DIGITALNA ARHIVA ŠUMARSKOG LISTA
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2004), and it plays an important role in Croatian spruce forests, which is also common for other part of Europe where spruce grows (Hrašovec et al., 2011). I. typographus is univoltine in northern Europe (Austarå and Midtgaard, 1986; Schroeder, 2013), and elsewhere in Europe at higher elevations. In central and southern Europe this beetles reproduces two times a year (Wermelinger, 2004; Faccoli and Stergulc, 2006; Jurc et al., 2006; Faccoli, 2009) and overwinters as an adult insect under the bark or in needle litter (Christiansen and Bakke, 1988; Weslien, 1992; Hrašovec et al., 2011; Wermelinger et al., 2012). Populations of I. typographus can remain endemic for a long period of time. In optimal conditions the populations may increase to epidemic proportions when growing trees are attacked and killed (Raffa et al., 2008). Large-scale disturbances, such as storms, e.g. Lothar and Vivian in Switzerland (Wermelinger, 2004), and Gudrun in Sweden (Schroeder, 2010) usually act as triggers of bark beetle outbreaks. I. typographus can quickly respond to the appearance of suitable hosts, resulting from favourable weather and stand conditions (Wermelinger, 2004), which happened in the mountain region of Croatia after the ice storm in the late winter of 2014 (Vuletić et al., 2014). The reproduction success of these insects usually differs between living and felled trees due to different host resistance (Elkin and Reid, 2004). For successful colonization of standing trees, beetle attacks need to overcome the defence system of host trees (Mulock and Christiansen, 1986). Living trees defend themselves against bark beetle attacks by a flow of resin-or through the mobilization of dead resin – rich cells around the infected sites to prevent further spread (Francheschi et al., 2000). The availability of these two types of substrate and beetle reproduction success determines their relative importance for the production of bark beetles at the landscape level (Hedgren and Schroeder, 2004). The reproduction rate is related to the intraspecific competition among larvae, particularly when bark beetles shifts from wind felled to live standing trees (Kommonen et al., 2011). A high bark beetle colonization density (maternal galleries per m2) usually leads to their low reproduction rate (Hedgren and Schroeder, 2004; Faccoli and Bernadinelli, 2011). However, some bark beetle species, precisely those finding additional feeding outside of the host bark can avoid this effect (Sauvard, 1989). Favourable weather conditions during summer usually allow I. typographus to start second or even third generation. In that case some individuals do not complete development before winter and some portion of offspring beetles are forced to hibernate as larvae or pupae. Winter mortality from freezing, especially in pre-adult stages (Faccoli, 2002), complex relationship between host trees and natural enemies (Christiansen et al., 1987), factors stressing trees or inducing variations in the natural enemy abundance (Weslien, 1992) could be an important determinant of population dynamics in I. typographus. The aim of this study was to research the influence of winter mortality and natural enemies on the reproductive success of bivotine populations of I. typographus with respect to their colonization density. MATERIAL AND METHODS Materijal i metode The study was carried out during winter 2014/15 in a 50-year-old spruce stand at 500 m a.s.l. in south-western Croatia (44˚36’49.41’’ N; 15˚19’13.89’’ E). Five spruce trees similar in size were felled and pruned in mid-July, and then left in a stand to be colonized by I. typographus. The mean diameter of trees was 29 cm (min-max = 27-32 cm), and medium tree height was 18 m. In the moment of felling all spruce trees were in good health condition without visible needles discoloration. Distance between felled spruces were 30 m to avoid the influence between them at the time of bark beetle colonization. Fist colonization were detected one week after felling. During January, all trees were first cut in 4 m long logs, and then transported to a storehouse where they had been stored at temperatures below 5 ˚C until undergoing analysis in February. For calculating the bark surface, the log diameter was to be measured first in the middle of each section. What need to be performed before debarking is counting exit holes per each section. During the bark analysis, the number of mating chambers and maternal galleries were counted first. Then the samples were carefully pulled apart, which was followed by counting of I. typographus callow beetles, pupae, the biggest larvae (third larval instar) and predator larvae (ant beetle-genus Thanasimus (Latreille) (Coleoptera: Cleridae), long-legged flies of the-genus Medetera (Fischer von Waldheim) (Diptera: Dolichopodidae) and predatory gall midges (Diptera: Cecidomyiidae)). Living and dead |