The spruce bark beetle is a natural part of Sweden's forests and affects forest dynamics. It also has a positive impact on biodiversity, provided that infestations are not excessive. More than 140 insect species live as predators, fungus eaters or decomposers in the bark beetle pathway. When the bark beetles abandon the trees after a year, more organisms will live in the dead wood until the log is completely decayed. It also creates openings in the forest, favouring plants and animals that thrive in sunny locations.

Three main factors will determine the extent of the infestation next year:

1. The weather
2. the number of bark beetles and
3. the vitality of the trees.

The weather is the most important factor. Dry and warm weather favours bark beetles, while the opposite favours spruces. The number of bark beetles depends on their reproductive success; if it has been low, one can expect a lower risk of damage in the coming season. Reproductive success is monitored by the Swedish University of Agricultural Sciences (SLU). One problem is that it is difficult to harvest most of the infested trees in time when infestations are extensive as in recent years. It is important to note that all outbreaks in Sweden so far have stopped after a few years when the forest has recovered and long before all the spruce forest has been killed.

Over the last 4-5 years, large areas of central Europe have experienced dry and hot summers that have favoured spruce bark beetles. In countries like the Czech Republic, this has led to disastrous consequences because the infestation was not responded to quickly enough. In 2017, 2.5 million cubic metres of wood were infested, the following year 20 million cubic metres. These examples show the importance of acting decisively.

Sweden and the Nordic countries have much more proactive forest management than Central Europe, which provides advantages in the fight against bark beetles. One advantage is that insect pests are detected earlier with more effective technology. Another advantage is that we have long worked with tree breeding, which creates the conditions for stronger trees that can better resist attacks.

Spruce bark beetle infestations can be difficult to detect. During spring and summer, it is important to be alert to spruce bark beetle in the forest. Signs of infestation include:

• Borage (similar to coffee powder or cocoa) on fallen trunks and at the trunk base of standing trees.
• Drops of flesh on the trunk
• Dry green needles on the ground
• Stripped bark, usually high up on the trunk
• Gnawing mould (white) in the bark pockets of the tree
• Drones can be useful when inspecting the forest. If you see groups of trees with rusty-red tops or other yellow-shifting colours, it is a sign of infestation.

Finding individual spruce bark beetle-killed trees from the ground in large forest areas is difficult. One way to get a better overview is to use remote sensing and time series of satellite images and drones. The technique of change analysis in satellite imagery is not new, but it is only recently, when the Sentinel 2 satellites, which are part of the European Copernicus programme, started to provide frequent information over the same area, that the method has been applied on a larger scale for forest damage analysis.

Information from remote sensing can also be used to produce comprehensive map products that show where the risk of new infestations is greatest. One example is the risk index map for spruce bark beetle infestations, which has been produced by combining several different map bases, including tree species information from SLU Skogskarta and volume data from basic forest data.

The best chance of finding individual damaged trees is when working with very high-resolution data, from drones for example.

It is important for landowners to continuously monitor their forest during the summer in case of ongoing spruce bark beetle outbreaks. The risk of infestation is highest in stands that were infested the previous year and in older, high-volume spruce stands. Here, the risk maps on the Swedish Forest Agency website be helpful. Groups of infested trees should be felled and transported away from the forest before new bark beetle generations spread. 

Early felling can save a large part of the timber value of infested trees. If significant infestations are detected in older stands, it may be wise to harvest the entire stand to save the timber value of trees not yet infested. In older stands without damage, thinning or gap creation should be avoided during outbreaks, as this may increase the risk of infestation.

After major storms, it is important to manage (fell and transport away) storm-felled trees before bark beetles multiply in them. In the long term, the most important thing is to establish and maintain spruce forests so that they are as resistant as possible to future storms and droughts.

Most commonly, infestations occur in several places in the forest. It is therefore important to prioritise which area to start with. The extent of the infestation and the risk of spread will determine how to act. Sometimes it may also be appropriate to combine several measures. The Swedish Forest Agency highlights a number of measures that may be relevant.

In case of minor infestation:

If only a few spruce trees are infested and you don't have the time or opportunity to move them from the forest, they can be cut down and left behind. If you are an experienced forester with the right equipment, you can mill the bark off the trees.

In case of widespread infestation:

If the forest has suffered major damage, it may be best to harvest the entire stand. These stands are usually in such poor condition that they are likely to be heavily infested again next year.

A variety of insects use Swedish forest trees as food and breeding grounds, such as the pith beetle, the shining beetle, the wood borer and the six-toothed spruce bark beetle.

To learn more about these insects and their tracks, visit
https://www.slu.se/skogsskada/ or
https://www.skogsstyrelsen.se/bruka-skog/skogsskador/insekter/

When planning for regeneration, it is important to choose tree species that are suitable for the location. If trees thrive where they grow, they will have a much greater ability to resist external threats. A healthy spruce is highly resistant to insect pests. If you are unsure which species is best suited to your site, consult your forestry advisor.

A basic rule is that a healthy and growing forest is more resilient. Here are some simple tips from the Swedish Forest Agency to create a more resilient forest.

- Ensure that the forest is healthy by establishing new forests with suitable tree species on the right land and managing them according to the recommendations of the forest management plan.

- Try to create mixed forest stands. Different tree species act as barriers to dispersal and research shows that bark beetles, for example, do not like the smell of birch.

- Promote the bark beetle's enemies by creating favourable environments for woodpeckers, ant beetles or the common bark beetle, for example.

There are many challenges related to the spruce bark beetle also in the future, but no one can safely predict developments in the coming years.

Research suggests that the risk of new infestations may increase with climate change. Warmer summers favour spruce bark beetle reproduction, and droughts weaken the resistance of spruce trees. Milder winters also result in less frost and increase the risk of storm traps - which could trigger new outbreaks.

For the 2025 season, inventories show that infestations have been greatly reduced compared to previous years, but experts warn that the threat is not over. The weather will determine how quickly the population can recover.

With the help of modern technologies - such as drones and AI-based image analysis - forest owners today have better opportunities than ever to detect attacks in time and act proactively.