The European Pine Sawfly (Neodiprion sertifer Geoffroy) is one of the most serious defoliators of Scots pine (Pinus sylvestris L.) in northern Europe. We studied the pattern in the regional occurrence of the outbreaks of N. sertifer in Finland in years 1961-90, and made predictions about the outbreak pattern to the year 2050 after predicted winter warming. We tested whether minimum winter temperatures and forest type and soil properties could explain the observed outbreak pattern. We analysed outbreak patterns at two different spatial levels: forest board- and municipal-level.
The proportion of coniferous forests on damage-susceptible soils (dry and infertile sites) explained a significant part of the variation in outbreak frequency at small spatial scale (municipalities) but not at large spatial scale (forest boards). At the forest board level, the incidence of minimum temperatures below -36 °C (= the critical value for egg mortality) explains 33% of the variation in the outbreak pattern, and at the municipal level the incidence of cold winters was also the most significant explaining variable in northern Finland. Egg mortality due to cold winters seems to be the most parsimonious factor explaining why there have been so few N. sertifer outbreaks in northern and north-eastern Finland. We predict that climate change (increased winter temperatures) may increase the frequency of outbreaks in eastern and northern Finland in the future.