosurus vulpecula) since the 1920s. In the mid-1990s, when the National Pest Control Agencies (NPCA) developed a national protocol for monitoring possums, they standardised how traps should be set, which lure to use and where to place it. These recommendations aimed to achieve standardisation (important for population monitoring), not necessarily high capture rates. Recent use of trail cameras has revealed possums eating the lure, but then walking away without stepping on the trap, suggesting that modifications to the way traps are set or lured might enable increased capture rates of possums. We compared the capture rate of four alternative trap sets (i.e. double sets, covered traps, hazed traps, enlarged trigger plates) with standard NPCA-recommended sets. None of these modified sets increased the captures rates. We then compared the capture rate of traps with the lure placed on the ground encircling the trap (including a vertical visual lure) against the NPCA-recommended method of placing the lure behind the trap. This modification resulted in a 33% increase in capture rate suggesting modifications to the luring method can increase capture rates and make ground-based possum control more effective.

Evaluating the effects of landscape structure on the recovery of an invasive vertebrate after population control

Effective landscape control of invasive species is context-dependent due to the interplay between the landscape structure, local population dynamics, and metapopulation processes. We use a modelling approach incorporating these three elements to explore the drivers of recovery of populations of invasive species after control. We aim to improve our understanding of the factors influencing the landscape-level control of invasive species. We focus on the case study of invasive brushtail possum (Trichosurus vulpecula) control in New Zealand. We assess how 13 covariates describing the landscape, patch, and population features influence the time of population recovery to a management density threshold of two possums/ha. We demonstrate the effects of those covariates on population recovery under three scenarios of population growth: logistic growth, strong Allee effects, and weak Allee effects. Recovery times were rapid regardless of the simulated population dynamics (average recovery time < 2 years), although populations experiencing Allee effects took longer to recover than those growing logistically. Our results indicate that habitat availability and patch area play a key role in reducing times to recovery after control, and this relationship is consistent across the three simulated scenarios. The control of invasive possum populations in patchy landscapes would benefit from a patch-level management approach (considering each patch as an independent management unit), whereas simple landscapes would be better controlled by taking a landscape-level view (the landscape as the management unit). Future research should test the predictions of our models with empirical data to refine control operations.

Kea survival during aerial poisoning for rat and possum control

Kea survival during aerial poisoning for rat and possum control

Control history, longitude and multiple abiotic and biotic variables predict the abundances of invasive brushtail possums in New Zealand forests

The factors affecting variation in the abundances of invasive mammal species have seldom been evaluated at large spatial scales. We used data collected in a large-scale biodiversity monitoring program to identify variables predicting abundances of common brushtail possums (Trichosurus vulpecula; ‘possum’), a marsupial folivore introduced to New Zealand in the 1800s. We sampled possum abundances, vegetation structure and composition, and abiotic variables at 395 spatially representative sites in natural forest on the North, South and Stewart islands. Records of possum control were obtained from management agencies. The large sample size enabled us to consider the relative importance of 27 variables for possum abundances. Twelve variables explained 80% of the contributions to the final boosted regression tree model. Control history had the largest contribution to the model, with sites subject to aerial 1080 poison baiting either once or more than once in the previous 7 years having possum abundances 64% and 74% lower, respectively, than sites without control. Possum abundances were also negatively related to humidity, stem density, tree cover density, slope, elevation and distance to pasture/crop. Abundances increased with increasing food plant species richness, Weinmannia racemosa (a preferred food tree) cover, and solar radiation. There was an interaction between longitude and control history. We conclude that invasive possum abundances in New Zealand’s natural forests are determined by multiple biotic and abiotic factors, sometimes in interaction. Recent control operations, however, have greatly reduced possum abundances. Increasing the spatial extent of aerial 1080 poison baiting will further reduce possum abundances in New Zealand forests.

Multi-scale dynamic maps for the management of invading and established wildlife populations: brushtail possums in New Zealand

Context The abundance and distribution of mammalian species often change in response to environmental variability, losses or gains in suitable habitat and, in the case of pest species, control programs. Consequently, conventional distribution maps rapidly become out of date and fail to provide useful information for wildlife managers. For invasive brushtail possum populations in New Zealand, the main causes of change are control programs by central and local government agencies, and post-control recovery through recolonisation and in situ recruitment. Managers need to know current, and likely future, possum population levels relative to control targets to help assess success at preventing the spread of disease or for protecting indigenous species. Information on the outcomes of government-funded possum control needs to be readily available to members of the general public interested in issues such as conservation, disease management and animal welfare. Aims To produce dynamic, scalable maps of the current and predicted future distribution and abundance of possums in New Zealand, taking into account changes due to control, and to use recent visualisation technology to make this information accessible to managers and the general public for assessing control strategies at multiple spatial scales. Methods We updated an existing individual-based spatial model of possum population dynamics, extending it to represent all individuals in a national population of up to 40 million. In addition, we created a prototype interface for interactive web-based presentation of the model’s predictions. Key results The improved capability of the new model for assessing possum management at local-to-national scales provided for real-time, mapped updates and forecasts of the distribution and abundance of possums in New Zealand. The versatility of this platform was illustrated using scenarios for current and emerging issues in New Zealand. These are hypothetical incursions of possums, reinvasion of large areas cleared of possums, and impacts on animal welfare of national-scale management of possums as vectors of bovine tuberculosis (TB). Conclusions The new individual-based spatial model for possum populations in New Zealand demonstrated the utility of combining models of wildlife population dynamics with high-speed computing capability to provide up-to-date, easily accessible information on a species’ distribution and abundance. Applications include predictions for future changes in response to incursions, reinvasion and large-scale possum control. Similar models can be used for other species for which there are suitable demographic data, typically pest species, harvested species or species with a high conservation value. Implications Models such as the spatial model for possums in New Zealand can provide platforms for (1) real-time visualisation of wildlife distribution and abundance, (2) reporting and assessing progress towards achieving management goals at multiple scales, (3) use as a decision-support tool to scope potential changes in wildlife populations or simulate the outcomes of alternative management strategies, and (4) making information about pest control publicly available.

Secondary poisoning risk for encapsulated sodium nitrite, a new tool for possum control

Secondary poisoning risk for encapsulated sodium nitrite, a new tool for possum control

Analysing the multiplicity of voices in the agora: using actor-network theory to unravel a complex issue

ABSTRACTThere is a need to develop an understanding of how science knowledge is interpreted and used when a Socially Acute Question (SAQ) is discussed on the ‘agora’ of the Internet.A case is made for using Actor-Network Theory (ANT) to unravel the diversity of participants taking part, their stance, source and expression of their justifications. This interaction can be visually expressed as a Cartography of Controversy. The SAQ of possum (Trichosurus vulpecula) control using aerial dispersal of 1080 (sodium fluoroacetate) was the context in which the participants (actants), their performative actions and affiliations (nodes) were mapped. Actants were identified via a Google search for groups’ websites with an opinion about 1080. Twenty-three groups were identified and their webpages analysed in terms how they perceived the issue. The construction of a cartography from these data showed the complexity of the group interactions. An analysis of justifications illustrated the range of stabilising mechanisms used. This research demonstrates the educational potential of ANT to develop a pedagogy that enables students to explore an SAQ and appreciate the multiplicity of perspectives.

Primary poisoning risk for encapsulated sodium nitrite, a new tool for pest control

ABSTRACTAcute toxicity of sodium nitrite (NaNO2) was assessed in chickens (Gallus gallus domesticus) and domestic mallard ducks (Anas platyrhynchos domestica) by oral gavage and in free-feeding trials with chickens, domestic mallard ducks, pigeons (Columba livia f. domestica), budgerigars (Melopsittacus undulates) and wētā (Family: Rhaphidophoridae). Free-feeding trials involved the presentation of toxic paste and pellet baits containing encapsulated NaNO2 developed for the control of common brushtail possums (Trichosurus vulpecula) and feral pigs (Sus scrofa). The oral gavage LD50 value for NaNO2 in solution was approximately 68.50 mg/kg (95% CI 55.00–80.00 mg/kg) for both chickens and ducks. In feeding trials, six out of 12 chickens consumed toxic paste bait and four of these birds consumed a lethal dose. When chickens consumed toxic paste bait, the LD50 value was approximately 254.6 mg/kg (95% CI 249.1–260.2 mg/kg). Of the other three species of birds presented with toxic baits only one duck consumed a lethal dose of paste bait. There was no evidence of wētā feeding on toxic baits.

Valuing conservation benefits of disease control in wildlife: A choice experiment approach to bovine tuberculosis management in New Zealand's native forests

We assess the non-monetary environmental benefits that accrue incidentally in New Zealand (NZ) from pest management conducted primarily to control an animal disease, bovine tuberculosis (TB). TB is an infectious disease that is one of the world's most serious animal health problems and, in many parts of the developing world, still a major mortality risk for humans. The incidence of TB in New Zealand (NZ) farmed livestock has been reduced progressively over the last 20 years, largely due to extensive and sustained population control of the main wildlife reservoir of disease, the introduced brushtail possum. Possums are also major pests that threaten indigenous forest biodiversity, and so extensive possum control for TB mitigation also incidental benefits conservation, but the extent and public value of this benefit has yet to be quantified. We conducted a choice experiment survey of the NZ public in an effort to value the native forest biodiversity benefits of TB-related possum control. We find strong public support for conservation outcomes consequent to TB-possum control in public native forests. The public place substantial value on the most observable biodiversity benefits of TB possum control, such as improved forest canopies and presence of native birds. The benefits, costs and values of TB-possum control are discussed in relation to the future directives of NZ's TB control programme, which is headed toward first regional and then national level disease eradication.

Long‐term response of temperate canopy trees to removal of browsing from an invasive arboreal herbivore in New Zealand

AbstractDefoliation of forest tree canopies by herbivores and other agents, leading to tree mortality and reduced productivity, threatens the ecological stability of forests globally. This study shows that long‐term control of a mammalian arboreal folivore (brushtail possums; Trichosurus vulpecula Phalangeridae) reduces crown dieback and increases foliage cover in browsing‐damaged canopy trees. We monitored indices of possum density, possum browsing, tree foliage cover and crown dieback for 20 years following initiation of possum control in 1994 that repeatedly reduced possum densities to near zero every 5–6 years and kept the population below 35% of pre‐control levels over the entire period. Observable possum browsing was recorded on 20–49% of individuals of three palatable tree species at the time of first control. Those percentages fell to zero after control and never exceeded 2–10% for individual species over the next 19 years. We recorded significant increases in foliage cover attributable to recovery from defoliation by possums for all three species during the first 10 years. Large increases in foliage cover occurred on individuals that were heavily browsed in 1994 (mean increases: 36–89%), but mean population increases were modest (3–19%) because only 10–19% of trees were initially heavily browsed. Twenty‐year mortality rates were similar for plants with, or without, initial possum browsing, indicating no residual impact of pre‐control browsing on tree mortality. Times for full recovery of crown foliage cover varied from 10 years for the youngest trees and faster growing species to more than 20 years for mature individuals of the slowest growing species.

Characteristics and behaviour of brushtail possums initially moving into a depopulated area

ABSTRACTBrushtail possums are controlled extensively in New Zealand because they are a livestock disease vector and have an impact on native biodiversity. Reinvasion of controlled areas and subsequent population recovery is a significant management problem but little attention has been paid to what influences the settlement of possums in depopulated areas. To address this gap we trapped possums out of an area of about 24 ha in native podocarp–hardwood forest and studied reinvasion and settlement in the central c. 14 ha over 22 months. Most new possums were young males, but adults were also trapped. Many of the new possums caught on the study site post-depopulation did not settle there, most likely because they continued to disperse, but some may have returned to their ranges nearby or were residents with a very low probability of capture. This finding highlights the need for better information about the origins and settlement of possums in depopulated areas to improve management of population recovery and long-term sustained control of possums.

A modelling framework for predicting the optimal balance between control and surveillance effort in the local eradication of tuberculosis in New Zealand wildlife

Bovine tuberculosis (TB) impacts livestock farming in New Zealand, where the introduced marsupial brushtail possum (Trichosurus vulpecula) is the wildlife maintenance host for Mycobacterium bovis. New Zealand has implemented a campaign to control TB using a co-ordinated programme of livestock diagnostic testing and large-scale culling of possums, with the long-term aim of TB eradication. For management of the disease in wildlife, methods that can optimise the balance between control and surveillance effort will facilitate the objective of eradication on a fixed or limited budget. We modelled and compared management options to optimise the balance between the two activities necessary to achieve and verify eradication of TB from New Zealand wildlife: the number of lethal population control operations required to halt the M. bovis infection cycle in possums, and the subsequent surveillance effort needed to confidently declare TB freedom post-control. The approach considered the costs of control and surveillance, as well as the potential costs of re-control resulting from false declaration of TB freedom. The required years of surveillance decreased with increasing numbers of possum lethal control operations but the overall time to declare TB freedom depended on additional factors, such as the probability of freedom from disease after control and the probability of success of mop-up control, i.e. retroactive culling following detection of persistent disease in the residual possum population. The total expected cost was also dependent on a number of factors, many of which had wide cost ranges, suggesting that an optimal strategy is unlikely to be singular and fixed, but will likely vary for each different area being considered. Our approach provides a simple framework that considers the known and potential costs of possum control and TB surveillance, enabling managers to optimise the balance between these two activities to achieve and prove eradication of a wildlife disease, or the pest species that transmits it, in the most expedient and economic way. This cost- and risk-evaluation approach may be applicable to other wildlife disease problems where limited management funds exist.

A review of biodiversity outcomes from possum-focused pest control in New Zealand

Worldwide, introduced vertebrate pests impact primary production, native biodiversity, and human health. In New Zealand, extensive pest control (~10 million ha) is undertaken to protect native biota and to prevent losses to the primary sector from wildlife vectors of bovine tuberculosis (TB), primarily possums (Trichosurus vulpecula). Control is conducted by TBfree New Zealand and by conservation agencies. Remote, forested terrain is treated using the toxin 1080 via aerial delivery in bait with a return time of ~5 years. Ground-based control is conducted annually using traps and/or poison bait. Possums are controlled to very low abundance by these operations. Aerial 1080 is effective against another forest-dwelling vertebrate pest, the ship rat (Rattus rattus). Possum control has reduced TB rates, but collateral benefits for native biodiversity have not been quantified, making it difficult to demonstrate a return on investment. We review information from 47 accounts of responses of native biota to possum control. Of these, 60% quantified responses to aerial 1080; the remainder were ground-based. Possum control benefited vegetation by increasing foliage and fruit production, and by reducing tree mortality. Controlling ship rats and possums together improved bird populations, but rats recovered rapidly and long-term outcomes for rat-vulnerable birds are unknown.Large-bodied invertebrates also benefited from extensive pest control. We conducted a meta-analysis of 84 response measures from 35 of these 47 studies in order to provide a quantitative assessment of these findings. The analysis demonstrated that both ground and aerial control of this invasive pest in New Zealand has provided substantial collateral benefits for native biota. Few studies have taken advantage of decades of extensive pest control in New Zealand to monitor ecosystem-level outcomes, which have received only short-term attention thus far. Non-treatment experimental controls and replicate sites that enable validated assessments of outcomes for native biota are vital. Future studies would benefit from a standardised set of biodiversity indicators from a range of taxonomic and functional groupings, and from standardising experimental designs so individual studies can contribute to future meta-analyses, to strengthen the evidence base for the impacts of invasive pests on native biota in New Zealand and worldwide.

A Contractor Industry that Underpins a National-Scale Pest Control Programme

In New Zealand, bovine tuberculosis is a major problem for livestock industries (i.e., dairy, beef, and deer). The disease is managed by TBfree New Zealand (formerly the Animal Health Board) under a National Pest Management Plan with a fixed annual budget of about $53 million. The strategy used to reduce herd infection involves: 1) herd testing and slaughter to remove infection from herds, 2) herd movement controls to minimise the risk of TB being spread between farms, and 3) control of the key wildlife TB vector, the brushtail possum. The management of brushtail possums is the most costly of the three strategy components, and those costs have driven the development of a highly cost-effective possum control contracting industry. Development of an integrated TB management programme involving possum control contractors has required: 1) a defined control target/threshold density for possums that requires numbers be reduced to and maintained at or below for five-seven years for TB to be eliminated, 2) an industry-accepted method for independently assessing whether the control target has been achieved, and 3) training and certification of contractors to independently monitor the possom control contractors. The successful implementation of these three requirements has enabled the industry to develop a performance-based control contract system in which control contractors get paid only if they achieve the required trap-catch target (i.e., contractors take all the risk). PDA/GPS systems with related online databases have been developed to support the management and auditing of contractors, and to capture data about both spatial coverage and control of possums (about 30,000 point data collected per month). The effectiveness of this integrated vector-control programme has enabled more than 1.5 million hectares to be declared TB-free, and reduced infected herds from about 1,700 in the early 1990s to 35 by February 2016.

A systematic approach to evaluating and ranking the relative animal welfare impacts of wildlife control methods: poisons used for lethal control of brushtail possums (Trichosurus vulpecula) in New Zealand

Context Control of unwanted wildlife (‘pest’ animals) is undertaken for conservation and economic reasons, and when such animals are considered a nuisance. Such control should be undertaken using approaches that minimise, as far as possible, detrimental impacts on the welfare of the animals. Using a scientific framework based on the Five Domains model, the relative welfare impacts of pest control methods can be compared across methods and pest species. Aims We demonstrate the application of a modified version of this framework to evaluate the relative impacts of seven Vertebrate Toxic Agents (VTAs) used to control brushtail possums (Trichosurus vulpecula) in New Zealand. The evaluation is used to produce a ranking of the seven VTAs based on their relative impacts on possum welfare. Methods Scientific literature describing mode of toxic action, specific effects in possums or other animals and reports from human poisonings was collated as reference material for a panel of six experts. The panel produced a median welfare impact score (‘none’ to ‘extreme’) for each of the Five Domains. The ‘Overall Grade’ (1 to 8) reflected the intensity and duration of all impacts of a VTA on possums. Key results All VTAs evaluated have at least moderate impacts on possum welfare, lasting for at least minutes. Cyanide was assessed as having the lowest welfare impacts (median grade 4), and cholecalciferol and the anticoagulants the highest impacts (7.5 to 8). Zinc phosphide was assigned an intermediate grade (6) with high confidence. While the overall impacts of sodium fluoroacetate (1080) and phosphorus were also assessed as intermediate (6), the panel’s confidence in these scores was low. Conclusions From an animal welfare perspective, anticoagulant poisons and cholecalciferol should be the least preferred options for controlling possums in New Zealand, as VTAs with less severe welfare impacts are available. Implications The results of such assessments allow animal welfare impacts to be integrated with other factors in wildlife management decision-making and policy development, and are thus useful for managers, researchers, regulators and operators. Evaluation of welfare impacts aligns with the goals and mandates of ethical wildlife control and may also be valuable in wider wildlife research and management activities.

Repellents with potential to protect kea and other native birds from aerial poisoning for possum and rat control

Repellents with potential to protect kea and other native birds from aerial poisoning for possum and rat control

Technology Advances for Vertebrate Pest Eradication

Author(s): Eason, Charles; Shapiro, Lee; Ross, James; Murphy, Elaine; Ogilvie, Shaun; MacMorran, Duncan; Jackson, Michael; Irie, Kenji; Inder, Shane; Clout, Mick; Rennison, David; Brimble, Margaret | Abstract: Internationally, over the last 20 years the number of tools available for the control of small mammals has declined. Through the efforts of research we have bucked this trend and retained and developed new tools. Three new toxins have been extensively researched and registered, namely para-aminopropiophenone (PAPP) in 2011 for stoats and feral cats; zinc phosphide for possums in 2012; and encapsulated sodium nitrite (ESN) in 2013, for possums and feral pigs. The development of PAPP and ESN, coined red blood cell toxins, developed for humaneness, represent the first new vertebrate pesticides registered for field control of mammalian pests anywhere in the world for g30 years. Research on rodenticides including diphacinone with the additive cholecalciferol (D+C) and a palatable form of norbormide continues. A New Zealand EPA application for D+C was filed in June 2015. More effective killing systems are being researched, and the first successful field trials of resetting toxin delivery devices for possum and stoat control were completed in 2013 and 2014. Improved deployment strategies, integration of humane and selective toxins, lures of greater potency, and improved killing devices aided by species’ recognition will transform ground control for endangered species protection. Sodium fluoroacetate (1080) and other important tools have been retained as new tools are emerging from a research and development pipeline. It is important for the future of New Zealand’s biodiversity that this focused research continues and we continue to learn and advance new technologies. Our goals are shifting to enable reduction in density of rat, stoat, and possum populations to zero over large scales (i.e., elimination at landscape scale), and to hold these at zero through detection and response, including the use of new technologies for perimeter control as part of barrier systems for conservation.