Possum Control Research Articles

Integrating Ecology and Technology to Create Innovative Pest Control Devices

Author(s): Blackie, Helen; MacMorran, Duncan; Murphy, Elaine; Smith, Des; Eason, Charles | Abstract: To achieve long-term suppression of pest populations, devices capable of continued control over extended timeframes are needed. Creating new pest management tools to achieve this goal requires the integration of animal ecology, toxicology, and design engineering. This research outlines the development and testing of a long-life, resetting, toxin delivery system for vertebrate pest control, coupled with advances in novel species recognition systems. Such devices have the potential to offer advantages over current labor-intensive control techniques. Resetting systems have been developed to target several of the most destructive vertebrate pest species in New Zealand, including stoats and weasels. Results of enclosure trials for these two species showed similar responses after a paste containing 40% para-aminopropiophenone was delivered onto the chest and stomach, following triggering by a treadle operated system. Both species groomed the paste off shortly after application and death occurred after an average of 42 minutes for stoats and 57 minutes for weasels. The applications of these resetting devices are now being extended for the control of brushtail possums, another major vertebrate pest in New Zealand. Coupled with this, developments in species identification systems are ensuring that risks to non-targets are substantially minimized. Resetting, long-life toxin delivery systems could be deployed to control a variety of pest species, and further enhancement of these tools are ensuring their use for widespread field applications in a cost-effective, safe and reliable manner.

Behavioural responses of brushtail possums to live trapping and implications for trap-catch correction

Context The behavioural response of animals to repeated trapping has implications for correction of population and monitoring indices that use catch per unit effort. Failure to account for sprung traps introduces biases into estimates of relative abundance. The time when animals get caught in live traps is often ignored, but it can provide important information about temporal movement patterns relevant to this issue. Aims We assessed changes in the behaviour of brushtail possums (Trichosurus vulpecula), a nocturnal marsupial, in response to repeated trapping and evaluated the potential benefit of correcting a commonly used index of abundance by using time-of-capture information. Methods Possums were live-trapped for three nights each month over a 20-month period in baited cage traps in a 6-ha area of native lowland forest in the southern North Island, New Zealand. Trapped possums were individually identified on first capture. Timing devices were attached to the traps to measure how long after sunset traps were sprung and how that time related to the duration of the trap-night (sunset to sunrise). Key results Possums were trapped, on average, ~1.25 h after sunset. Traps triggered other than by possums were sprung on average 1–2 h later. Possums caught on the first night of a trapping session were caught significantly earlier than those caught on subsequent nights. Previous capture influenced the time of subsequent capture in a trapping session in complex ways, and recapture times were generally earlier than times of first capture. Possums were captured, on average, after 11% of the duration of a trap-night and traps were triggered by animals other than possums, on average, after 22% of the duration of a trap-night. Conclusions The data on time of capture of possums and triggering of sprung traps suggested a need to alter the commonly used correction factor for population indices for possums, because, on average, traps were sprung for significantly more of each trapping interval (i.e. trap-night) than the half a trap-night assumed in the correction factor. Implications Better understanding of possum foraging behaviour is a key to more effective control using traps. In that context, more research is needed to understand the reasons for individual differences in trappability. Although it is theoretically desirable to account for sprung traps when trapping is used to index populations, to reduce biases in estimates of relative abundance, correction of the standardised residual trap-catch index for possums is probably unimportant in practical terms, because most possum control reduces numbers to levels (2–5% trap catch) at which correction of the index is unimportant. The principal exception to this is likely to be when there is a high level of non-target interference from rodents.

Progressing the Possum Pied Piper Project

Author(s): Kavermann, Matt; Ross, James; Paterson, Adrian; Eason, Charles | Abstract: Improving vertebrate pest control operations relies on increasing pest animal interactions with control devices (e.g., bait stations, bait bags, and/or traps). Interactions are encouraged using a variety of baits and lures that stimulate an animal’s visual, olfactory, or auditory sense, orientating the target species towards a control device. On a generalised spatial scale of conspicuousness, an auditory lure will function over a greater distance for mammals in forested ecosystems than both visual and olfactory lures, suggesting auditory lures could have the greatest luring potential. In New Zealand, there is an overabundance of the introduced Australian brushtail possum that is the subject of ongoing control. Ground-based control operations typically use visual (e.g., a flour blaze), and to a lesser extent olfactory (e.g., cinnamon) lures for attracting possums to control devices. However, the potential for an auditory stimulus remains largely unexamined and underutilised. Research presented here expands on previous studies with captive animals and examines the development and field testing of an audio lure for possum control. The results from three preliminary field trials show that possums found audio-lured devices sooner than un-lured devices, and that a greater proportion of lured devices were located over time. In addition, possums were recorded investigating lured sites at a higher rate compared to un-lured sites, suggesting that possums were more likely to interact with a control device if it has an audio-lure than if it does not.

Why 0.02%? A review of the basis for current practice in aerial 1080 baiting for rabbits in New Zealand

Sodium fluoroacetate (1080) has been used as an aerially distributed toxin against mammalian pests in New Zealand since the 1950s. Although its use for rabbit (Oryctolagus cuniculus) control ceased temporarily after the illegal release of rabbit haemorrhagic disease virus (RHDV) in 1997, there has been a recent resurgence in the use of aerial baiting with 1080 to control rabbits as the efficacy of RHDV has fallen. Current practices for rabbit control using 1080 have changed little since the 1980s, with high sowing rates and low toxin loadings commonplace. The lack of ongoing development in baiting practices for rabbit control contrasts sharply with continued improvements in the aerial 1080 baiting practices for brushtail possums (Trichosurus vulpecula) in New Zealand, such as a shift to a comparatively high 1080 loading and using much lower application rates of prefeed and toxic bait. These modifications have resulted in an overall reduction in the amount of toxin used for possum control. The disparity in these two approaches prompted a formal review of the rationale on which the current 1080 baiting practices for rabbits are based. Two issues emerged strongly. First, the current low toxin loading used (0.02–0.04% 1080 in bait) is not based on experimental optimisation in New Zealand but, rather, on research conducted several decades earlier in Australia. Second, despite long-standing concerns about the quality of carrot bait used in New Zealand, current bait manufacturing and distribution practices still produce large numbers of small sublethal fragments. Thus, the current New Zealand practice of multiple prefeeds and very high sowing rates of bait with a low 1080 loading used against rabbits seems to have resulted from the need to compensate for the low toxic loading and poor quality control of the bait (carrots). We, therefore, suggest that there is considerable potential to improve current aerial 1080 baiting practices for controlling rabbits in New Zealand. More generally, these findings also help illustrate that ‘best’ pest-management practice may sometimes be based on pragmatic solutions aimed at overcoming unrecognised internal constraints that are in fact avoidable. Refining and modernising vertebrate pest-control programs, so that they better meet efficacy requirements and contemporary public expectations, therefore requires understanding not just that a solution works, but also how it works.

Reduced spillover transmission ofMycobacterium bovisto feral pigs (Sus scofa) following population control of brushtail possums (Trichosurus vulpecula)

In New Zealand, bovine tuberculosis (bTB) is present in domestic cattle and deer herds primarily as the result of on-going disease transmission from the primary wildlife host, the brushtail possum (Trichosurus vulpecula). However, bTB is also present in other introduced free-ranging mammalian species. Between 1996 and 2007, we conducted a series of studies to determine whether poison control of possum populations would have any effect on the prevalence of Mycobacterium bovis infection in sympatric feral pigs (Sus scrofa). We compared trends in the prevalence of bTB infection in feral pigs in six study areas: possum numbers were reduced in three areas, but not in the other three, effectively providing a thrice-replicated before-after-control-intervention design. Before possum control, the overall prevalence of culture-confirmed M. bovis infection in feral pigs was 16.7-94.4%, depending on area. Infection prevalence varied little between genders but did vary with age, increasing during the first 2-3 years of life but then declining in older pigs. In the areas in which possum control was applied, M. bovis prevalence in feral pigs fell to near zero within 2-3 years, provided control was applied successfully at the whole-landscape scale. In contrast, prevalence changed much less or not at all in the areas with no possum control. We conclude that feral pigs in New Zealand acquire M. bovis infection mainly by inter-species transmission from possums, but then rarely pass the disease on to other pigs and are end hosts. This is in contrast to the purported role of pigs as bTB maintenance hosts in other countries, and we suggest the difference in host status may reflect differences in the relative importance of the oral route of infection in different environments. Despite harbouring M. bovis infection for a number of years, pigs in New Zealand do not sustain bTB independently, but are good sentinels for disease prevalence in possum populations.

Hoarding behavior by ship rats (Rattus rattus) in captivity and its relevance to the effectiveness of pest control operations

Hoarding of food items is well known among muroid rodents, but evidence for hoarding behavior among ship rats (Rattus rattus) is scant. Here, we characterize hoarding behavior in ship rats maintained in captivity after capture from the wild. After acclimatization to captivity, 40 ship rats (21 females, 19 males) were presented with baits in experiments designed to emulate a typical poison control operation for vertebrate pests in New Zealand: this involved first offering rats nontoxic cereal baits (of 2- or 6-g size) as a prefeed for three nights consecutively, followed by 6- or 12-g cereal baits laden with 0.15% 1080 on the fourth night. Seventy-eight percent of rats (31/40) hoarded food in distinct cache sites when presented with nontoxic baits although there was no significant effect of bait size or type on hoarding behavior and nor did hoarding behavior vary according to rat gender. When rats were presented with 1080-laden baits, the incidence of hoarding was reduced to 40%, due to the onset of toxicosis. This study indicates that R. rattus will show hoarding behavior analogous to other rat species when presented with an excess of cereal-based baits, at least under conditions of captivity and free from competition. This finding may have practical relevance: since 1080 is the principal toxin used against the major vertebrate pest species in New Zealand (the brushtail possum, Trichosurus vulpecula), ship rats have the potential to deplete supplies of prefeed and/or toxic baits intended for possum control. However, based on typical rat densities recorded in New Zealand native forest (c. 5 rats/ha), the degree of removal and manipulation of toxic baits observed by ship rats here is unlikely to impact adversely on the efficacy of possum control operations.

Could recombinant technology facilitate the realisation of a fertility-control vaccine for possums?

While lethal control remains the primary tool for possum control in New Zealand, there has been substantial research effort over the last decade into controlling possum populations by reducing the animals’ breeding potential. In particular, the potential of immunological control to reduce fertility has been investigated, whereby the possums’ immune system is induced to react against its reproductive system, in order to block or destroy key components of the reproductive process. Two of the most targeted key components have been the blocking of circulating hormones (e.g. gonadotrophin-releasing hormone) and the inhibition of functional egg surface proteins (e.g. zona pellucida). While some success has been achieved for each approach, three main obstacles remain to the development of a working fertility-control vaccine for possums: first, ensuring that the vaccine remains efficacious in oral-delivery formulation; second, ensuring sufficient levels of fertility reduction; and third, ensuring that the induced immune response is sustained for a duration sufficient for long-term suppression of reproduction. The use of a genetically modified recombinant organism (parasite, bacterium or virus) to deliver a fertility-control vaccine could satisfy these requirements. A strong precedent for this approach has been set already in wildlife biology, namely the oral rabies vaccine, which is based on a recombinant vaccinia virus and which has been used successfully for the last 25 years as a disease-eliminating vaccine in mesocarnivores in Europe and North America. This review outlines the different forms and examples of recombinant organisms with potential for engineering into recombinant fertility-control vaccines to reduce possum reproduction; non-transmissible agents or fully-transmissible vectors are discussed, with shortcomings and benefits outlined for each.

The relationship between possum density and browse damage on kamahi in New Zealand forests

To minimize the impacts of introduced pests and to justify and prioritize pest control, managers need to know the relationship between pest density and damage. This relationship can be difficult to quantify because pest impacts can be highly variable. In New Zealand, introduced brushtail possums (Trichosurus vulpecula) browse a wide range of native forest species. However, possum browse is extremely patchy making it difficult to quantify the relationship between density and damage, meaning the benefits of reducing possum densities are poorly understood. We quantified patterns of possum browse on kamahi (Weinmannia racemosa), a common canopy tree species, at 21 forest sites that were repeat-measured over an 8-year period in the North Island, New Zealand, during which time possum densities fluctuated widely. We fitted a multilevel statistical model in order to quantify the relationship between possum density and browse damage while simultaneously quantifying how browse varied among trees, sites and years. Higher possum densities were associated with greater browse damage, but browse was also patchily distributed among trees at the same site, and among sites and years for a given possum density. This heterogeneity meant there was no simple density damage relationship, with the relationship differing from tree to tree and among sites and years. Our results show that at most sites reductions in possum density would have little benefit in reducing the probability of heavy browse on kamahi trees, but at a few sites there would be substantial benefits. This approach provides insights into the pattern and potential causes of variability in possum impacts, and a quantitative basis for prioritizing sites for possum control.

Effect of prefeeding, sowing rate and sowing pattern on efficacy of aerial 1080 poisoning of small-mammal pests in New Zealand

Context Aerial poisoning using sodium fluoroacetate (1080) is an important but controversial technique used for large-scale control of brushtail possums (Trichosurus vulpecula) and other pests in New Zealand. The technique reliably produces near total kills of possums and rats, provided that many tens of baits (and therefore many tens of individually lethal doses) are sown for each target animal present. Aim The aim of this study was to further refine aerial 1080 poisoning by determining the effect of prefeeding, sowing rate, and sowing pattern on effectiveness. Methods Eighteen experimental treatments comprising all possible combinations of three sowing rates (1, 2, and 5 kg/ha of bait), three frequencies of non-toxic prefeed (0, 1, and 2) and two sowing patterns (parallel and cross-hatched) were applied to each of two forested areas. Treatment effectiveness was assessed from changes in the rate of interference recorded on baited cards for three species: possum, ship rat (Rattus rattus) and mouse (Mus musculus). Key results Outcomes were highly variable, ranging from increases in pest activity to near total reductions. Possum reductions were highest where one or two prefeeds were used, and at the higher sowing rates (2 or 5 kg/ha), but with some interactions between these factors. For rats, two prefeeds resulted in the highest reductions but sowing rate had no effect. For mice, post-poisoning indices were often high, indicating low effectiveness. Conclusions Some treatments were highly effective so poor kills were unlikely to have resulted from pests not encountering bait, or the bait being unpalatable. Rather they appeared to reflect sub-lethal poisoning either as a result of low acceptance (as a result of a lack of familiarity and/or satiation) or bait fragmentation. We infer that for possum and rats prefeeding helps reduce this risk of sub-lethal poisoning not only by increasing familiarity, but also (in conjunction with high sowing rates) by increasing the bait encounter rate, particularly for possums. Implications There is scope to further reduce the amount of toxic bait sown and the cost of poisoning, without compromising efficacy, by fine-tuning the balance between prefeeding and sowing rate based on which species are being targeted and, for possums, reducing bait fragmentation.

Management of bovine tuberculosis in brushtail possums in New Zealand: predictions from a spatially explicit, individual‐based model

Summary 1. Bovine tuberculosis Mycobacterium bovis (Tb) in brushtail possums Trichosurus vulpecula in New Zealand continues to pose a threat to the livestock industry. While significant reductions in cattle reactor rates have been achieved by widespread possum control, there is still a need for significant improvement, particularly in relation to limiting the further spread of the disease. 2. A recently developed oral Tb vaccine based on bacille Calmette–Guérin (BCG) offers a possible alternative to poison baiting for widespread disease control in possums. In addition, there is ongoing development of orally delivered fertility control vaccines (FC) as an alternative method for possum control. However, there is some doubt about whether control strategies based on oral BCG or FC can offer cost‐effective solutions for disease control in possums. 3. We developed a spatially explicit, individual‐based model (IBM) of Tb in possums to address perceived shortcomings in existing non‐spatial models, particularly their failure to model realistically the spatial scale of transmission. The spatial IBM was also used to compare the cost‐effectiveness of control strategies using BCG and combined BCG/FC to existing strategies based on lethal control. In particular, we explored the effectiveness of strategies to contain a localized outbreak. 4. Considering the cumulative cost of control to contain a localized Tb outbreak, the most cost‐effective strategy combined an initial cull of possums with both BCG and FC applied every 3 years. In addition, the combined culling/BCG/FC strategy required only a 2‐ to 3‐km control buffer around the outbreak site to achieve successful containment. By comparison, control strategies that used BCG or BCG/FC alone were not as cost‐effective as they required at least a 5‐km buffer around the outbreak site to achieve containment. 5. Synthesis and applications. Current strategies for Tb control in possums rely heavily on lethal control techniques, but there is public antipathy in New Zealand to the widespread use of toxicants for pest control. Strategies for Tb control that use oral vaccination of possums with BCG, or combined BCG/FC, in combination with a single initial cull of the population offer a cost‐effective alternative for local eradication and containing disease spread. Such strategies would greatly reduce the need for repeated application of toxicants.

Alternatives to brodifacoum and 1080 for possum and rodent control—how and why?

The risks of vertebrate pesticides to non-target animals are determined by intrinsic susceptibility, the toxicokinetics of the compounds used, and the degree and frequency of exposure. Metabolism and persistence studies coupled with field surveys have provided us with an improved understanding of the toxicokinetics and non-target effects of different anticoagulants. This has enabled improved choice of tools for island versus mainland use in New Zealand, and has stimulated the development of low-residue tools. Brodifacoum is a potent second-generation rodenticide used worldwide for commensal rodent control, and has been widely used to eradicate rodents from islands. The risks associated with ‘one-off’ application of baits containing second-generation anticoagulants for rodent eradication on islands are considered to be outweighed by the potential benefits to their ecosystems. Possums are susceptible to bait containing brodifacoum, but not to first-generation anticoagulants, hence brodifacoum has been the only alternative to 1080 that effectively targets both possums and rodents. On the mainland, contamination of wildlife and game species and secondary poisoning of non-target species has been substantial where brodifacoum has been used repeatedly. We are extending the current range of low residue alternatives to reduce reliance on brodifacoum and sodium fluoroacetate (1080). Currently, suitable alternatives available include encapsulated cyanide Feratox® and cholecalciferol paste Feracol®. Other compounds being developed as alternatives include micro-encapsulated zinc phosphate and low dose cholecalciferol and coumatetralyl, or C+C.

A method for considering the acceptability of novel biotechnologies for the control of brushtail possums

Public acceptance and regulatory approval of biotechnologies is increasingly dependent on consideration of cultural, ethical and spiritual effects, and the opportunity for community involvement. The objective of this study was to articulate a means for researchers to identify relevant issues during the development of the technologies. It encompassed the substantive issues, such as safety, the provision of credible and adequate public information, and the opportunity for citizens, as well as stakeholders, to give society's informed consent. In this initial analysis of novel toxins and infertility or sterility-inducing agents being developed to manage pest populations of brushtail possums, several issues were identified. They included the risk associated with fertility controls entering the food chain, the severity of animal welfare compromise with toxins, and the adequacy of terminology used to describe pest control agents. This approach provides a means for researchers to acknowledge the issues contributing to public acceptance of biotechnologies during their development and with the involvement of those with interests in biotechnology.

Trends in Vertebrate Pesticide Use and New Developments: New Zealand Initiatives and International Implications

Author(s): Eason, Charles; Murphy, Elaine; Ogilvie, Shaun; Blackie, Helen; Ross, James; Kavermann, Mathew; Sam, Shona; Statham, Mick; Statham, Helen; Lapidge, Steven; Humphrys, Simon; Henderson, Ray; MacMorran, Duncan; Gibson, Troy; Gregory, Neville; Harrison, Joanne; Giles, Gregory; Sammut, Ivan; Jansen, Paul; Conole, Daniel; Rennison, David; Brimble, Margaret | Abstract: In New Zealand, sodium fluoroacetate (1080) has been used for vertebrate pest control for several decades. Since the 1990s, some 1080 users have switched to brodifacoum for possum and rodent control because of its ready availability and ease of use. An awareness that field use of brodifacoum results in persistent residues provides the impetus to develop alternatives and provide new tools and greater flexibility. Looking to the future, we seek toxins which increasingly combine “low-residue” characteristics with humaneness, and more selective bait and delivery systems enabling better and more acceptable control of possums, wallabies, mustelids, rodents, feral cats, and rabbits. Experience gained in the 1990s with the introduction of cholecalciferol (Feracol®) and a cyanide pellet (Feratox®), which both kill possums without secondary poisoning, underpins the extension in 2009 of the Feratox® registration to include introduced Dama wallabies. To date, zinc phosphide has not been registered in NZ, despite its field use in Australia and the U.S. and low secondary poisoning risk compared with 1080. Research and registration dossiers are being assessed in 2009-10 for zinc phosphide containing products for possum and rodent control. Registration documents are also being prepared for a combination of cholecalciferol and coumatetralyl to provide a slow-acting alternative to brodifacoum for the field control of possums, rodents, and rabbits with low risk of bio-accumulation. Anticipated timelines for product availability are 2010 (zinc phosphide) and 2011-13 (cholecalciferol and coumatetralyl). Our intention now is to move beyond these conventional rodenticides and develop new vertebrate pesticides. For example, we are pursuing the registration of para-aminopropiophenone (PAPP) for humane control of stoats and feral cats, and a series of related novel toxins and other compounds that target the red blood cell for other pest species including rodents. PAPP products should be available in 2010, subject to registration approvals. New research initiatives in 2010 will increasingly result in a shift in focus to the development of novel rodenticides aided by new international research collaborations.

Cost Effectiveness and Efficacy of Ground-Control Techniques for Pest-Control in New Zealand

Author(s): Ross, James G.; Eason, Charles T.; Ogilvie, Shaun C. | Abstract: The brushtail possum, introduced to New Zealand in 1858, is a significant conservation pest and a major vector of bovine tuberculosis. Previous control-simulation studies have suggested that aerial delivery of bait containing sodium fluoroacetate (1080) is the most cost effective (large scale) possum control strategy. Over the past decade, considerable improvements in groundcontrol techniques have been developed by private contractors and bait manufacturers. These techniques are not reliant on 1080, and there have been major cost reductions as the new baits and delivery mechanisms have been optimized. In addition to this, our research team (in collaboration with Connovation Research Ltd.) have recently developed a new bait station design that has the potential to be left out in the field for up to 5 years without the need for servicing. These devices are self setting and have the ability to deliver gel or liquid spray, and ‘target-specific’ toxicants. Preliminary cost analysis suggests that this new bait station design has the potential to save NZ$21 million per annum from the amount currently spent on possum ground control.

Feratox® as a Humane Control Agent for Wallabies in Tasmania

Compound 1080 has been used to control native wallabies and possums in Tasmania for over 50 years. Public concern in relation to humaneness and its effects on domestic dogs and nontarget species has led to opposition to its use. Feratox®, a form of encapsulated cyanide pellet registered for brushtail possum control in New Zealand, was considered as a replacement toxin. Trials in New Zealand showed that the material is fast-acting and humane in wallabies. In Tasmania, protocols were developed using bait stations that would minimise access by nontarget macropods and wombats. In field trials using Feratox®, however, there was excessive spillage of toxic pellets and variation in bait take between seasons by Tasmanian pademelons. Further work is under way to resolve these issues to see if protocols can be developed for the safe and effective use of cyanide pellets in areas where nontarget mammals are prevalent.

Predicting the unexpected: using a qualitative model of a New Zealand dryland ecosystem to anticipate pest management outcomes

AbstractPest management is expensive and there is often uncertainty about the benefits for the resources being protected. There can also be unintended consequences for other parts of the ecosystem, especially in complex food webs. In making decisions managers generally have to rely on qualitative information collected in a piecemeal fashion. A method to assist decision making is a qualitative modelling approach using fuzzy cognitive maps, a directed graphical model related to neural networks that can take account of interactions between pests and conservation assets in complex food webs. Using all available information on relationships between native and exotic resources and consumers, we generated hypotheses about potential consequences of single‐species and multi‐species pest control on the long‐term equilibrium abundances of other biotic components of an ecosystem. We applied the model to a dryland ecosystem in New Zealand because we had good information on its trophic structure, but the information on the strength of species interactions was imprecise. Our model suggested that pest control is unlikely to significantly boost native invertebrates and lizards in this ecosystem, suggesting that other forms of management may be required for these groups. Most of the pest control regimes tested resulted in greater abundances of at least one other pest species, which could potentially lead to other management problems. Some of the predictions were unexpected, such as more birds resulting from possum and mouse control. We also modelled the effects of an increase in invasive rabbits, which led to unexpected declines of stoats, weasels, mice and possums. These unexpected outcomes resulted from complex indirect pathways in the food web. Fuzzy cognitive maps allow rapid construction of prototype models of complex food webs using a wide range of data and expert opinion. Their utility lies in providing direction for future monitoring efforts and generating hypotheses that can be tested with field experiments.

Comparison of two luring methods for trapping brushtail possums in non-forest habitats of New Zealand

A national trap–catch monitoring protocol been developed to standardise the settingand luring of leg-hold traps for monitoring brushtail possum control operations in New Zealand. The methods recommended were initially developed for use in forest and scrub areas, but a suitable luringsystem was needed to monitor possum populations in grassland (tall tussock) areas. A backing board in a metal frame was developed to which the standard dry flour and icing sugar lure could be applied. Because this added cost to monitoring operations, a trial was carried out to assess whether backing boards could be omitted and the lure placed directly on the ground. Results showed that traps with groundset lure had a capture rate of only about 50% of the rate achieved with lured backing boards. Traps with backing boards are more visually attractive, which probably explains their higher capture rate. To increase the capture efficiency of trapping, we recommend further research on improved visual lures rather than olfactory ones.

Effect of prefeeding on foraging patterns of brushtail possums (Trichosurus vulpecula) about prefeed transects

Context. Brushtail possums (Trichosurus vulpecula) are a major pest of native biodiversity and agricultural production in New Zealand. To maximise the effectiveness of control operations, prefeeding (free-feeding) of non-toxic bait before poison is often used, but the mechanisms by which it does so, remain unclear. One possibility is that prefeeding changes foraging patterns and space use in ways that increase the likelihood of possums finding and eating a lethal dose of poison bait. Aim. To determine whether prefeeding along transects increases possum activity on the transect, and if so, how long the effect lasts. Methods. We monitored the time that radio-collared possums spent within a few metres of 350 m of aerial wire laid along a transect. Key results. Initially, possums spent only 2.6 min per night in the vicinity of the wire, but after 20 kg of bait per kilometre were placed along the wire, that time increased 20-fold on the first night and remained high for the next four nights (by which time all of the bait had been consumed). After that there was a gradual decline in time spent near the wire over a further 18 days. The increased amount of time spent near the wire was the product of both an increase in the number of visits and the duration of those visits. Conclusions. We conclude that sowing prefeed in concentrated strips is likely to greatly increase the probability of possums rapidly encountering toxic bait sown along the same strips, especially where the toxin can be sown immediately after all of the prefeed has been eaten. Implications. Possum control operations can now be designed to apply much smaller quantities of toxic bait that will potentially reduce concerns about 1080 poisoning because of a move away from broadcast sowing operations that are intuitively disliked by many, to much more localised baiting regimes.

Vesicle-associated protein 1: a novel ovarian immunocontraceptive target in the common brushtail possum, Trichosurus vulpecula

Ovarian-based immunological research is currently restricted to proteins of the zona pellucida. This study examined the immunocontraceptive potential of a novel vesicle-associated protein, VAP1, previously isolated from the vesicle-rich hemisphere of the brushtail possum oocyte. Seven female possums were immunized against recombinant glutathione S-transferase-VAP1 fusion protein. Control animals (n=3) received antigen-free vaccinations. Following immunization, regular blood sampling determined the level and duration of immune response. Animals were monitored daily, pre- and post-immunization, to determine estrous cycling activity and the percentage of reproductive cycles yielding viable young. The reproductive tracts and somatic organs of VAP1-immunized (n=7), control-immunized (n=3) and non-immunized (n=5) animals were collected and examined by histology and transmission electron microscopy. VAP1 immunization caused a strong and sustained immune response. Elevated levels of VAP1 antibody binding were detected in sera following initial injections, and immune titers rose as boosters were administered. Immunization had no adverse effect upon animal behavior or body condition. Immunized females demonstrated no major change in annual estrous cycling activity; however, the percentage of reproductive cycles resulting in pouch young decreased significantly (P<0.05) by 40%. Histological and ultrastructural analyses revealed an abundance of lipid-like degradation bodies within the ooplasm of developing oocytes and the cytoplasm of failing uterine zygotes. Active macrophage invasion of enlarged endometrial glands was observed in the uteri of two females. Reproductive tract changes are discussed in relation to observed fertility decline. The results of this study indicate that VAP1 has exciting potential as an immunocontraceptive target for possum control in New Zealand.

Risk factors for bovine tuberculosis in New Zealand cattle farms and their relationship with possum control strategies

This paper reports the investigation of farm-level risk factors for confirmed bovine tuberculosis (TB), based on a retrospective cohort study of a population of cattle in the lower North Island of New Zealand. Data were obtained from the TB testing surveillance programme operational in this area since the mid-1970s and comprised 190,665 cattle-years at risk from July 1980 to June 2004 (inclusive). A mixed-effects Poisson regression model was used to investigate the influence of farm-level covariates on the number of cattle confirmed with TB throughout the study period. This model was interpreted in context of depopulation strategies for the wildlife reservoir for TB, the brushtail possum Trichosurus vulpecula, that were applied in this area. The model showed that, despite intensification of possum control strategies over time, proximity to forest parks (a principal possum habitat in this area) remained a significant predictor of the number of confirmed cases of TB detected per farm per year. Our analyses showed a significant, three-fold increase in TB risk in dairy cattle relative to beef conditional on the size of local possum habitat, and confirmed the positive influence of cattle population size and the presence of previous infection status as a determinant of the number of confirmed TB cases per farm per year.