The spruce bark beetle’s sibling broods are often established in June when the spruce has the poorest defensive capacity against attack. Researcher Petter Öhrn has studied bark beetles for several years and is now publishing his thesis.
Photo: Elin Fries/Bitzer, Illustration: Alvilda Öhrn
Petter Öhrn’s doctoral thesis at SLU shows that the spruce bark beetle’s sibling broods are often established when the trees have the poorest defensive ability against attack, namely when the spruce grows most in June. This makes it particularly important for the forest owner to find fresh infestations during that period. The thesis also shows that the spruce’s defences are weakened by drought and that it takes a long time for the spruce to recover from periods of poor water availability. Drought in the previous year also results in poorer defences the following year. With ongoing climate change, knowledge of threshold values for both the development of the spruce bark beetle and the drought stress of the trees is crucial to better predict the extent of spruce bark beetle outbreaks.
How bark beetles attack the spruce
In recent years, carbon sequestration in European forests has decreased as a result of a changing climate that has led to extensive drought and outbreaks of bark beetles. The spruce bark beetle is an approximately 5 mm long beetle that drills its way through the bark of spruce trees (Picea abies) to reproduce there. Mass infestation, i.e. many penetrations in a short period of time, is made possible by fragrances that allow the bark beetle to overcome the defenses even in living trees. When the trees are stressed by drought, the resistance can be reduced and fewer penetrations are needed to kill the tree, which is necessary for the spruce bark beetle to be able to reproduce in the bark.
To help it, the spruce bark beetle has fungi that attack the tree’s defense system. After the extreme drought that hit large parts of Europe in 2018, including Sweden, the spruce bark beetle has attacked and killed hundreds of millions of cubic meters of spruce.
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The spruce’s defensive ability varies
Removing infested trees before the new generation leaves the trees is the best known method for reducing the number of bark beetles. Knowledge of when the spruce bark beetle attacks trees and to what extent the trees can defend themselves is important knowledge for practical forestry. The spruce’s defensive capacity can probably vary over the summer depending on water availability but also on the growth phase the spruce is in, as some phases are very energy-intensive and leave less energy left over for defence against bark beetle attacks.
Flying habits and fungal growth provide important information
The aim of this thesis was to provide more knowledge about when the bark beetle flies and when it attacks spruce trees in southern Sweden and how the spruce’s defense ability against attack varies over the growing season depending on water availability and growth rhythm.
Traps grazed with fragrances (pheromones) were used to determine the flight activity of the spruce bark beetles during the summer. Felled trees that have been attacked by spruce bark beetles during three periods were used to determine 1) when overwintering beetles start to fly in the spring,
2) the time when the overwintering parent beetles leave their first brood to fly to new trees and to establish a new brood of spruce bark beetles there (so-called sibling litter), and 3) when hatching of the new generation starts and how this is affected by temperature.
The spruce’s ability to defend itself against attacks was investigated by measuring the growth of a fungus carried by the spruce bark beetle. A culture of the fungus was injected into the bark and how much the fungus grew was measured during different periods during the growing season at three locations in 2019 and 2021. A large growth of the fungus in the outer wood is assumed to correspond to a low defence against the spruce bark beetle.
The spruce bark beetle began to fly at the end of April when a first brood was established. Construction of a second litter (sibling litter) began at the end of May. Continued high flight activity later in the season showed that sibling litters were common. The new generation began to leave the infested trees at the end of June to begin a second generation.
Conclusions
The spruce’s defences were on average lower early in the season (June) than later, but varied with both the amount of precipitation and local soil moisture conditions. Defence was higher in 2021 than in 2019, indicating a recovery after the severe drought in 2018. For both years, there was a clear correlation between the trees’ defensive capacity and the availability of water the previous autumn.
In the prevailing climate in southern Sweden, spruce defences are at their lowest during the season in June, when attacks from sibling litter-warming spruce bark beetles are most common. This means that during this period, it is particularly important for the forester to identify fresh infestations so that infested spruce trees can be quickly felled and taken care of.
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