The pine tree lappet – a greedy needle feeder

By Bjørn ØklandNorwegian Forest and Landscape Institute

The pine tree lappet has had serious outbreaks in Norway in the past but may cause more frequent damage in a changed climate. Long periods of drought may trigger outbreaks of this species.

The pine tree lappet (Dendrolimus pini) is an example of a species which is already common in Norway, but may become more expansive in the event of climate change. This large butterfly eats pine needles during its larval stage, and hibernates twice before pupation. The larvae move down the stem and into the ground to hibernate. Early in their third summer they will have reached full size and be ready to pupate, emerging shortly after as a fully grown butterfly. The larvae eat a vast amount of needles during their development, particularly during the second summer when they have the largest appetite. Pine trees can withstand losing most of their needles if this is restricted to one season. If repeatedly stripped of needles however, the trees will die.

Rare but extensive outbreaks in the past

Large outbreaks of the pine tree lappet occurred in Elverum and Løten from 1812-16 and from 1902-04. The most recent outbreak stretched from Åsnes in the south to Stor-Elvdal in the north, and from the Swedish border in the east to Romedal and Løten in the west. During the outbreak, local forest owners applied rings of glue to the tree trunks to trap the larvae as they moved up or down the trunk from their hibernation sites in the ground. The forest owners sought governmental funding for the gluing operation, but their application was rejected. Subsequently – after the outbreak had been combated – they received compensation for their expenses. More than a hundred years later, you can still see the marks of these glue rings on some tree trunks.

Potential for mass destruction

Together with its relatives Dendrolimus sibiricus and Dendrolimus superans the pine tree lappet has caused damage and death to trees over vast forest areas in Russia, China and East Asia. During a 30-year period, these insects damaged or destroyed trees over an area corresponding to 84% of the total land area of Norway. The pine tree lappet thrives on dry pine barrens on moraine, and outbreaks seem to be triggered by drought. During the two summers preceding the 1902-04 outbreak water levels in the Glomma river had been unusually low. Long periods of drought may therefore be the most important climatic factor triggering outbreaks of this species.

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Picture: Pine tree lappet (Dendrolimus pini) (Photo: Vladimir Kononenko, Norwegian Forest and Landscape Institute)