In 1991, severe damage to the foliage of the birch Betula pubescens L. forest was observed along the north Norwegian coast in the county of Nordland. Many leaves were wilted. Between the living, leaf-carrying shoots (short and long shoots; for definitions, see Maillette 1982) many short shoots were dead, giving the foliage a sparse appearance. Apparently, the damage had accumulated for some years. In 1992, similar damage was observed at Alta Fjord, about 400 km NE of the first area (Solberg et al. 1994). In both instances, the attacking insect was larval Argyresthia retinella Zell. (Lepidoptera, Yponomeutidae), mining and killing the buds of birches (Johansen & Kobro 1996). This species is indigenous to Norway and the rest of Fennoscandia (Svensson et al. 1987) but was not previously known as an outbreak insect in Scandinavia or elsewhere.
Birch forest is the dominant natural ecosystem in northern Norway. Its significance as a source for building material, fuel and fencing, and for shelter, reindeer husbandry, game hunting, recreation and tourism is fundamental. The seriousness and seeming novelty of this outbreak and its coincidence with a sequence of years (1987–93 1995) with mild (see Vaer 1989–95) and short (cf. Myneni et al. 1997) winters make it interesting in several respects, including its possible dependence on climate variability and future climate-dependent incidence.
Here, we report on the geographical distribution of the damage caused by A. retinella, changes in attack intensity over years, compensatory reactions and structural changes of birch foliage, and on the climatic background of the outbreak. Occurrence of earlier outbreaks in the region and the dependence of outbreaks on climate variability, natural or anthropogenic, are discussed. We wish to call the attention of forestry authorities to this threat to the forest.