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Warmth brings more pests and pathogens to Finland's forests

Unfortunately, many of the species that pose the worst threat to Finland's forest environment love the prolonged heat.

Bark beetles may be small, but they have strength in numbers Image: Erkki Oksanen / Metla
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European spruce bark beetles (Ips typographus) have become so prevalent in the southern forests of Finland that most of the spruce trees there have been destroyed.

Antti Pouttu, a researcher with the National Resources Institute of Finland, says that climate change has been a real boon for the pests, who reproduce in the inner bark of the trees when conditions are right.

Once the larvae hatch, they feed and pupate under the bark, spreading a species of blue stain fungus that kills healthy trees by cutting off the water supply. The fungus also stains the wood with blue streaks, stripping it of its commercial value.

"They usually don't make their homes in the best trees, but this year has given them plenty of dried out ones to choose from. This of course contributes to their numbers rising," Pouttu says.

He says there will likely be two or more generations of the insects born this summer because the weather has stayed so warm.

"Most of the old growths of spruce have already been harvested in areas south of the Salpausselkä ridge because of the beetle. Forest owners have wanted to salvage as much of the healthy trees as possible. Trees infested with the bark beetles cannot be used as timber for paper and pulp making, so they are burned for energy instead. This means their selling price falls considerably," Pouttu says.

He says humans have also contributed to the problem. Landowners are too fastidious about removing trees that have been felled by storms, for example, so pests make their homes in the healthier trees that are still standing.

Pine sawflys, nun moths and fungi

But Antti Pouttu says the bark beetles aren't the only pests and pathogens that stand to endanger Finland's forests as the summers grow longer.

"For example, there have been large epidemics of the common pine sawfly (Diprion pini) from time to time. It seems they are also growing in number and moving north," he says.

A new forest pest that wasn't as well known in Finland until now is the black arches or nun moth (Lymantria monacha).

A nun moth can sport a five-centimetre wing span. Image: Reima Leinonen / Ely-keskus

"The moth's larvae eat spruce as a rule, but they can eat anything in theory – even blueberries. It was a very rare sight in Finland still in the 1970s, but now it is a common sight in areas south of the Vaasa-Kuopio line," Pouttu says.

He says that between the nun moth and the bark beetle, things aren't looking good for the future of coniferous trees in Finland.

"Then there are the decay fungi that will spread easily because the ground doesn't necessarily freeze solid in the wintertime anymore."

Species in the forest decay fungi Heterobasidion genus can carry several diseases. The mushroom-like growths cause large-scale destruction of forests with trees that host them.

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