According to the Finnish Forest Centre, more forests were felled due to spruce bark beetle in 2022 than ever during the history of the current statistics. In the monitoring by Natural Resources Institute Finland (Luke), more spruce bark beetles than before were found in southern Finland, as well as in the north.
The spruce bark beetle is not yet an acute problem outside south-eastern Finland. It is still a growing problem of which forest owners should be aware.
The increase in the damage caused by the spruce bark beetle is explained by both climate change and the abundance of spruce. As climate change progresses, hot and dry summers are expected to become more common. The change is beneficial for spruce bark beetle, not for spruce.
The spruce bark beetle starts to swarm in the spring, when temperatures rise to 18 degrees Celsius. The warmer the summer, the more likely it is that it will manage to produce two generations during a single growing season.
As a shallow-rooted tree species, spruce suffers from heat and drought. The issue becomes even more challenging if the spruce-dominated forest has been cultivated on the wrong site type to begin with. In a forest with only one tree species, damage spreads uncontrollably.
Healthy spruces can protect themselves against the spruce bark beetle thanks to their resin. The risk is therefore higher in spruce-dominated forests with a lower defence capability. Risky sites include areas with windthrows or storm damage, edges of clearings, sun-drenched slopes, and areas surrounding roads, ditches and power lines. Spruce-dominated forests affected by root rot are also more exposed to the spruce bark beetle.
Timely forest management maintains the forest’s vitality. If the second thinning is carried out too late, on the other hand, it increases the risk of damage. If the spruce-dominated forest is more than 50 years old, the necessity for the second thinning should be carefully considered.
The older the spruce-dominated forest, the more likely the damage.
If the damage caused by the spruce bark beetle gets started, it can spread rapidly through the forest. In warm summers, the situation must be monitored regularly. We can monitor the development of the spruce bark beetle situation in Metsä Group’s Metsäverkko, for example. We will publish a new application in Metsäverkko for identifying insect and storm damage, probably in August 2023. The application will be visible in Metsäverkko as a new map layer.
When spruce bark beetle damage starts, it should be stopped in time. Individual trees that have died can be left in the forest, and how the situation develops should be monitored. If dead or damaged trees appear in groups or in several different locations, the situation must be monitored closely.
If the number of damaged trees is more than 10 cubic metres per hectare, i.e. about 20 trunks or more, they should be removed from the forest by the deadlines set in the Forest Damages Prevention Act. Dead trees can be left in the forest.
Many are unsure about leaving them, but they do not increase the risk of damage but serve as habitats for the spruce bark beetle’s natural enemies.
When planning to renew the area damaged by the spruce bark beetle, renewal with spruce-dominated forest should be avoided if possible.