Overwintering of Eight Toothed Spruce Bark Beetle (Ips typographus) in Spruce Forests of North Velebit

The eight toothed spruce bark beetle (Ips typographus) plays an important role in spruce forests of Croatia, in the same way as it does in the rest of Europe wherever spruce grows. The size and intensity of its outbreaks is considerably lower though, and compared with the majority of countries scoring huge timber losses, Croatian forestry in general is much less affected, both economically and ecologically, by this bark beetle. Mixed tree species composition, natural origin and uneven age structure of spruce stands surely adds to the less pronounced and sporadic outbreaks in the Croatian territory. The recent rise of bark beetle populations in general, and above than average attacks of this bark beetle in the central parts of Dinaric mountain chain, initiated some new, targeted research projects.In the northern part of the Velebit Mountain, in the spring of 2008, a field trial was set up aimed at investigating the overwintering strategy of the eight toothed spruce bark beetle on two experimental plots in different elevation zones. The core idea was to test the already described phenomenon of different hibernation niche choice within the northern dwelling populations (predominantly choosing forest litter) and southern populations (staying within their growing niche, under the spruce bark). The general objective of the research and the expected outcomes fall within the area of sanitation felling and mechanical procedures in a wider concept of integrated bark beetle suppression tactics that could be used by Croatian foresters.Each experimental plot was characterized by the freshly attacked group of spruce trees (last summer – second generation of spruce bark beetle). The aerial distance between the lower tirova a plot (1080 meters a.s.l.) and the higher Zavi an plot (1404 meters a.s.l.) was 17.5 kilometers ensuring that the two were well separated in space. The altitudinal difference of 324 meters might not seem striking but climatic conditions in the area of experimental plots are quite different, notably in terms of temperature profile and height and duration of snow cover. On each experimental plot a set of forest floor eclectors was set up within the vicinity of standing spruces hit by the last summer bark beetle attack. Also, on each of the plots, one tree–mounted cylindrical eclector was wrapped around the lower tree trunk of a standing spruce, collecting beetles emerging from the bark (details in Table 1, Fig. 2). In the close vicinity of the plots a pheromone trap was installed (commonly used Theysohn type trap baited with Pheroprax aggregati