The pine sawyer beetle – the deadly messenger

By Bjørn Økland, Norwegian Forest and Landscape Institute

The pine sawyer (Monochamus sutor) is a moderately large (2 cm long) longhorn beetle with impressive antennae. It lives on both Scots pine and Norway spruce throughout most of Norway. The larvae tunnel deep into the wood and can cause severe technical damage to timber. Despite this, the pine sawyer is not considered an important forest pest in Norway. It usually only attack dead or dying trees, or live trees that are weakened by for example forest fire. The main reason for the pine sawyer’s notoriety is that it is a potential vector for the pine wood nematode (Bursaphelenchus xylophilus), if this was ever to arrive in Norway. Despite its microscopic size (< 1 mm), the pine wood nematode can cause severe damage to forests, as infected trees quickly wilt and die. The nematode hitches a ride with the insects and depends upon longhorn beetles in the genus Monochamus to spread between trees.

In Japan, where the pine wood nematode was accidentally introduced more than 100 years ago, it is considered the most damaging tree killer in conifer forests. The nematode also causes significant losses in other countries where it has been introduced, such as China, Korea and Taiwan. In Europe, the pine wood nematode was first detected south of Lisbon, Portugal in 1999. The insect vector in Portugal is the black pine sawyer (Monochamus galloprovincialis), which also occurs in some parts of south-eastern Norway. Despite extensive countermeasures, the nematode has spread to other areas in Portugal and across the border into Spain. Norway and many other European countries have monitoring programs in place to detect if the pine wood nematode should arrive. If the pine wood nematode gets established in Norway the economic consequences could be severe. This is partly because the nematode might kill trees, but most importantly because all export of timber from infested areas would be prohibited. There have been plans for dramatic countermeasures if the nematode were to be found in Norway. One proposal was to destroy all potential host trees within a 28 km2 circle around the site where the nematode is found! However, simulation models have suggested that even such extreme measures are unlikely to prevent the spread of the nematode, and the plans have been abandoned.

Climatic conditions seem to determine the risk of widespread tree-killing by the pine wood nematode in Europe. Swedish studies have shown that the current climate in Scandinavia tends to be too cold for the pine wood nematode and that the nematode therefore will not induce wilt disease in our area. Still, the nematode can probably survive in dead timber, and it may also be present in live trees for many years without causing wilt. A warming climate can dramatically amplify the impact of the pine wood nematode in Norway because it will both increase the incidence of tree-killing and facilitate the introduction of new and more effective beetle vectors.

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Photo: Steinar Melby, Norwegian Forest and Landscape Institute

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