Twenty per cent of the spruce areas that have died due to the spruce bark beetle are also observed to be attacked by another pest, the double-eyed bark beetle. This was revealed in the latest annual report on forest damage from The Forestry Commission, which also identifies spruce bark beetle and grazing damage by elk and other cervids as continuing major problems in Swedish forests.
During the latest survey of the damage caused by the spruce bark beetle to spruce trees in southern and central Sweden, conducted by SLU on behalf of the Swedish Forest Agency, the first inventory of the extent of damage caused by the double-eyed bark beetle was also carried out in the area. Like the spruce bark beetle, this insect reproduces under the bark of spruce trees, damaging and eventually killing the tree. It was found that twenty per cent of the spruce forests that died and were surveyed were infested by either the double-eyed bark beetle alone or in combination with the spruce bark beetle, further increasing the risk of damage to the spruce trees. In total, half a million cubic metres of forest are affected by the bark beetle.
Lennart Svensson, National Forest Damage Coordinator at the Swedish Forest Agency, points out that the dry and hot summer periods, which started with the 2018 drought, favour more insect pests than just the spruce bark beetle. This emphasises the importance of planting spruce in places where they thrive best to increase their vitality and resistance to pests.
Choosing the right tree for the right place is crucial to reduce the damage from the double-eyed bark beetle. It is recommended to avoid planting spruce in dry areas. Unlike the spruce bark beetle, the number of double-eyed bark beetles can be significantly reduced by felling infested trees during winter, especially if the bark can be removed from the forest where the larvae of the double-eyed bark beetle overwinter.
The forest damage report for 2023 indicates that damage from spruce bark beetle is still remarkably high, although the total damage of 2.4 million cubic metres of forest is one third of the worst years of 2020 and 2021. Since 2018, around 34 million cubic metres of spruce have died as a result of infestation. Damage volumes have decreased in Svealand but remain at the same level in eastern Götaland as the year before.
New observations include the unprecedented spruce bark beetle attack on Norway spruce, and a young Norway spruce stand in Hudiksvall being affected by the pest diplodia, believed to be the first known case in Sweden and the northernmost in the world.
Damage from grazing elk and other cervids also exceeds targets, with inventories showing that 11% of young pine trees (1-4 metres high) are damaged annually, which is more than double the societal target.
A recent major storm in the interior of Västerbotten County caused extensive damage to the forest, again emphasising the need for diversification in forestry to increase resilience to pests, storms and fires.
Facts about the double-eyed bass borer:
It is found all over Sweden, is 2-3 millimetres long and mainly attacks spruce trees, especially those that are weak or stressed by drought. It prefers to swarm and attack during July to August and overwinters as a beetle or larva in the trees, pupating in spring.
