The climate change may indirectly affects the forest ecosystems through the activity …

• Abstract
• Climate change and insect distribution
• Climate change and host-insect interactions
• Box 1 - The pine processionary moth Thaumetopoea pityocampa
• Box 2 - The spruce webspinning sawfly Cephalcia arvensis
• Box 3 - The larch bud moth Zeiraphera diniana
• Acknowledgements
• References

The outbreaks of the spruce web-spinning sawfly Cephalcia arvensis in the Southern Alps are a good example of what may happen when favourable climatic conditions interfere with the mechanism of the induction of extended diapause, allowing an exponential growth of the population and consequent damage to trees. This species, as many others in this genus, is monophagous on Picea and endemic to the spruce range in Eurasia, where outbreaks have been seldom recorded ([37]). Cephalcia species generally show low fecundity and spreading of the cohort over many years by mean of an extended diapause, which is induced by low temperature at pupation time ([5]). However, in the period 1985-1992 there was a sudden outbreak in the Southern Alps (Asiago and Cansiglio Plateaux - Fig. 2), during which the populations developed an annual life cycle and grew exponentially, causing repeated defoliations, which ultimately caused tree death over hundreds of hectares ([6]). The most likely reasons for such a change in the life cycle of the insect have been explored through an analysis of the local climate, which showed that the years preceding the outbreak were characterized by an abnormally warm and dry weather during the feeding period of the larvae. We hypothesized that favourable conditions promoted the survival and speeded up the development, making it possible to pupate when soil temperature was high enough to start pupation immediately, skipping in this way from the extended diapause. Later, an experiment showed that the soil threshold temperature for the induction of the extended diapause was about 12°C ([5]), well below the values recorded in the forest at the beginning of the outbreak. The sudden increase of the population density was not quickly followed by that of natural enemies, which were unable to limit population growth ([6]). It seems likely that the increase of the temperature in June and July of 1983-85 is the major factor promoting the outbreaks, as they occurred simultaneously at two sites at a distance of about 100 km, through switching the insects to an annual generation.