Possums and other animal pests

What is a pest?

An animal or insect that has been introduced to New Zealand becomes a pest if it has the potential to cause significant national economic, environmental or cultural effects.

In their home environments overseas, these organisms often create no problems. However, New Zealand's warm climate and lack of natural predators have meant some species have made themselves too much at home at the cost of our native plants and animals.

The Council's role

The Northland Regional Council (NRC) has primary responsibility for the management of plant and animal pests in Northland. This responsibility comes from provisions in the Biosecurity Act 1993. As part of its functions, the Council identifies major pests and sets out methods for dealing with them in a pest management strategy.

Hundreds of thousands of dollars are spent each year on controlling some of Northland's worst pests. However, it is not enough to eradicate them. At best the Council hopes to keep numbers down and stop pests spreading.

The Council also has to take steps to control new infestations of pests or the spread of existing pests into new areas.

Northland's biggest pests

Possum.There are estimated to be 10 million possums in Northland - that's 72 possums for every person in the region! Some experts think there might be up to 15 million possums in the region.

Northland possums eat nearly three tonnes of leaves, twigs and berries every night. They tend to target new shoots and systematically destroy forest canopies. In combination with deer and goats, they can eventually decimate tall forests. Rata, kamahi and tawa are especially vulnerable, as well as pohutukawa on the coast. Possums strip native forests of fruit, flowers and leaves which are vital food sources for birds. They also eat the eggs and chicks of native birds.

In Northland, the average adult possum weighs 2-3 kilogrammes, and lives for about seven to nine years, often breeding twice every year. The heaviest concentration of possums is usually along the outer edges of bush or scrub, in horticultural areas or where there are good nesting sites close to food sources.

The Regional Council undertakes initial possum control work in specified areas. The Council encourages the land user to work alongside the contractor to receive training in the use of poisons to control possums. The land user can then go on to gain a licence in the use of poisons such as phosphorus and cyanide. Land holders are encouraged to keep possum numbers low after the initial poisoning is finished.

Find out more: Download possum resources from our publications section

Diagram of weasel and stoat.Mustelids are weasels, stoats and ferrets. They were introduced into New Zealand in the late 1880s to try and control rabbits. It didn't work and now rabbits and mustelids are both classed as pests in New Zealand! Mustelids are secretive animals that are rarely seen by humans.

Weasels are the smallest of the mustelids at 20 centimetres long. They are brown with a white belly, and like to eat mice, small birds, eggs, lizards and insects.

Stoats look similar to weasels but they are about 40 centimetres long, which means they can tackle larger prey. They have a longer tail with a black tip. Stoats are more common than weasels because they survive on a wider variety of prey.

Ferrets were farmed for their fur in the mid-1980s. When the fur industry collapsed, many were released into the wild and quickly became established so that New Zealand now has the largest known population of wild ferrets in the world. Ferrets are large and stocky animals, with yellow-white colouring and a black face mask. They like to eat small animals such as rabbits, rodents, possums and birds, as well as eggs, lizards, frogs and insects. They will kill kiwi and eat their eggs.

Find out more: Download mustelid resources from our publications section

 Diagram of ferret

Feral Cats:
Feral cat.Many people love cats, but most do not understand that these great hunters are estimated to kill up to 100 million birds in New Zealand each year.

Cats do not realise that some of their prey might be rare native species. When cats attack birds, they will often kill the parent, chicks and also eggs in a nest. Cats also eat lizards and frogs.

In the early 1800s, the Stephen's Island wren was discovered in Cook Strait. It was the only flightless perching bird known in New Zealand. Shortly after its discovery, the lighthouse keeper's cat wiped out the last of the species so it is now extinct.

Pet owners are encouraged to keep their cats in at night, which is when birds are most vulnerable and cats like to do their hunting. A bell on the cat's collar can help warn birds that danger is near.

Wild or feral cats are widespread in Northland. These have become established through people dumping unwanted pets. Under the Northland Regional Council Pest Management strategy it is illegal to dump live cats in the wild. The Council also urges people to control wild cats on their properties.

Find out more: Download feral cat resources from our publications section

Wild goat.Goats were brought to New Zealand in the 1770s by Captain Cook in the hope of establishing a food source, setting up a fibre industry and controlling weeds. These animals quickly flourished.

By the 1930s, goats had become a major forest pest, and they still are.

A goat population can double in size every two years as females start breeding at about six months of age and twins are common.

Find out more: Download wild goat resources from our publications section


Wasps have good and bad characteristics. Beekeepers don't like them because they compete with bees for food. People don't like them, either, because of the risk of being stung, which can be a serious health risk for anyone who is severely allergic.

However, wasps also kill other pests such as flies and spiders.

There are three main types of wasps in Northland - Australian and Asian paper wasps, and German wasps. The common wasp is not actually that common in the region yet, although there are fears that they will become more of a pest soon.

German and common wasps look very similar - they both have yellow and black bodies. Nests are often underground, although they can also be found in trees.

The Australian paper wasp is brownish black or yellowish, sometimes with yellowish or whitish bands on the abdomen. The Asian paper wasp is yellow and black in colour.  Paper wasps build umbrella-shaped nests of wasp paper.

Find out more: Download wasp resources from our publications section

Deer.Any deer sighted outside a farm should be reported to the Northland Regional Council.

Wild deer are a pest because they are prolific breeders, they seriously damage native bush and they are potential carriers of bovine tuberculosis. This disease is not present in Northland, and any outbreak would be a serious threat to the cattle industry in the region.

Control of wild deer is a specialised job for experienced hunters.
There are known to be a few wild deer still eluding hunters in Northland, and the Northland Regional Council will respond immediately to any reports of sightings.

Find out more: Download wild deer resources from our publications section


Mynah bird.Mynah birds can be seen everywhere in Northland. These imports from the tropics were first introduced to New Zealand in the 1870s and have made themselves right at home in the warm climate of the North.

They are aggressive and territorial, and have been known to move into the nests of other birds.

Mynahs can be controlled by poisoning and shooting, but care needs to be taken with both of these methods. Bird scarers can be used to protect commercial crops, although the devices need to be moved frequently as these smart birds quickly learn to ignore them.

Find out more: Download bird resources from our publications section 

Wild rabbit.Rabbits were introduced to New Zealand by Captain Cook in 1777, and other settlers brought more for food, fur and sport.

Wild rabbits munch the best grass in pasture, damage young trees and create a nuisance in vegetable gardens and orchards.

The saying ``breed like rabbits'' comes from the fact that rabbits can produce up to 30 young throughout the year, although the main breeding season is spring to early summer. A female usually mates again within 12 hours of giving birth.

Rabbits have been controlled by shooting, poisons and trapping.

However, the introduction of Calicivirus - a virus that only kills rabbits - has reduced the numbers of rabbits all over New Zealand to the point where they are no longer considered a major pest in Northland. This situation could change if rabbits gain immunity to the virus, as has been reportedly occurring in some parts of the South Island.

Find out more: Download rabbit resources from our publications section

Hare.Hares are much larger animals than rabbits with their characteristic large ears. They were introduced from England in 1851 and have become well established.

Even one hare can do considerable damage to trees by munching on young trees, tree bark and new shoots.

Hares are mostly controlled by shooting, as they are wary of poison baits.

Find out more: Download hare resources from our publications section

Magpie.These Australian imports were introduced in the 1860s. There are two subspecies: white backed and black backed. The white backed magpie is the most common in Northland. The two interbreed to produce individuals with varying amounts of black and white.

Magpies have become pests because they rob smaller birds' nests, taking eggs and baby birds. They are territorial and aggressive and are known to attack, kill or drive off other birds, including native species. At nesting time, they will attack humans and can cause serious harm.

Studies are underway to devise the best method to control magpies, and to find out what damage these pests are causing to native bird populations. The Northland Regional Council is taking part in this study. The magpie is notoriously difficult to trap or kill, because they learn very quickly. While these birds have been largely tolerated in the past, their status among Northland's pests may be raised, depending on the outcome of the trials.

Find out more: Download magpie resources from our publications section

Future pests

Rook.There have been reports of about six rooks in Northland, and Northland Regional Council staff are trying to track down the birds before they become established in the region.

These large black birds are well established in the rest of the country but had previously only been found as far north as the Waikato, with some individuals seen near Helensville and Muriwai.

Rooks nest in tall trees and eat a wide variety of insects and vegetation, including sown and germinating seed. They are a serious pest in cropping areas.

Find out more: Download rook resources from our publications section


Tropical Grass Webworm:
This grass pest was only recently established in Northland and has quickly spread as far north as Te Paki and as far south as Taupo Bay. Damage was first seen near Houhora in early March, 1999.

Webworm.The caterpillars can completely chew out a five-hectare paddock in 48 hours. They completely strip grass of green tissue, often leaving peeled stalks.

This pest was probably blown over from Australia in this latest infestation. It has been recorded in New Zealand before. However, previously it had failed to become established as it cannot survive frost. With climate warming, northern parts of Northland are frost-free, giving this pest a chance to settle in to some serious pasture munching.

The tropical grass webworm is pale in colour and feeds at night. Caterpillars feeding on pasture during the day are likely to be army caterpillars.

Find out more: Download tropical grass webworm resources from our publications section

Common wasps:
The common wasp is actually not very common in Northland. However, this pest has recently been seen at Lang's Beach, near Waipu, and the Northland Regional Council fears that it could spread further into Northland. The common wasp has nests about the size of a football, and each nest produces about 80,000 queen wasps. (See picture earlier in article).

Argentine Ants
Argentine ants were first found in Auckland in 1990 on the site of the Auckland Commonwealth Games. They can now be found in a large area of Auckland with satellite populations in Northland, Bay of Plenty, Christchurch and Wellingon.

Although it has a low rate of spread, humans can accidentally move them to a new area by moving infested pots or vehicles.

The honey-brown coloured ants travel in wide columns in their thousands, eating everything in their path and stripping plants bare. They farm aphids and leafhoppers for the honeydew they produce. They will even move aphids to different areas on a plant. The ants protect these insects from their natural predators.

The ants pose a significant threat to native plants and insects, and have also been known to kill baby birds in their nests.

Find out more: Download argentine ant resources from our publications section

The threat of pests

New Zealand has lost a large number of its native species in the past 1000 years since humans settled on these islands. In many ways humans have been the biggest pests of all because they have totally transformed much of the land into farms, roads and settlements.

Scientists say that extinctions include:

  • 32 percent of endemic land and freshwater birds;
  • three out of seven native frogs;
  • one of the three native bats;
  • three of the 64 reptiles;
  • 11 of the 2300 known plants; and 
  • at least 12 invertebrates, such as snails and insects.

The populations of most surviving native species have been heavily reduced, including 1000 that are now considered threatened.

Invasive pests and weeds are now the single greatest threat to biodiversity in New Zealand.
Biological diversity, called biodiversity for short, has been described as the variety of all biological life - including plants, animals, fungi and micro-organisms, the genes they contain and the ecosystems on land or in water where they live. It is the diversity of life on earth.

Browsing animals such as possums and goats eat native plants. Introduced predators such as stoats, ferrets, rats and cats prey on native birds, reptiles, frogs and insects.

Some garden plants have escaped into the wild to become major weed problems in the native bush and on offshore islands. These plants threaten to choke out native species.

The Council's work

Before starting pest control work on a property, the Regional Council contractor will talk to the owner and occupier about the most appropriate control methods for their particular circumstances.

Control techniques can include poisons, trapping or shooting. The quickest and most effective method is poisoning, but where this is not possible, the contractors will use other methods.

If the landholder has a strong preference for one particular method that may be time-consuming or expensive, they may be asked to contribute to the cost.

The poisons most commonly used by pest control contractors are phosphorus paste, cyanide in capsule or paste form and 1080 in jam form or cereal pellets. All poisoned baits are coloured green.

The paste or pellets are set in bait stations or poison bags in trees, or on the ground on overturned earth. Pellets may also be thrown by hand into areas of bush. The contractor will often lay non-toxic "pre-feed" baits for several nights to attract possums before poison is laid to kill them.

Poisoning will be followed up with night shooting, if necessary. Very occasionally, where it is inappropriate to poison or shoot, traps may be used.

Every effort is made to ensure that pet owners are told how to keep their animals safe during the control operation. However, dog owners, especially, must be aware that poisoned possum carcasses are toxic to dogs, even when they are well rotted.

Once the possum control operation is over, landholders are responsible for possum control on their own properties. Regional Council pest management staff are happy to teach groups of people the most effective methods of pest control.

Experts believe the tropical grass webworm is likely to be here to stay. Farmers, the Northland Regional Council and other authorities are working together to minimise its effects.

At its height in late summer 1999, the webworm stripped hundreds of hectares of pasture in the Far North, with up to 2000 of the wriggly worms per square metre. The webworm was not so devastating last summer, but it still did substantial damage.

Since then, the Tropical Grass Webworm Task Group has been working with the Regional Council and other authorities to establish the best ways of dealing with the pest.

Field trials have shown that some pesticides can control the webworm, but unfortunately the most effective chemicals are organophosphates, which are of environmental concern. Other "softer" pesticides can be used if applied early enough - for example synthetic pyrethroids, insect growth regulators and (BTs) biological toxins.

Other alternatives include changing farming practices - either to grow crops that are unpalatable to the webworm, or to accept that stock-carrying capacity will be reduced in the summer and autumn.

Meanwhile, the Regional Council and the Task Group will continue to monitor webworm populations, and farmers will be warned when numbers start to increase. The Council will also fund research into the webworm, including possible biological control.

The Regional Council has also agreed to put another $25,000 into wild deer eradication work in 2000. The project, jointly run by the Northland Regional Council, Department of Conservation and Animal Health Board, aimed to completely eradicate wild deer in Northland by 2007.

It is estimated that only eight wild deer are left in Northland - but there is always the risk that more will escape from farms.

The Department of Conservation operates a 24 hour toll-free hotline - 0800 346333 90800 FIND DEER) - for people to report deer in the wild.

The Council is also involved in a national trial on magpie control. The trial is taking place on two farms in Northland, and scientists hope to find out the extent of the damage to native birds from these aggressive intruders.  

How can you help?

Call the Environmental hotline if you see any of the new pests that are threatening our special Northland plants and animals. The number is 0800 504 639.

Northland Regional Council staff can help with advice on any of these pests and with identification.

Support Landcare projects in your area, and help keep pests and weeds - especially possums - down to manageable numbers. Help with revegetation and clean-up projects.

The native trees and animals of Northland are too precious to be lost forever to these foreign invaders.

Further information

Find out more: Read more about animal pest control

Visit the Biosecurity New Zealand website: www.biosecurity.govt.nz

Sources: NRC Animal Pests fact sheets, NRC Possum Control booklet, My Cat Did That? DOC video, Whangarei Report column by Derek Keene (19 October 2000 edition), National Draft Biodiversity Strategy.