07.03.2023 | Beate Kittl/Sophie Stroheker/Simon Blaser | News WSL
*** Correction from 2.5.2023: In the 3rd paragraph, it now reads "Since in western Switzerland in particular only 60 percent of the normal precipitation fell regionally..." instead of "Since in western Switzerland in particular up to 60 percent less precipitation fell than in normal years...". ***
The European spruce bark beetle (Ips typographus) benefited from the warm and dry weather last year. Three percent more infested wood than in 2021 had to be used and the number of reported infestation sites rose by around 1000 to over 8000 nationwide, shows the annual review of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.
2021 had given forestry operations a brief respite: The cool and damp weather was unfavourable for the European spruce bark beetle - the economically most important bark beetle species in Switzerland. In 2022, however, the salvage logging of beetle wood in summer increased again in 14 cantons, the most in Vaud (+125 %), Neuchâtel (+95 %) as well as in the canton of Zug (+77 %). Compared to the previous year, salvage logging in summer decreased sharply in the cantons of Jura and both Basel (-60 % each), as well as in the cantons of Appenzell Ausserrhoden and Lucerne (-45 % each).
The number of infestation sites reported by forestry operations rose more sharply. Once again, the cantons of Vaud and Neuchâtel are in the lead (Vaud: +137%, Neuchâtel +107%). "Last year offered exceptionally good development and flight conditions for the beetles," says Simon Blaser of Forest Protection Switzerland, the WSL's specialist and advisory office for forest health. With 1.6 °C above the norm from 1991-2020, 2022 was the warmest year since measurements began in 1864. In part, this enabled the beetles to establish an additional generation, as model calculations show (www.borkenkaefer.ch, in German or French).
Forest Protection Switzerland explains the increase in infestation sites, among other things, by the pronounced drought in many regions from June to August 2022. "It can be assumed that the spruce trees suffered from drought stress, at least in some areas, which may have reduced their defense potential against bark beetle attacks," says Blaser. Since in western Switzerland in particular only 60 percent of the normal precipitation fell regionally, the high increase in salvage logging and infestation sites in the cantons of Vaud and Neuchâtel may be partly due to this.
Severe hail damage
Another cause for the stronger reproduction of the spruce bark beetle is severe hail damage to forests in many regions of Switzerland in June 2021. Among others, spruce stands in the cantons of Bern, Fribourg, Neuchâtel, Schwyz and Zug were affected. Hailstones with diameters of up to 7 cm and strong gusts of wind caused bark flaking, windthrows and weakened or destroyed entire spruce stands. Where the damaged wood could not be quickly removed, it provided ideal conditions for the spruce bark beetle to multiply.
The increase in infestations suggests that spruce infestations will rise again in 2023. Especially if the weather continues to be warmer and drier than average. With climate change, it can be expected in the long term that spruce will be increasingly infested by spruce bark beetles outside its natural range - which in Switzerland lies in higher altitudes. This development will make forest conversion necessary, leading away from pure spruce stands.
In order to optimally support forestry practice in the planning of bark beetle management, WSL is currently working on a further development of the Bark Beetle Simulation Online (BSO, www.borkenkaefer.ch, German or French).
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