Bird-cherry ermine: Ghost trees on the march

By Paal Krokene, Norwegian Forest and Landscape Institute

The bird-cherry ermine is a leaf-eating butterfly which has become increasingly prevalent over the last decade. While mass attacks previously were confined to Eastern Norway, recent years has seen this butterfly spread its wings northwards.

From time to time, many trees across Southern Norway are covered by a white silk web. Sometimes the entire tree trunk is covered in such an elaborate web that the tree resembles a mummified ghost tree. Other times, you will find small silk nests at the tip of the branches. This is the work of the bird-cherry ermine (Yponomeuta evonymella) – a small butterfly with a wing span of 20-25 mm. The bird-cherry ermine flies in the summer and lays its eggs on the branches of the bird-cherry tree. The eggs hatch in the autumn and the tiny larvae overwinter in the tree crown. The next spring they search up the flower buds and eat the leaves which are about to emerge. Later in the summer, they eat the grown leaves protected under a silk web which can cover twigs, branches or entire trees.

Historically, the bird-cherry ermine was most common in Eastern Norway and the largest numbers were registered south of Dovre (about 62˚25' N). Over the past 10 years however, there has been an increasing number of reports of mummified bird-cherry trees further north. During the most recent outbreaks, in 2006 and 2008, Trøndelag in mid Norway suffered as much as Eastern Norway. In 2007, the bird-cherry ermine was registered as far north as 66˚10' northern latitude in Northern Norway, the northernmost find ever made in Norway.

The bird-cherry tree is widespread throughout Norway, all the way north to Hammerfest (70˚40' N) and high up into the mountains. The distribution of the host tree is thus much wider than that of the bird-cherry ermine, which seems to be restricted by climate. Minimum winter temperatures are particularly critical, as the hibernating larvae cannot survive temperatures lower than -20°C. If the climate becomes milder, the bird-cherry ermine will probably continue to extend both northwards and to higher altitudes. This development provides little cause for concern, however, since bird-cherry trees are of little economic importance and the trees generally survive even complete and repeated defoliation by the bird-cherry ermine.

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Picture 1: The bird-cherry ermine has made its mark in recent summers, with vast numbers over large parts of South Norway. (Photo: Dan Aamllid, Norwegian Forest and Landscape Institute9

Picture 2: The bird-cherry ermine is a frail, beautiful insect with white forewings featuring black dots. (Photo: Kai Berggren, Norwegian Forest and Landscape Institute)

(Pictures can be provided as high resolution)

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