Bark beetle outbreaks – From bad to worse

By Paal Krokene and Bjørn Økland, Norwegian Forest and Landscape Institute 

The spruce bark beetle (Ips typographus) has caused significant losses for forestry throughout northern Europe. Research carried out at the Norwegian Forest and Landscape Institute indicates that these problems may become exacerbated in the event of climate change. Warmer weather will most probably result in two periods of attack by the beetle each year, and more frequent storms may trigger an increased number of outbreaks.

The spruce bark beetle is a premier example of how climate change may lead to increased insect damage in the future. It is the only insect in Norway that can kill large numbers of spruce trees, and in the 1980s alone it destroyed 15 million trees in southeast Norway. It may be surprising that such a tiny insect – the beetle is the size of a grain of rice – can kill off such large trees, but the beetles’ secret weapon is teamwork. When a spruce bark beetle has found a suitable tree, it produces a special pheromone scent which attracts other beetles to the area. This may result in thousands of beetles gathering to attack and break down the tree’s defences in only a few days. Moreover, the beetles engage in another form of teamwork. Each beetle carries a multitude of fungal spores into the tree, and these fungi play an important role in breaking down the tree’s defences.

Two attack waves every summer

Increased summer temperatures will speed up the development of the spruce bark beetle in Norway. This may permit two beetle generations to be completed each year, instead of the current rate of one. Today, the spruce bark beetle flies in May and lays its eggs under the bark of suitable spruce trees. If we get two beetle generations per year the beetles will also attack trees in late summer. Two attack periods each summer will most probably result in an increased number of trees being destroyed. Since spruce trees appear to be more susceptible to attack in the late summer, the scope of the problem becomes even greater. Research carried out at the Norwegian Forest Research Institute in the 1980s indicated that spruce trees are much more susceptible to attack in July-August than in May-June.

Powerful storms

Climate change scenarios suggest that we may see an increase in the frequency of powerful storms in the future, although there is still much uncertainty surrounding this. Large-scale windfellings by such storms often trigger bark beetle outbreaks, as observed after the recent storms in Sweden and Slovakia. Simulation models suggest that an increased frequency of storms may result in more frequent but short-lived bark beetle outbreaks. In addition, parts of Norway that so far have been spared of bark beetle outbreaks may experience outbreaks in the future. Areas with much old spruce forest, such as Trøndelag in mid-Norway, may be particularly exposed.

What can we do?

Although future climate change may give more frequent bark beetle outbreaks it is possible to reduce the risk by optimizing the way we manage our forests. However, in order to develop silvicultural practices for a future world with two beetle generations per summer we need improved model tools and more knowledge about the links between forest stand structure and outbreak risk. Such enhanced tools and insights will be important contributions to successful climate adaptation for Norwegian forestry.

Photo: Lars Sandved Dalen, Norwegian Forest and Landscape Institute

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