Feature

Fighting the Furry Invader

As the possum problem spreads, what hope is there that we can win the war against this furry invader?

By David R. Morgan

Not only are some of our finest native forests being devastated by browsing possums, but our economy is also seriously threatened. This introduced marsupial spreads bovine tuberculosis, putting at risk $8 billion worth of annual agricultural exports. Possums can be controlled and even eradicated, but because of the cost, control is restricted to areas of highest priority. Extermination is attempted only on islands of outstanding conservation value, such as Kapiti and Rangitoto, where natural reinvasion cannot occur.

Aerial sowing of poisonous 1080 baits has been the main control technique used since the 1960s, but it costs $15-20 per hectare, and its effectiveness is reduced in wet weather. Furthermore, 1080 can only be used by registered pest controllers, not individual farmers, hunters, or other members of the public. With the spread of bovine Tb and growing awareness of its implications, most farmers are eager to carry out possum control. In answer to this, scientists at Landcare Research are developing "farmer-friendly" techniques.

For the longer term, the focus is on alternative control technologies that do not rely on pesticides. These may lead to even more efficient control, and perhaps eradication. The research forms part of the national science strategy programme funded by the Foundation for Research, Science and Technology and the Animal Health Board, with extensive collaboration with other research organisations and industry. The multidisciplinary teams formed are essential for the rapid advancement of possum control technology.

Public opinion helps direct research. Although there is a high level of support for the need to tackle possum problems, humaneness is seen as an important element of any new technologies. An Animal Ethics Committee discusses all of Landcare's research into possum control. This committee has three public representatives. Much of the work is centred at Landcare's new animal facility at Rangiora, where possums can be maintained in controlled-climate cages indoors or in free-run pens outdoors.

New Bait Development

Most conventional possum baits are made from grains, which need little processing and are palatable to possums. Such baits are, however, quite difficult to waterproof without affecting the taste, storage life and presentation characteristics. Ray Henderson, Bruce Warburton and I are investigating a wide range of grains and other materials to improve the formulation of pellet baits. We are also studying how possums respond to bait types based on gels and plastics.

In collaboration with Dr Tony Woolhouse of Industrial Research and Development, we are studying possums' responses to a range of possum scents. We know very little about the function of the cloacal, chest, and mouth glands of the possum. It is likely, however, that scents play an important role in communication since possums are mostly active during the night.

The work on improved baits complements that being done on new toxins. The goal is to develop toxic baits that can be used without a licence as is required for 1080. Brodifacoum and pindone have recently become available in pellet form suitable for on-farm use, but these materials are not ideal. They are expensive and can leave residues in stock inadvertently exposed to sublethal doses. A research team led by Dr Charles Eason is investigating the suitability of some 20 other toxins. This will ensure that safe and efficient chemical control of possums can continue if 1080 becomes unavailable.

Cyanide was the only alternative to 1080 available in the past. This has been used widely by hunters harvesting possum furs, but the cyanide paste is rejected by many possums because they can detect the cyanide gas that is released. Bruce Warburton and Geoff Wright are collaborating with Dr Bob Welch of AgResearch to reduce the emission of cyanide gas. Minute microcapsules of cyanide are wrapped in polymers, then formulated into a tablet. This makes the cyanide less detectable. Another approach that the team is following is to mask the cyanide with favoured odours such as cinnamon. This technique was previously used to solve the problem of possums rejecting 1080 baits.

Within 12 months, the researchers expect these studies to yield some products for use by official control agencies and some for farmers.

Environmental Safety

Because of increasing public concern over the impact of pesticides on the environment, a series of investigations are being carried out to find out if possum poisoning operations have other environmental impacts. Eason has shown that 1080 does not persist in soil or water [see Old Pesticide -- New Data, March 1992] and it does not persist longer than 48 hours in stock that may have been exposed to sub-lethal amounts of bait. Analysis of 1080 residues relies on instruments that will identify 1080 at concentrations as low as three parts per billion.

Bird populations are considered by many to be at risk from aerial control operations. However, Dr Eric Spurr's extensive series of surveys during the last decade showed that populations of the more common bird species are unaffected by control operations. Consequently emphasis has now shifted to rarer species.

In a search for other possible environmental impacts, Spurr and Mike Meads are monitoring the effect of 1080 on invertebrate populations, which form the food resource of many birds and reptiles. Early results after a single control operation showed that temporary reductions occurred in populations of some insect species, but these recovered in the year after poisoning. Laboratory tissue analyses showed that traces of the toxin were found in some insect species captured soon after the control operation, but were absent in samples collected after one to five months. As these studies develop, a clearer picture of the effects of 1080 on whole ecosystems will emerge, enabling managers to minimise the environmental risks of using the compound.

Also aiming at increasing the environmental safety of 1080 operations, Dr John McLennan and Dick Porter are investigating the use of potent bird repellents in 1080 baits. This technique may enable possum poisoning to be used safely as a tool in the conservation of rare birds such as kaka and kokako, both of which compete with possums for fruits.

Any alternative toxin to 1080 will also require close scrutiny to ensure that effects on non-target species are minimised and that residues do not accumulate in soil or water.

Taking the Bait

In surveys after aerial 1080 operations, up to 50% of target areas were missed when sowing of baits depended on the pilot's visual assessment of coverage. In collaboration with the Department of Conservation, I am testing a new aerial navigation system based on the use of GPS (Global Positioning System) technology. The system uses triangulation of satellite signals to locate an aircraft's position in relation to a predetermined route. It can dramatically improve the coverage achieved once a pilot has gained a few hours experience.

By using precision sowing, we have been able to reduce the application rates of baits from the 20 kg/ha of just a few years ago to 5 kg/ha. Recent trials indicate that this can be reduced further to 1 kg/ha if complete coverage can be attained. Such reductions in sowing rate decrease operational costs by more than half, so that larger areas can be treated for the same cost. Precision sowing means less pesticide is introduced into the environment, contributing to safety.

A robust, inexpensive bait station has been developed by Malcom Thomas at Landcare Research for use on farms. With Dr Graham Hickling of Lincoln University, he has tested the best way to space bait stations to ensure that all possums encounter them. The use of bait stations and the new inexpensive baits being developed should enable more "self help" on farms, freeing up the control agencies to focus on critical Tb hot-spots.

Non-lethal Control

Rather than killing possums, it may be more appropriate to prevent some of the damage they cause by changing their behaviour with the use of repellents. This approach has potential for horticultural crops, forest plantations and home gardens.

Dr Woolhouse and I are evaluating a range of synthetic odours based on scents and waste products of predators such as foxes and stoats. When these odours are sprayed on highly palatable pine seedlings, possums are repelled. As possums evolved in Australia in the absence of these mammals, it is likely that a generalised mechanism is at work. We suspect that herbivores can recognise carnivorous predators by odours that are probably by-products of meat digestion.

Amongst other repellent compounds being tested is a substance known as Bitrex. This compound is one of the most bitter substances known, and is incorporated in many household detergents to deter children from drinking them. We are testing the uptake of this compound by pine seedlings to determine if enough of it gets bound into the plant tissues to prevent browsing by possums. If this is successful, the use of Bitrex may lead to alternative ways of establishing and protecting not only pine plantations but also native reserves and home gardens.

Long-term Solutions

Some exciting prospects for long-term possum control are being explored by Dr Simon Jolly, who is investigating the potential for reproductive control. Contraceptive vaccines are being assessed in the medium term, but in the long term he is looking at biological control of possums by a genetically modified virus. Such a virus would have to be possum-specific, disrupting a reproductive process specific to the possum. If these stringent safety requirements can be met, Jolly believes high levels of control could be achieved nationwide, without the ongoing costs associated with current control methods.

As control technology develops, it is likely that more control will be done by farmers. Combinations of different methods will be used because integrated pest management strategies offer the best hope for effective and sustained possum control. Individual possum's chances of survival are reduced when confronted with more than one control technique, and the likelihood of resistance developing to any particular technique is also reduced.

The prospects for improvements in the control of possums are therefore very good. Current and future research promises the further development of new, less expensive, and environmentally acceptable technologies.

David Morgan works for Manaaki Whenua-Landcare Research, Lincoln