An insect (Argyresthia retinella, Lep., Yponomeutidae) outbreak in northern birch forests, released by climatic changes?
In the early 1990s, birch (Betula pubescens) forests in north-western Norway were damaged by the bud- and shoot-mining larvae of Argyresthia retinella not previously known for outbreaks. In 1993-96, the outbreak was mapped and changes in attack intensity and foliage structure were quantified by sampling birch twigs along transects from coast to inland. Results were considered in relation to a variable climate. The outbreak extended 400 km along the coast, mainly within the lowest 100-150 m a.s.l. It started in the late 1980s and the attack intensity culminated in 1993-94. Repeatedly, 30-50% of the leaf-carrying shoots were damaged or killed. Trees compensated by sprouting clusters of shoots from undamaged shoots and, hence, increased foliage clumping. Eventually, twigs and branches died. The overwintering post-diapause egg, the larva and the adult stages were considered the most sensitive to changes in temperature climate. The air temperature, averaged for a combination of these stages, was calculated for each year, as well as deviations from the long-term (1868-1996) average. The 5-year running average of deviations approached or exceeded + 1°C three times: in the 1930s, around 1960 and around 1990. The third peak coincided with the outbreak of the 1990s. There are no reports of an A. retinella outbreak in either the 1930s or 1960s. However, in one photographic documentation from 1940, clumped foliage structure of birch suggests an outbreak in the 1930s. Severe A. retinella attacks appear to be a fairly new phenomenon, possibly connected to recent high temperature deviations. If so, with the present air temperature climate, outbreaks may occur at intervals of ∼ 20-25 years. With a trend of decreasing air temperature, outbreaks may be less frequent. At higher temperatures, natural or from anthropogenic global warming, the outcome is more uncertain although more frequent outbreaks may occur initially. It is recommended that foresters learn to identify damage caused by A. retinella and include observations on this insect in their reporting. Monitoring A. retinella damage in the North and elsewhere, should contribute to the understanding of the outbreak ecology of this insect.