Climate, Soils and Cephalcia-arvensis Outbreaks On Picea-abies In the Italian Alps

The recent outbreaks (1986-1992) of the spruce web-spinning sawfly Cephalcia arvensis Panzer (Hymenoptera, Pamphiliidae) extended over 1500 ha of spruce (Picea abies Karst.) stands in two areas of the Venetian Pre-Alps (north-east Italy). Among the possible causes of these outbreaks, the climatic and pedological factors seem to predominate: (1) the period 1983-1986 showed a hot and dry climate in both areas, especially during the developmental time of the pest (June-July); (2) severe water stress occurred in the same years preceding the outbreaks; (3) inside each area the insect attack was irregularly distributed, reflecting the variability in soil and stand features. The relationships between climate and attack levels could be interpreted in two complementary ways: (1) lower mortality and faster development of the insect induced by favourable climatic conditions (high temperatures and lack of precipitations); (2) increase of food quality as a consequence of the water stress suffered by the trees, according to the soil and stand characteristics.