Opossums (Possums)
Brushtail possums were among the earliest animals introduced into New Zealand by European settlers. They were first brought from Australia in 1837 to establish a fur industry.
During the 1940s, evidence of damage by possums to New Zealand’s forests increased, and in 1947 all restrictions on possum hunting were removed and penalties for releasing them were increased.
The number of possums in New Zealand was estimated some years ago at 70 million, significantly more than the number of sheep.
Today possums are considered the major animal pest in New Zealand. In farming areas they spread bovine tuberculosis to beef and dairy cattle and to farmed deer, damage crops and orchards, kill poplars and willows planted to control hill-country erosion and stabilise riverbanks, and eat pasture.
Effects of Possums on Native Animals
Until recently, possums were seen as serious conservation pests mainly as a result of their effects on native plants. Although they had sometimes been seen eating birds and insects, few people previously appreciated the role possums played as a direct predator (rather than just a plant-eating competitor) in reducing populations of native birds.
That has now changed following a study of the kokako (a rare bird) in which Land-care Research filmed possums eating kokako eggs and killing chicks. Four of 19 nests recorded on time-lapse video failed because of possum predation, and possums were probably responsible for one third of the nests suffering predation found during this 4-year study.
Possums have also been seen killing or eating eggs, chicks, or adults of at least five other native birds including kukupa (native pigeon). During 1996 and 1997, three more sequences of possums threatening adult kukupa and eating their eggs were filmed by night time video cameras.
TRICHOSURUS VULPECULA
- Over 70 million Possums now live in New
Zealand ( 1993 estimates ) - Digesting in excess of 21,000 tonnes of
green matter daily - Possum number are highest near bush
edges because of good quality pasture - Usually solitary animals, possums are
cat like in size - Cool southern areas breed bigger
animals than northern areas - Nocturnal, omnivorous marsupials,
possums sleep during the day - Possum dens are normally found in
thickets of gorse, flax, scrub and among the roots of trees. Occasionally in the forks of trees, but
mainly close to ground level - When travelling, possums dislike dense
undergrowth and do not like wet feet - Possums numbers are regulated by the
availability of a suitable food supply.
Consequently, due to high density of suitable vegetation here in New
Zealand our possum density is more than 10 times that of Australia - Possums are very selective browsers and
often return again and again to the same trees and shrubs, stripping them
completely of all new growth shoots and fruit often to the stage where the tree
will simply die. - 75% of all RATA – KAMAHI forests in New
Zealand have suffered severe damage - More than 70 New Zealand native trees
and shrubs form an integral part of the possums diet with numerous other ferns,
vegetables, fruits, grains and grasses topping off their menu. - Possums however cannot stomach KAURI
because of the leaves which contain high levels of TERPENES. - Possum fur trade was at its peak in the
early 1980’s with over 3.2 million pelts exported annually worth over 23
million dollars in exports - The current anti-fur sentiment world
wide has caused a drastic downturn in demand resulting in only 100,000 pelts
being exported in 1992.
Possums … the Pros and Cons of Different Poisons
Possums can be controlled by shooting, trapping, and poisoning. A range of toxic bait formulations are available, containing one of the six poisons currently registered for possum control: 1080, phosphorus, cholecalciferol, cyanide, brodifacoum, or pindone.
1080 (Sodium fl uoroacetate)
Compound 1080 is the most widely used poison (in carrot, cereal and paste baits) for situations where possum numbers need to be reduced rapidly over large areas. Carrot baits are screened to remove small pieces so as to reduce the risk of birds eating baits. Cereal its are used for both aerial and bait station control. Paste baits are used extensively for ground-based follow-up maintenance control. Cinnamon is sometimes added to possum baits to act as a lure and mask the taste of 1080
- Highly effective for achieving a rapid reduction
- Registered for aerial broadcast in mainland
- Biodegradable in the environment, ongoing monitoring of waterways following aerial operations
- Residues in sub-lethally poisoned animals do not have prolonged persistence
- Its broad-spectrum toxicity can be an advantage in targeting multiple pest species, e.g. possums, rodents
- Proven track record in possum control – high-quality efficacy data exist to support both aerial and ground-baiting techniques, with ongoing potential to reduce application rates
Phosphorus
Phosphorus is used as a paste and is generally applied to turf spits on the ground.
- Effective (kills of >90% achieved)
Cholecalciferol
All registered bait formulations (pellets, paste and gel formulations) contain 0.8% cholecalciferol (vitamin D3) for field application in bait stations for possum control and do not require a licence to use.
- Low risk of secondary poisoning to dogs and birds
- Lower toxicity to birds than mammals reduces primary poisoning risk to birds
- Residues in sub-lethally poisoned animals do not have prolonged persistence
Cyanide
Cyanide paste baits are applied at times and in locations where exposure to rain or moisture is minimised. Pastes are generally applied with lures in various types of weatherproof bait station above ground level but may also be applied as an ‘unprotected’, roughly pea-sized portion of paste.
- Humane (very rapid action)
- Suitable for skin/carcass recovery
- Low secondary poisoning risk
- Achieves moderate to high kills (70–90%)
- Biodegradable in the environment
Anticoagulants (Brodifacoum and Pindone)
Anticoagulant cereal baits containing brodifacoum and pindone are registered for use against possums. These baits are generally used to maintain low possum numbers following use of fast-acting poisons for the initial population reduction.
