A new study conducted by Skogforsk and the Swedish University of Agricultural Sciences (SLU) on behalf of the Swedish Forest Agency has shown that spruce bark beetles in Götaland and large parts of Svealand overwinter less than half in the bark of trees regardless of the growing zone. The study has also identified several factors that influence spruce bark beetles' choice of overwintering site, including bark thickness, tree age, forest type and time of infestation.
The study showed that spruce trees with thick bark and standing some way into the forest had a higher proportion of spruce bark beetles in the bark. The spruce trees that still had some green needles also had a higher incidence of spruce bark beetles. This means that if infested trees are felled in winter, the best control effect is likely to be achieved if the bark is thick, the trees have green needles and are inside the stand.
However, summer felling has been shown to have the best control effect because spruce bark beetles tend to be more entrenched in the bark at this time of year. If the temperature is not below freezing, the bark can easily fall off the trees during winter felling, which means that the spruce bark beetles remain and survive in the forest.
The study also showed that there was no difference in the choice of overwintering site between different growing zones for spruce bark beetles in Götaland and most of Svealand. However, a lower proportion overwintered in the bark further north (northern Värmland and Jämtland) where winters are usually colder.
Older trees also had thicker bark, which may explain why spruce bark beetles prefer to overwinter in the bark of older trees. Bark thickness is an important variable for spruce bark beetles' choice of overwintering site.
Although winter felling of infested spruce trees may have some control effect, it should not be the main reason for felling. Felling is usually done to save timber value, for safety reasons or to make way for new forest.
