The nun moth: Nuns heading north?

By Paal Krokene, Norwegian Forest and Landscape Institute

Many insects are going to extend their range northwards if the climate grows warmer. This may result in the introduction of new and destructive species to Norwegian forests. One of the species we have most reason to fear is a beautiful moth with an innocent-sounding name.

A nun we’d rather not see more of

The nun moth, or simply the nun as it is often called, is a butterfly. It is quite common in South Norway today but does not cause any damage. Farther south in Europe however, it is one of the worst insect pests in coniferous forests. The species’ core area is Poland and Germany, but you do not have to go farther than Sweden to find forests damaged by the nun.

The nun is a generalist species, and its larvae may feed on oak, birch, hornbeam, linden and hedge apple in addition to pine and spruce. However, destructive mass attacks tend to be limited to pine and spruce. The spruce is particularly vulnerable to attacks, as it is much more sensitive to needle loss than pine. As a rule, a spruce tree will die if it loses 50-80% of its needles, while pine trees can easily survive 95% needle loss or more. Trees weakened by nun attacks will also be more vulnerable to subsequent attacks by, for example, the spruce bark beetle or the pine shoot beetle.

Night-time flights ...

The nun flies at night during August and lays its eggs in the bark on tree trunks and branches of pine and spruce. One female can lay 100-200 eggs. The larvae hatch in May when the new shoots begin to expand, and feed on flower buds, shoots and needles up in the tree crowns. In July, the larvae pupate in a loose web on trunks or branches, and the new generation hatches a few weeks later.

In Norway, the nun is most prevalent along the south coast from Kristiansand to the Swedish border, but has been found farther inland. The northernmost find was at Starmoen near Elverum. All insect pests have a distribution area or range which is much larger than their outbreak area – the area where they occur in large enough numbers to cause economic damage. In the northern end of their range, such species are able to survive and reproduce despite the marginal living conditions, but never become numerous enough to cause damage.

Nuns on the move?

Model calculations from Finland indicate that if summers become 3-4 degrees warmer than today the distribution area of the nun may move 500-700 km further north. If the nun’s range expands several hundred kilometres to the north, the outbreak area may expand correspondingly. Such an expansion may result in mass attacks by the nun in the central spruce forests of South and East Norway by the end of this century.

In the worst case scenario, this means that we could risk similar nun outbreaks in Norway as those experienced by Poland in the early 1980s, when 25% of the country's forest area was attacked and 6.5 million hectares were sprayed with pesticides.

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