The scarce umber – butterfly leapfrogging northwards

By Bjørn ØklandNorwegian Forest and Landscape Institute, and Arne C. NilssenUiT The Arctic University of Norway

The scarce umber has traditionally been a southern species in Scandinavia, but has recently had a dramatic northward expansion. Will it become yet another moth that is feeding on Norway’s northern birch forests?

The scarce umber (Agriopis aurantiaria) is a familiar insect in southern Norway. This golden triangular butterfly is a common sight on house walls when the outdoor lights are turned on in the autumn. In Norway the species is also known as the golden-yellow winter moth, which is a suitable name given the colour of the males and the meaning of the species’ scientific name (aurantiaria = golden). The females, however, are darker in colour and are almost wingless. They cannot fly and move around by crawling along the stem and branches.

The scarce umber belongs to the family Geometridae or geometer moths, so called because the larvae appear to measure the trees as they crawl along in a looping fashion. The larvae also have a special talent for camouflage. If disturbed they freeze in position, with their bodies stretched out in the air, giving the appearance of a small twig.

The scarce umber feeds on several deciduous tree species in southern Norway and elsewhere in Europe. In Alf Bakke's writings on forest insects in Norway from the 1960s, the scarce umber is mentioned as a potential pest insect in the oak forests along Norway’s southern coast, between 58 and 59˚ northern latitude. Until recently, we only had a few scattered observations of the species further north, in Nordland and Troms counties.

It was therefore quite a surprise when large numbers of scarce umber were observed at almost 70˚ north in Tromsø in 2004. In the following years, the species occurred in masses on birch, rowan and roses throughout Troms County. This dramatic northward expansion is being closely monitored by a group of ecologists at the Arctic University of Norway, seeking to understand how climate has influenced the distribution of the scarce umber and other moths in the north (see www.birchmoth.com). Perhaps the scarce umber will become a new “lauvmakk” or leaf-eating worm, helping itself to our northern birch forests?

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Photo: Karsten Schnack, Scanpix Danmark