- Outbreak stage
- Susceptible species
- Reporting suspected cases
The great spruce bark beetle (Dendroctonus micans) is found in forests throughout mainland Europe. It damages spruce trees by tunnelling into the bark of the living trees to lay its eggs under the bark, and the developing larvae feed on the inner woody layers. This weakens, and in some cases can kill, the tree.
The beetle was first discovered in Great Britain in 1982 after it was accidentally introduced, most likely via a consignment of imported timber. It has become an established pest in Wales and western England, and has more recently expanded its range to southern Scotland.
The rate and extent of damage to individual trees and forests is variable. Neither the beetle nor its larvae burrow into the wood itself and, consequently, provided the wood is salvaged before the tree is completely dead, the timber is not spoiled in any way. Trees are killed by being completely girdled, at one or more points along the stem, although this may take several years of sustained attack. However, large breeding populations can be building up long before individual trees are killed, creating a risk of spread to adjacent and nearby trees.
The beetle attacks and breeds in all species of spruce grown in Britain.
Look out for signs of poor tree health. Check especially for isolated or small groups of dead or dying trees characterised by browning foliage over some or all of the crown.
The entry of female beetles into the bark of trees gives rise to characteristic ‘resin tubes’ on the trunk. Resin tubes and granular resin at the base of the tree are reliable signs of stem or root attack. Resin tubes vary in colour from white and cream, to shades of purple and brown. They may be accompanied by copious resin bleeds. Loose bark with exposed beetle galleries usually indicates older infestations that have been attacked by woodpeckers.
Inspect the bark around resin tubes, particularly those that are purple to brown. A hollow sound when the bark is tapped often indicates successful attack. Remove the bark carefully and inspect for signs of the beetle. The most characteristic indicator is the presence of a mixture of insect faeces (frass) and bark packed into ‘islands’ creating a quilted appearance. All beetle stages, from egg to adult, might be present.
The beetle has a long life cycle, ranging from 12–18 months under British conditions. This results in extensive overlap of generations so that it is possible to find any stage at any time of year. However, there are periods, particularly in the winter, where most can be at the same stage.
Adult beetles are 6–8 mm long and 2.5–3 mm wide.They are black when mature, with a covering of orange hairs. The large size of the beetle enables the females to withstand the resin flow produced when they bore into the bark of trees.
Eggs are laid within a small egg chamber in the cambium of the tree. Each female can produce up to 300 eggs, laid in groups of 50–80, in interconnecting chambers. Eggs are usually laid on one side of the chamber.
Larvae - The beetle has five larval stages (instars), which each become progressively larger. All larval stages feed under the bark in a similar manner: larvae feed side-by-side, packing powdery wood debris (or ‘frass’) and diseased or dead larvae behind them into islands away from the main feeding site. The mixture of resin and frass forms a distinctive quilted pattern.
Pupae are the immobile, resting stage of beetle development before the larvae moult to the adult stage. Pupae are found in pupal cells among the larval frass. They are often found in close proximity and give rise, upon emergence, to aggregations of adults under the bark. These stages can be prolonged over several weeks or months, depending on temperature.
The newly emerged adults are light brown. As they mature the colour darkens to brown and black. Adult beetles move within and between trees mainly by crawling (at temperatures of 12°C or greater), but they occasionally fly (at temperatures of 22.5°C or greater).
Information on treating infected trees can be found in the guidance document Minimising the impact of the great spruce bark beetle
Statutory controls against the spread of D. micans were first put in place in Great Britain in 1982. Following the discovery of three new outbreaks outside the main infested area, the Forestry Commission reviewed the controls and consulted the forestry industry on two options: extension of the designation of the infested area and continuing the policy of movement restrictions into Scotland and north-east England, or revocation of our EU protected-zone status. The second of these options was agreed, and from 15 May 2005 the movement within Great Britain of conifer wood and conifer bark was no longer subject to any treatment requirements.
Part of the west of Scotland is designated as a ‘pest-free area’ out of which wood and bark can be moved without treatment under the EU plant passport regime.