THE SIX-SPINED SPRUCE BARK BEETLE

The six-spined spruce bark beetle (Pityogenes chalcographus L.) is two millimetres long dark brown bark beetle. It is our most common bark beetle species, and it occurs throughout the country. Unlike the European spruce bark beetle that uses Norway spruce as a host, the six-spined spruce bark beetle can also use other hosts like pines (Pinus), fir (Abies) or larch (Larix). Six-spined spruce bark beetle often appears together with the European spruce bark beetle. The presence of the European spruce bark beetle may increase the reproductive possibilities of six-spined spruce bark beetle.

Young, 25 to 45 years old spruces are optimal hosts to the six-spined spruce bark beetle together with logging waste and the canopy of a large standing tree. After overwintering as a larva, a pupa or an adult, the six-spined spruce bark beetle reproduces under the thin bark. Fine, brownish dust at the base of the tree trunk is a sign of the bark beetle infestation. Mother galleries range from 4 to 8 per mating chamber. The gallery of the six-spined spruce bark beetle therefore resembles the shape of a star. The mother galleries and later the larval galleries affect the fluid flow of a tree that causes the drying of the canopy. The bark comes off easily and the galleries can be seen in the surface of the trunk.

Swarming happens from the end of May till the end of July when the temperature is above 18 degrees Celsius. Studies show that the six-spined spruce bark beetle is able to swarm in temperature up to 35 degrees Celsius and it has higher cold tolerance compared to the European spruce bark beetle. Thinning temporarily lowers the resistance of the trees left to the plot, therefore thinning should not be done in young spruce dominant plots at the time six-spined spruce bark beetle is swarming.

References

Hedgren, P. O. 2004. The bark beetle Pityogenes chalcographus (L.) (Scolytidae) in living trees: reproductive success, tree mortality and interaction with Ips typographus. Journal of Applied Entomology, 128(3), 161–166.

Kuusentähtikirjaaja, Metsäinfo, Luonnonvarakeskus. Viitattu 15.8.2024. https://metsainfo.luke.fi/fi/cms/opas/tuhonaiheuttajaluettelo/kuusentahtikirjaaja. (In Finnish).

Schebeck, M., Schopf, A., Ragland, G., Stauffer, C., & Biedermann, P. 2023. Evolutionary ecology of the bark beetles Ips typographus and Pityogenes chalcographus. Bulletin of Entomological Research, 113(1), 1–10.

Terhonen,E. ym. 2023. Metsätuhot vuonna 2022. Luonnonvara- ja biotalouden tutkimus, Luonnonvarakeskus. Viitattu 15.8.2023. (In Finnish).

Uotila, A., Kasanen, R. & Heliövaara, K. 2020. Metsätuhot. Tapio. 206 s. (In Finnish).