Tree pests

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A great deal of our British wildlife relies on trees for a place to live or feed. Trees provide homes for many insects, birds and mammals within their roots, bark and branches. During the lifespan of a tree – which may be many hundreds of years – some species can cause problems. However, these tend to be alien species (those that are not native to our environment) or those whose populations may be unnaturally high.

Insects

Some species of insect have leaf-eating larvae capable of completely defoliating a tree; the caterpillars of the oak processionary moth are a good example. Native to southern Europe, it has been found in recent years in southern parts of the UK. The caterpillars can completely strip the leaves from an oak tree, but usually the tree is able to recover the following year. The hairs of these caterpillars are toxic to humans and can cause significant human health problems. Do not handle them or disturb their nests. For more information, visit the Government's Forest Research information pages.

For specific information and advice on the oak processionary moth, visit www.oakprocessionarymoth.co.uk, a website dedicated to information about this pest. Other useful pages on this pest can be found on the websites of Kew Gardens and the London Tree Officers Association.

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Oak Processionary Moth
Caterpillars of the oak processionary moth
© Forestry Commission

Wasps

Gall wasp larvae can also cause problems for trees; particularly oaks. Tiny gall wasps lay eggs on bark, leaves, fruits or flowers (depending on the wasp species) causing plant tissue to swell up and form a gall. The gall provides the larvae with protection from the elements and a nutritious food supply. The wasp Biorhiza pallid lays its eggs on oak leaves forming spherical, knobbly growths called ‘apple galls’ which cause little lasting damage. Others, such as the wasp Andricus quercuscalicis, can destroy a whole seed crop by causing distorted ‘knopper galls’ to grow on acorns.

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Oak apple gall
Oak apple gall
© Forestry Commission

Grey squirrels

Tree damaged by grey squirrels
Grey squirrel damage
© Forestry Commission

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The common grey squirrel was introduced from North America over 100 years ago and they are now found throughout much of Britain. Grey squirrels have developed the habit of stripping bark from broadleaved trees and in some cases they have been known to kill mature trees. For this reason grey squirrels are humanely controlled in many woods to keep damage to an acceptable level.

There is a usful article on the impact of grey squirrels in the Woodshed area of this website, or visit the Government's Forest Research website www.forestresearch.gov.uk/greysquirrels

Deer

Where population density is high, deer can have a serious impact on trees and woodlands. Deer can damage trees by browsing them, stripping the bark or fraying the trees with their antlers. Over the long term this can prevent tree regeneration, change the canopy structure and alter the characteristic ground flora of ancient woodland.

In Britain, our two native species of deer have large populations: 350,000 red deer and 800,000 roe deer. In addition, we have the long-established fallow deer and three other recently introduced species: the muntjac, sika and Chinese water deer, which add a further 400,000 to these figures. The Government has reported that deer numbers are now higher than they have been for 1000 years and therefore supports sustainable management, which includes humane control.

There is an article about deer in the Woodland animals area of this website, or visit the home page of the Deer Initiative

Other mammals

Other mammals such as field voles and rabbits may also damage trees and saplings by gnawing bark around the stem. This can happen when trees are located in fields or orchards and surrounded by long grass.

Invasive plants

There are also some invasive plant species which can cause problems for trees; the most notable of these is Rhododendron ponticum. Dense thickets of rhododendron can prevent regeneration of young trees and, in woodland, only those trees which manage to grow above the level of rhododendron can survive. Rhododendron is also host to the disease organism Phytophthora ramorum which has been implicated as a causal agent in ‘sudden oak death’.

See also, Tree diseases

For more information, or if you are concerned about an invasive species, please visit the Government’s Forest Research website at www.forestresearch.gov.uk