D. R. Morgan

Sodium monofluoroacetate and alternative toxins for possum control

Abstract Sodium monofluoroacetate (1080) is still an essential tool for possum control. We have reassessed the fate of this compound in the environment, and found no evidence of water contamination after large-scale possum control operations. The toxin is biodegradable in all living systems and will not accumulate in the food chain. Nevertheless, overreliance on a single toxin for a particular pest, such as 1080 for possum control, is unwise, and we are evaluating alternatives. Possums are susceptible to some non-anticoagulant toxins, including gliftor, cholecalciferol, calciferol, and alpha-chloralose. Of the anticoagulant toxins, brodifacoum is more effective than pindone. Integrated pen and field trials will determine the most cost-effective alternatives to 1080 for use in bait stations and for aerial application. Any alternative toxin will need to be subjected to the same scrutiny as 1080 for its environmental fate and impact on non-target species.

Responses of captive and wild possums to lures used in poison baiting

Abstract Possum hunters believe that scented flour lures possums to cyanide poison paste baits, resulting in better kills. We tested 20 odorous compounds for attractiveness to possums in pens, then field‐tested five of the most effective (cinnamon, aniseed, plum, cherry, and orange). Only cinnamon increased kills achieved with cyanide by acting as a lure and improving bait palatability. However, neither cinnamon nor any of the other four attractive compounds masked the hydrocyanic acid gas that is released by cyanide paste and detected by possums. When cinnamon was tested as a lure for attracting possums to bait stations, the numbers of possums visiting bait stations did not increase. Cinnamon functions as a lure only over short distances (i.e., up to 2 m). However, this may be useful in guiding possums to cyanide baits used by hunters or aerially sown baits, many of which become hidden in the vegetation. Most compounds commonly used by hunters are unlikely to improve kills.

Benefits and feasibility of local elimination of possum populations

Introduction of the brushtail possum (Trichosurus vulpecula) to New Zealand has resulted in serious ecological and economic impacts and considerable control efforts. Recovery of possum populations after control occurs through immigration from adjacent areas and breeding of survivors and immigrants. If complete local elimination can be achieved, the recovery of populations will depend solely on immigration and therefore should be substantially slowed (particularly in very large areas). To compare the cost-effectiveness of four control strategies over the long term (60 years), we constructed a deterministic bioeconomic model based on 23 variables describing population characteristics, sizes of the sink (i.e. area controlled) and source (of reinfestation) areas, and costs. Sensitivity analysis showed that the most influential variables related primarily to cost and effectiveness of control, whereas factors describing immigration after control had relatively little influence. When the most influential variables were varied, the model predicted that local elimination of possums followed by ‘perimeter’ control is likely to be a more cost-effective control strategy under most scenarios than the current ‘knockdown-then-maintenance-control’ approach. Possum-control technology and its application have improved greatly in the last three decades such that it now appears that local elimination is, technically, a realistic goal, and is possibly already being achieved occasionally. Constraining factors include unreliable monitoring/detection at ultralow densities, inappropriate selection and use of control options, lack of incentive under the present contracting system, initial cost, contracting capacity, and the future regulatory status of poisons. However, these difficulties can be overcome, facilitating the adoption of long-term local elimination strategies that are better suited to managing possum populations in perpetuity.

Investigation of sodium monofluoroacetate (1080) bait shyness in wild brushtail possum (Trichosurus vulpecula) populations

Bait shyness is a significant threat to the sustainable control of vertebrate pests. New Zealands foremost vertebrate pest is the introduced brushtail possum (Trichosurus vulpecula). Bait shyness was identified in two wild possum populations by comparing the consumption of two non-toxic bait types before and after each population was presented with one of the bait types containing the toxin sodium monofluoroacetate (1080). Before poisoning, approximately 60% of total bait consumption was of the type which 1080 was later presented in, whereas after poisoning this bait type made up only 2-4% of total consumption. This shyness persisted for at least 11 weeks in one of the populations. No individual component (bait base, cinnamon lure or green dye) of the toxic bait could be isolated as the primary cue eliciting bait shyness as the response. Possible means of overcoming bait shyness are discussed in light of these findings.

Improving the efficacy of aerial poisoning of brushtail possums (Trichosurus vulpecula) through reduced fragmentation of bait

In New Zealand, aerial delivery of sodium fluoroacetate (1080) baits to poison introduced brushtail possums (Trichosurus vulpecula) is a key pest management tool. GPS-guided broadcast baiting systems should expose all possums to large amounts of toxic bait, but some possums still survive despite being poisoned. This sub-lethal poisoning might reflect bait fragmentation, so we measured bait size before and after broadcast sowing from helicopter-slung buckets with spinners used to fling bait laterally. Mean bait size was smaller after sowing than before. The amount of fragmentation differed between operations and between buckets. Small fragments were concentrated under the flight path suggesting that they were caused by the spinner. Fragmentation increased with spinner speed. Removing the spinner resulted in less fragmentation. Use of spinners may therefore adversely affect poisoning efficacy by producing sub-lethal bait fragments. Solutions to this bait fragmenation problem include reducing spinner speed, removing the spinner, or development of shatterproof baits. A reduction in the amount of fragmentation is likely to reduce the need to sow large numbers of baits, so our findings could enable major reductions in toxin use.

Enhancing maintenance control of possum populations using long‐life baits

Abstract Possum populations must be maintained at very low density if the aims of Tb eradication and conservation of particularly vulnerable native species are to be met. The present tactic of initial “knockdown” followed by annual maintenance control allows for reinfestation in between annual operations. Development of toxic baiting methods for “continuous” control of possum populations is therefore desirable. Six bait‐type/presentation‐methods designed for prolonged field life were exposed to field conditions at a forest edge site in Westland. Samples of baits were collected at 2‐monthly intervals for up to 26 months, and assessed for palatability to possums and toxicant concentration. Regression analysis was used to predict the time at which reductions in palatability and toxicity would make the bait ineffective. Field life varied between 2 and >26 months. The most durable option tested was a solid gel bait containing cholecalciferol and presented in a purpose‐designed bait station. This option was selected for assessment in continuous control of possums in forest‐edge habitat in West‐land. Baits were placed up trees beyond the reach of livestock and left in place for 10 months. Trap‐catch monitoring showed these baits did reduce the number of possums, but only after improving the visibility, attractiveness, and accessibility of bait stations. As the study predicted that cholecalciferol gel bait should remain effective for >2 years in the field, there appears to be considerable potential for improving the efficiency (and probably environmental safety) of long‐term control using this new tool.

Multi-species control by aerial baiting: a realistic goal?

Abstract Two advantages could be gained by developing multi-species control by aerial baiting. It should assist pest managers to achieve sustainable resource management by improving the efficiency of operations; and it should reduce the likelihood of unwanted ecological side-effects (such as prey-switching) that sometimes follow when pest species are controlled individually. The technology to achieve efficient multi-species control is already well advanced, aided particularly by the refinement of aerial control of possums and by the development of baits for individual species. More effort will, however, be required to develop predictive models for planning the integration of multi-species control with conventional single-species control, and for predicting the ecological effects of such intervention.

Aerial Prefeeding Followed by Ground Based Toxic Baiting for More Efficient and Acceptable Poisoning of Invasive Small Mammalian Pests

Introduced brushtail possums (Trichosurus vulpecula) and rat species (Rattus spp.) are major vertebrate pests in New Zealand, with impacts on conservation and agriculture being managed largely through poisoning operations. Aerial distribution of baits containing sodium fluoroacetate (1080) has been refined to maximise cost effectiveness and minimise environmental impact, but this method is strongly opposed by some as it is perceived as being indiscriminate. Although ground based control enables precise placement of baits, operations are often more than twice as costly as aerial control, mainly due to the high labour costs. We investigated a new approach to ground based control that combined aerial distribution of non-toxic ‘prefeed’ baits followed by sparse distribution of toxic baits at regular intervals along the GPS tracked prefeeding flight paths. This approach was tested in two field trials in which both 1080 baits and cholecalciferol baits were used in separate areas. Effectiveness of the approach, assessed primarily using ‘chewcards’, was compared with that of scheduled aerial 1080 operations that were conducted in outlying areas of both trials. Contractors carrying out ground based control were able to follow the GPS tracks of aerial prefeeding flight lines very accurately, and with 1080 baits achieved very high levels of kill of possums and rats similar to those achieved by aerial 1080 baiting. Cholecalciferol was less effective in the first trial, but by doubling the amount of cholecalciferol bait used in the second trial, few possums or rats survived. By measuring the time taken to complete ground baiting from GPS tracks, we predicted that the method (using 1080 baits) would be similarly cost effective to aerial 1080 operations for controlling possums and rats, and considerably less expensive than typical current costs of ground based control. The main limitations to the use of the method will be access to, and size of, the operational site, along with topography and vegetation density.

Field efficacy of cholecalciferol gel baits for possum (Trichosurus vulpecula) control

Abstract The efficacy of a cholecalciferol gel bait against high‐density possum populations was assessed in two replicate field trials conducted in beech forest in the Hopkins Valley. The possum populations were monitored by (i) trap‐catch before and after control at both treated and non‐treated sites and (ii) recapture of possums tagged during pre‐control trap‐catch. Control was conducted at each treatment site by presenting a non‐toxic gel as a prefeed in mid April, followed a fortnight later by gel bait containing 0.9% cholecalciferol; these baits were checked and replenished at intervals of decreasing frequency (i.e., 1–8 weeks) until late July. Monitoring of interference at bait stations suggested that the population was greatly reduced within the first fortnight of toxic baiting (i.e., by early May). The mean corrected trap‐catch reduction at the two treatment sites was 81%, while there was a 100% reduction in the recapture of ear‐tagged possums at both treatment sites. This difference was due to an average 68.5% drop in trap‐catch at the non‐treatment sites, which reduced the effect of the otherwise very large declines (average of 94.1%) in the treatment sites, while the mark/recapture‐based estimate was not affected because no tagged possums were caught after control at the treatment sites. The unexpectedly large population reductions in the non‐treatment sites are believed to be mainly (possibly entirely) because possums were removed by unauthorised hunters during June, as verified by Department of Conservation staff. Therefore, the effectiveness of the cholecalciferol gel was markedly underestimated by the trap‐catch monitoring. Conversely, the 100% reduction assessed by mark/recapture was overestimated because some possums were caught in the treatment sites after control. Consequently, the true reduction achieved by the gel bait was between 81 and 100%, at a cost of about

RISKS TO NON-TARGET SPECIES FROM USE OF A GEL BAIT FOR POSSUM CONTROL

35/ha. This figure is similar to the cost of using other ground‐based control methods, but there is potential for the development of more efficient use of cholecalciferol gel baits, particularly where sustained control at low density is the aim.