Controlling possums to manage TB
Since the early 1970s, possums have been controlled in New Zealand as part of an ongoing strategy to manage bovine tuberculosis (TB) in livestock. The frequency and intensity of such control is driven by a requirement to reduce populations to very low levels, then to hold them at or below this level for 5–10 years to ensure the disease is eradicated.
Possum control is implemented using aerial applications of toxic bait and ground-based applications of toxic bait and traps. Both are applied under various regulatory and operational constraints. Aerial applications use bait loaded with sodium fluoroacetate (1080) at 0.15% and sown at rates of 0.5 to 2 kg/ha. Aerial control is the preferred option for controlling possums over extensive and rugged areas of forest that are difficult to access by foot. Ground-based control uses a range of toxins (primarily cyanide, as Feratox®) and light-weight traps (e.g. Victor No 1).
Although possum control tools have been available for many years, control programmes didn’t make significant inroads into possum populations and into the incidence of TB in livestock until the mid 1990s, when control shifted from essentially being carried out by regional council pest control staff to a competitive contracting industry.
Possum control contracting systems
For TB management, TBfree New Zealand publicises a range of control operations for competitive tender. Although price is an important selection criterion, other criteria include competence in health and safety management, relevant experience and track record, technical and management skills, equipment, and proposed methodology. Initially, all contracts were performance-based and used an index of possum density to determine if control targets were achieved and whether contractors should be paid. Most performance-based contracts have few restrictions on what control methods can be used, and it is up to the contractor to select the most cost-effective method to achieve the contracted target density. However, before such a competitive and performance-based system could be implemented, it was necessary to have a robust, standardised, and independent monitoring system for measuring whether the contractor had achieved the contracted target reduction.
Monitoring contractor performance
A standardised trap-catch index (TCI) was developed in the 1990s by Bruce Warburton and colleagues for monitoring contractor performance. It was based on a defined number of randomly allocated trap lines, each containing 10 leghold traps spaced at 20 m and set for three contiguous nights of fine weather. From this method, for example, two possums caught over 100 trap nights would indicate a TCI of 2%. To ensure the methodology was applied in a standardised way, a national protocol and training courses were developed by the National Pest Control Agencies.
From 2011 to 2013, the mean TCI achieved from all recorded performance contracts each year was 0.65%, 0.46%, and 0.4%, respectively, and was considerably lower than the contracted targets of 1–2%. Additionally, 93% of c. 200 performance-based operations contracted each year achieved their targeted TCI on their first post-control monitor, with the remaining contracts requiring extra work to achieve the desired target.
In areas that have had several years of performance-based control, contracts generally shift to input-based control to eliminate the cost of having to independently monitor the operation. Input contracts are essentially method-driven, with contractors being required to apply a particular control method at a prescribed intensity, such as a stipulated trap-spacing and number of trap nights. Input contracts are often used to ensure control is applied across all habitats to decrease the probability of clusters of possums being missed, and recently have included the use of detection devices, such as chewcards, to better target trapping effort.
Capturing the data
For data recording and auditing purposes, all contractors must use GPS-capable personal digital assistants (PDAs) to record the location of all detection devices and traps. These data can then be uploaded, checked against habitat maps, and used in models to determine if TB has been eradicated from possums. TBfree New Zealand has developed ‘bespoke’ databases (i.e. VectorNet and VectorTrax) to manage the large amounts of spatial and activity data collected by contractors. Such data are now used to support TB management decisions based on probability predictions from the ‘proof of freedom’ utility (viz. a computer model that generates the probability that an area is free of TB). Because of the need for these data, in the last 5 years there has been a shift from contractors simply carrying out possum control to them collecting data on possum presence/absence and collecting possum carcasses for checking their TB status.
Delivering an effective control programme
The strategies for managing TB in New Zealand wildlife operate on four major principles: (1) having a target threshold for possum population reduction that is known to result in TB being eliminated, (2) an objective methodology for assessing whether the target reductions have been achieved, (3) cost-effective control tools for achieving possum population reductions, and (4) the necessary legislative support to ensure compliance. TBfree New Zealand’s possum control programme meets these requirements, and provides an example of a very effective pest and disease control programme underpinned by significant and ongoing research.
This work was funded by TBfree New Zealand.