The common pine sawfly – a troublesome relative

By Paal Krokene, Norwegian Forest and Landscape Institute

The red pine sawfly is well known for its widespread outbreaks in Norway’s pine forests. The common pine sawfly is, despite its name, a less common species in Norway today, but may become more widespread if the climate turns warmer and drier.

Sometimes a name may be misleading, as for the common pine sawfly (Diprion pini), which in fact is less common than its relative, the red pine sawfly (Neodiprion sertifer). And for that we should be thankful, as outbreaks of the common pine sawfly tend to be much more damaging to pine trees. This is because the larvae of the common pine sawfly continue to eat pine needles into the late summer and autumn, leaving the pine trees completely stripped of needles as winter arrives. This results in much higher mortality rates and much greater losses for forestry.

Similar to the red pine sawfly, the common pine sawfly attacks most pine species growing in Norway, and it attacks trees of all ages. Attack by the common pine sawfly also increases the risk that other insect pests, such as pine shoot beetles (Tomicus), will attack the trees. Although the common pine sawfly may be found throughout Norway as far north as Vega in Nordland, to date it has only had sporadic and local attacks.

Finland has had several large-scale outbreaks of the common sawfly. Between 1998 and 2001, its larvae stripped the needles off 500,000 hectares of pine forest – the largest insect outbreak ever recorded in Finland. Attack by the common sawfly can result in extremely high mortality rates. In Lauhanvuori National Park in south-west Finland, 75% of the trees in certain diameter classes died in the years following a severe outbreak. Climatic conditions appear to be the main reason why Finland experiences much stronger attacks than Norway, by both the common pine sawfly and the red pine sawfly. Warm summers with little rainfall provide perfect conditions for these insects. As climate change is expected to make such conditions more common in South Norway we may see an increase in damages by the common sawfly. So in the future the common sawfly may actually become more true to its name than it is today.

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Photo: Norwegian Forest and Landscape Institute