In the municipality of Östhammar, Uppland, an infestation of spruce bark beetle on living contorta pines has been reported for the first time in Sweden. Two forest areas with dead contorta pines have been discovered, raising concerns at the Swedish Forest Agency.
Patrik Karlsson, an expert on insect and fungal damage at the Swedish Forest Agency in Uppsala, expresses his concern: "It is worrying that the spruce bark beetle is now attacking the contorta pine. We urge forest owners in Uppland to pay extra attention to dead trees and contact us."
Near Öregrund, on the coast of Uppland, two stands of 30-year-old contorta pine have been affected. These stands, which are about half a hectare in size, are 500 metres apart. In one stand, about 10 trees have died, and in the other about 25. Interestingly, there are no signs of infestation in neighbouring spruce stands or on common pine.
Despite the fact that the infested trees are dead, spruce bark beetles have had difficulty reproducing in the contorta, which is consistent with previous studies. There are also previous cases where the spruce bark beetle has attacked and killed pine.
A similar phenomenon was observed in 2015 in Mellannorrland on storm-felled contorta pine, but infestations on standing, living contorta are something new for the Swedish Forest Agency.
"We are puzzled why the spruce bark beetle chose these trees over the neighbouring spruces. We are now working to gather more information and map the extent of the infestation," says Patrik Karlsson.
The smaller spruce bark beetle, which is common in Finland and Norrbotten and favours contorta, has not been observed in the area. The soil in the area is medium moist with elements of rocky outcrops, and the level of infestation is not higher than in other places in Uppland.
Following the 2018 drought, spruce bark beetle infestation in Swedish forests increased significantly, with an estimated 32 million cubic metres of spruce damaged between 2018 and 2022. The contorta pine, which accounts for five percent of Sweden's forest area, is a North American tree species introduced to Sweden because of its faster growth compared to ordinary pine. In September this year, a pest fungus was also discovered on contorta in Gävleborg, which had never been observed before.
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