Pest control in forestry
Managing potential pests is an important part of caring for a forest. There are many pests that can affect our forests – from plants and flowers, to insects and animals. Some pests are more common in New Zealand than others.
Forestry pests database
Ministry for Primary Industries (MPI) and Farm Forestry New Zealand have pest databases. Check these and read about the common pests below.
Borer and termites
Borer and termites are insects that burrow into timber to build their homes. They damage wood and can affect how much harvested timber can be sold for production or export.
Caterpillars and moths
Caterpillars are the larval stage of moths – winged insects resembling butterflies. Caterpillars feed on the green growth of plants, including both hard- and softwood trees.
Looper caterpillars are native to New Zealand and particularly common in forests and gardens. At normal population levels, they're unlikely to cause significant damage. In certain weather conditions, populations can explode and cause large-scale forest damage.
Other types of caterpillars
Leafroller and light brown apple moth caterpillars can cause more damage than loopers in plantation forests. These caterpillars bind pine needles together in webs and feed on new growth. The damage they cause, particularly to buds on young trees, can cause malformation and stunted growth.
Weevils are small beetles whose larvae tunnel holes into dead logs or trees, planks of wood and treated timber buildings.
Weevils tend to be a problem only in harvested timber.
Eucalyptus variegated beetles
The Eucalyptus variegated beetle is an Australian leaf beetle. Its larvae feed on the leaves of eucalyptus trees and can cause significant damage. It has been detected across the Hawke's Bay region.
Aphids are small insects that feed on the sap of new plant growth. They can reproduce asexually (without a male and female coupling) and can occur in large numbers.
Most species of aphid are considered of little economic importance in New Zealand forests. However the Giant willow aphid affects willows (and attracts wasps) it can:
- cause willow branches and leaves to be covered by black sooty mould fungi
- weaken willow trees and kill branches.
Manaaki Whenua's factsheet on the Giant willow aphid has more information.
There are 3 types of wasp in New Zealand:
- social wasps
- parasitic wasps
- sawflies and wood wasps.
Wasps don't normally cause problems for foresters. But large populations may cause stunting and damage to small, closely packed stands of trees.
Social wasps include German wasps, common wasps and paper wasps. They have a varied diet, feeding on small insects, fruit, nectar, honey, or fish and animal carcases. They create nests out of wood fragments and mud. Their nests are often found attached to houses and other buildings. These wasps are the most likely to sting humans.
Parasitic wasps lay their eggs in other insects. There are many known species of parasitic wasps in New Zealand.
Sawflies and wood wasps bore into trees to lay their eggs. Fungus then gets into the trees through the holes, rotting the timber. When the larvae hatch, they burrow out through the rotted wood, leaving more holes in the trees.
Pukeko can cause significant damage to newly planted native seedlings by pulling them out of the ground. They may repeat the behaviour following replanting. The best prevention is to plant larger, well grown plants, as they’re less able to pull heavier plants out of the ground. Some species of geese will also pull out and eat native seedlings when planting around waterways.
Feral animal pests
Feral pests like hares, rabbits, goats, deer, wallabies, possums and pigs can impact forests by eating:
- leaves and shoots
- the seed crop of some plants.
Hares and rabbits
Hares and rabbits are more abundant near farmland and cause damage to freshly planted seedlings. Rabbits will browse leaves and stems and leave digging signs. Hares will slice off seedlings cleanly on a 45 degree angle, damaging them.
Goats usually leave cleanly cut leaves and shoots as feeding signs. However, when the stems are too tough to bite through they can be torn off leaving ragged wood fibre edges. They:
- browse the understory
- browse new seedlings, consuming most of the plant
- ringbark poles and consume the bark and branches of some species
- can feed by standing on their hind legs and reach vegetation up to 2m high
- climb trees.
Deer browse the understory and rub bark of trees with their antlers. They can eat seedlings and saplings.
When deer have been browsing leaves, several of the leaves will be removed from the stem, and some remaining leaves will have been bitten in half.
Wallabies browse the understory and seedlings. They can bite off part of a leaf or a whole leaf.
Possums can be very damaging to young stands of trees, particularly in winter and spring when nutrition sources are harder to find. Possum damage may be minimal over a single season but can become economically significant if it continues to occur over several years.
Pigs root (dig up the soil with their noses) which damages the ground surface. Rooting mainly occurs in damp areas.
Pigs will often rub on trees, logs and fence posts. These often become coated with mud and pig hair, and sometimes tusk marks are evident. Pigs can kill trees if they rub against them too much, particularly if they remove all the bark.
Having a pest management plan in place to control animals is essential to protect your forest.
Tane's Tree Trust has information on the impacts of livestock and pests on native plants. We also have a page on Managing livestock damage.
Monitoring pests, weeds and diseases
Tell the Ministry for Primary Industries if you see anything unusual.