The double-spined bark beetle (Ips duplicatus) is native to Scandinavia, Eastern Europe, Russia and Asia, but for a number of years now, there have been increasing reports of its presence in Central Europe, including Germany and Austria. In addition to spreading by natural means, the beetle's dispersal further south is probably mainly due to the transportation of spruce logs by road and rail. The newly discovered beetle species mainly attacks Norway spruce.
Fig. 1 - The double-spined bark beetle (Ips duplicatus). Photo: Nina Feddern (WSL)
Biology
The development of the newly discovered beetle is similar to that of the European spruce bark beetle (Ips typographus), the most common species of bark beetles found on spruce trees in Switzerland and Liechtenstein. However, being adapted to lower temperatures, the double-spined bark beetle tends to take flight in spring earlier than the European spruce bark beetle. Furthermore, its larval development is also slightly faster, and the second generation of beetles usually leaves their host trees in autumn and overwinters in the soil litter. Its rapid development could enable this species to produce a third generation in hot and dry years at low altitudes, which would greatly accelerate the population's development and increase the risk of infestation.
The two species of bark beetle are direct competitors for breeding sites under the bark of spruce stems. However, it remains to be seen whether the double-spined bark beetle will cause additional infestations in the future and what the economic consequences of beetle infestations will be.
In northern countries, the double-spined bark beetle is considered less aggressive than the European spruce bark beetle. Although the double-spined bark beetle can also attack healthy trees, it seems to cause less damage in other countries than the native spruce bark beetle. An unanswered question is how the increasingly warm climate in Switzerland will affect the beetle's infestation behaviour.
Fig. 2 - Norway spruce bark with bore holes of the Nordic spruce bark beetle. Photo: Beat Wermelinger (WSL)
Bark beetle control is becoming more complicated
For now, WSL's entomologists recommend taking the usual measures to control bark beetle infestation during the warmer seasons. After discovering boring holes and boring dust on the bark and discolouring or dying crowns, infested trees should be felled and either removed in due time from the forest or debarked for storing. In winter, felling and removing trees from the forest is virtually ineffective against the double-spined bark beetle, because the majority of beetles spends the cold season in the soil litter. However, since the double-spined bark beetle has always co-occurred with the European spruce bark beetle, measures taken in winter may still reduce bark beetle infestations in spring.
The double-spined bark beetle and the European spruce bark beetle can hardly be distinguished by the naked eye. The two species also have very similar breeding galleries. The double-spined bark beetle, roughly 3-4 mm long, is usually a little smaller than the native spruce bark beetle. In addition, both species often colonise the same host tree, however, double-spined bark beetles prefer the mid- and upper tree trunk, whereas the spruce bark beetle colonise the lower part. The main characteristic differentiating the two species – which is only visible with a good magnifying glass – is that they have differently shaped spines on their elytral declivity at the posterior end of their body.
