Pests - the possum

Pests that affect plant growth - possums

What is it?
The possum is high on cuteness, and equally high in nuisance value. The Australian brush tailed possum was introduced into New Zealand in 1837 to establish a fur trade.

In its native land the possum is up against dingoes, bush fires and less palatable vegetation, but in New Zealand conditions are so favourable it often breeds twice in one year. It is estimated that the New Zealand possum population now tops 70 million and chomps its way through seven million tonnes of vegetation per year.

Why are possums a problem?
The damage to native forests can be seen all too clearly in many areas. Possums ignore old leaves and select the best new growth. In some areas they have eaten whole canopies of rata, totara, titoki, kowhai and kohekohe.

Possums compete with native birds for habitat and for food such as insects and berries. They also disturb nesting birds, eat their eggs and chicks and may impact on native land snails.

Dairy and deer farmers have the added worry of possums spreading bovine tuberculosis. Possums are a nuisance in suburban gardens, and sometimes even indoors.

Methods of control
Prevention: Deprive possums of nesting sites on your property by blocking off sheds and barns.

Individual trees can be protected with metal strips placed about a metre above ground level, as seen on telephone poles, to stop possums from climbing up.

You can protect trees with repellents.

Bait Stations: Bait stations are widely used by possum control agencies to give sustained control of possum populations, with minimal risk to humans, pets or livestock. A wide range of baits and bait stations are now available.

A range of bait stations are also available. Most are made from plastic and are sealed at the top but open at the bottom for feeding. Protection from the weather increases the effective life of the poison. Bait stations should be fixed above ground, such as in a tree, where the possums can easily reach the bait. They need to be placed high enough to be clear of children, pets, livestock and ground-feeding birds, such as weka.

Each bait station will cover a range of 100 metres. Site the station where the possum is looking for food or on identifiable possum runs.

Timms Traps: This trap is a strong bright yellow plastic `box' which in itself acts as a lure. The possum puts its head into a keyhole opening, attracted by a bait such as apple, kiwifruit or orange. Fruit baits make it unlikely that animals such as cats will be caught. When the possum investigates the bait a trigger is released which brings a striker rod up under the possum's neck.

If the bait is too large or too small it won't set off the trigger. An ideal size is 25mm in diameter.

Set Timms traps close to dry nest sites, around trees known to be possum favourites, indoors in the rafters or nailed to fenceposts or trees. Take care to set them out of reach of pets, children and ground-feeding birds, such as weka.

Cage Traps: These are steel mesh cages with a trap door which is triggered when the animal takes the bait. Small pieces of fruit or a trail of curry and flour can be laid outside the trap to attract the possum. Tie the main bait to the trigger arm and set the trip pin only far enough to prevent a light wind from releasing the door.

The disadvantage of the cage trap is that you still have to kill the possum. Given the destruction possums are wreaking on the environment and economy it is irresponsible to catch and release.

Cyanide: To buy cyanide you need to be licenced by MAF, at a cost of about $100, and have the written approval of the landowner where you intend using the poison.

MAF will supply further details on the regulations covering this poison and the licence requirements.

Unlike bait stations, cyanide causes near-instant death of possums and allows easier recovery and monitoring of any operation. Cyanide is a highly volatile and toxic poison, which needs to be used with care.

It is available either in tubes and is mixed with feed paste, or in encapsulated form as Feratox.

FOLLOWUP

1. Look up in the library to find out more information about possums.

2. Can you explain why possums are a problem in New Zealand, and not in Australia?

3. Find out about "Project Crimson", which is a huge project to try and restore the wonderful crimson colours of flowering pohutakawa trees in summer. What effect has the possum had on pohutakawas? If you write to the Department of Conservation, you may be able to get more information.

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