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The bad bug guide

Stephen Sweeney looks at the growing array of threats to Britain’s trees.

A multitude of pests and diseases, often introduced with imported plants or timber, currently affect forests all over the UK.

Climate change means the country’s temperatures continue to rise, the last decade being the warmest since records began, so it is likely that our forests will be exposed to an increasing number of pests and diseases. however, the correct treatment in conjunction with appropriate active management can reduce the danger. Checks are also in place at ports and timber processors to limit the likelihood of new pests and diseases being introduced through imports of either planting material or timber.

Here is what we know about the most common culprits, how to look out for signs of infection and successful control methods to minimise their impact.

Phytophthora ramorum 

One of the most serious and well-known of the diseases currently affecting forestry both in Scotland and further afield is Phytophthora ramorum. It is commonly referred to as larch disease because the species is particularly vulnerable to infection. 

This disease was first found in 2009 and has since spread to forests across most of the UK. Phytophthora ramorum is a water mould and is therefore more common in the wetter western regions of the country, the largest concentration of infected sites in Scotland being located in the Dumfries and Galloway area. 

The disease is spread by tiny spores dispersed by the wind, movement of felled timber and travelling mammals. It can readily infect other common plant species such as rhododendron and has been found infecting isolated Sitka spruce trees within stands of infected larch. The infection causes bark lesions often oozing dark fluid as well as wilting and withered shoots with blackened foliage which eventually kills the tree. 

There is no cure or effective chemical treatment to combat this disease, and current control methods focus on limiting its spread. Statutory Plant Health Notices (SPHNs) can be issued to force the felling of infected specimens and any other vulnerable trees within a 250 metre buffer zone, The movement and processing of infected timber can be controlled through licensing. These measures have brought a decrease in the number of infected sites in Scotland in recent years. 

Phytophthora ramorum has had a marked effect on forestry in Scotland with large areas of larch being felled under SPHNs. Larch is now seen as unsuitable for use in woodland creation projects due to the high likelihood of infection.

Dothistroma septosporum 

Commonly known as red band needle blight, Dothistroma septosporum affects a variety of coniferous species but is most often found in pine. 

This fungal infection causes premature needle defoliation leading to decreased annual growth of the infected tree. A severe infection can lead to the death of the host tree. Symptoms of infection are first visible on the needles from September onwards. The disease causes discolouration of the needles with tan-coloured spots before the reddish-brown bands start to appear.  

There has been a rapid increase in the number of reported incidents in the UK since the 1990s with lodgepole, Scots and Corsican pine being particularly vulnerable.

Dothistroma is treatable, good stand management being the primary method used in the UK. The Dothistroma fungus needs moisture and humidity, so regular thinning of pine stands to increase air flow through the crop will reduce the production and spread of spores. The disease is rarely fatal, its primary influence on forestry being to decrease timber yields.

Hymenoscyphus fraxineus 

Hymenoscyphus fraxineus is another fungal disease affecting forestry in Scotland that primarily targets species of ash tree. It is more commonly known as Chalara ash dieback. 

The disease causes continual dieback, weakening mature trees and making them more susceptible to other pests. It is often fatal for young or coppiced ash trees.

The first visible symptoms of infection by Chalara are the blackening and wilting of foliage in mid to late summer. From this point the disease will progress along the shoots and branches to reach the stem, forming dark lesions that will eventually girdle the tree and kill it by blocking the supply of fluids and nutrients from the roots.  The Chalara infection is spread by wind-borne fungal spores or through the movement of infected material, particularly leaf litter. There is no cure and no method to fully prevent the spread of the disease, but efforts have been made to limit its spread.

Early identification and removal of infected trees can slow the progress of the disease in mixed productive stands, and regular removal and burning of leaf litter in areas such as parklands can also help. Leaf litter should be burned or deep composted on site to avoid spreading spores. 

The spread of Chalara has led to the introduction of prohibitions on the movement of ash planting material. Ash is no longer a species choice for woodland creation, but natural regeneration of existing ash stands is encouraged as resistance to the disease is thought to be hereditary. 

Hylobius abietis 

Hylobius abietis, the large pine weevil, is one of the most widespread and damaging pests that threatens the UK softwood timber-growing industry. 

The pine weevil is 13mm long and dark brown in colour with patches of light brown or yellow. Adult beetles feed on the bark of any species of young conifer or broadleaved tree and other woody plants, leading to patches of bark removal. They lay their eggs in or near the stumps and roots of felled or dead conifer trees. 

Severe damage by pine weevils is mostly limited to commercial clear-fell sites, where the felling of large areas of conifers creates an abundance of food and breeding opportunities for the weevil leading to a population explosion. The young trees used to restock the area are then exposed to an abnormally high weevil population resulting in trees being girdled and dying. 

Losses of young restock trees due to weevil damage can be significant if no action is taken.  The main method of control is an application of chemical insecticide both in the nursery prior to planting and via top-up spraying. A long fallow period before restocking can also help to reduce weevil damage.

Ips typographus 

The larger eight-toothed European spruce bark beetle, Ips typographus is a non-native pest common throughout Europe where it is responsible for widespread damage in commercial conifer crops.

One outbreak has been recorded in Kent, with tight control measures being introduced to prevent further spread. 

Mature adult beetles are dark brown/black in colour with light brown or orange hairs. Adult females bore into the bark of weakened or windblown trees and lay eggs. When the larvae hatch they radiate outwards leaving a distinctive pattern of damage in the bark and surface wood. 

This species of bark beetle is not currently widespread in the UK, but it poses a significant threat to the commercial softwood timber industry. It also has an effect on global timber markets as outbreaks can result in high volumes of timber entering the market over short time periods

Dendroctonus micans 

The great spruce bark beetle, Dendroctonus micans is the native equivalent of Ips typographus. 

This bark beetle is another secondary pest in small numbers, preferring to attack windblown or already weakened trees. But when populations increase to a significant level they will target and overwhelm healthy trees. 

Infestations are identified through visual inspections with resin bleeds on the truck or roots being an early indicator. The great bark beetle attacks trees in a similar way to its European counterpart.

Prolonged infestation by bark beetles will eventually show with the crown of the tree browning, the tree becoming girdled under the bark and eventually dying. The most effective method of control is biological in the form of the bark beetle’s natural predator – another beetle, Rhizophagus grandis.