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ŠUMARSKI LIST 1-2/2019 str. 23     <-- 23 -->        PDF

The percentage of beetles remaining under the bark differs significantly between sections [K–W H(DF = 4, N = 55) = 21.55579, p = 0.0002]. The proportion of adult bark beetles remained in the bark varies between 79.4 and 95.1 % among trees, with the average proportion to 84.7 %.
There is a positive correlation between the number of adults beetles which left the bark and the abundance of predator larvae (r = 0.487693, N = 55, p < 0.05). In total, 4622 predator larvae were detected, among which Medetera larvae were the most abundant larval form.
The results indicate that the proportion of a beetle population which stays under the bark during the winter increases with a decrease of elevation. It can be explained by the genetic variability (Stauffer et al., 1999) and plasticity (Hrašovec et al., 2011; Dworschak et al., 2014) within the oldest European populations of I. typographus which are present in south and central Europe (Stauffer et al., 1999). This species is adaptable to spatially and temporally changeable habitat conditions. The beetles of southern latitudes and lower elevations are not forced to leave the bark during the winter to avoid chilling injuries caused by long-lasting low air temperatures, which are typical for northern Europe and higher elevations elsewhere. Higher portion of beetles leave the bark in sections with their higher total production or predator abundance. This fact can be viewed as a further confirmation of the plasticity of species and the possibility of adaption to changeable habitat conditions. Most of the natural enemies of I. typographus overwinter as larvae in bark beetle galleries, which is similar to the situation in Switzerland (Wermelinger et al., 2012).
Overwintering behavior is the key factor in planning sanitation felling as recovery measure within hot spots of I. typographus attacks. In regard to spruce stand growing at 550 m. a. s. l., the second generation of spruce bark beetles finishes full development in late September or mid-October (Kasumović, 2016), but a shortage of photoperiod in mid-August can cause a swarming, copulation or gradual disruption of egg hatching (Baier et al., 2007; Kasumović, 2016). Moreover, 15 % of beetles which leave the logs actively through exit holes overwinter in the needle litter. As far as the zone of Velebit peaks is concerned, half of bark beetle population overwinter in the needle litter while in altimontane region, this figure is smaller, around 40 % (Hrašovec et al., 2011). The life cycle and voltinism of I. typographus is elevation adaptable (Faccoli 2002; Kasumović, 2016), and have significant influence on the share of beetles which leave the bark during the winter (Wermelinger et al., 2012).
Despite temperature fluctuation and super cooling injuries (Annila, 1969), bark is believed to be a good overwintering niche. This thesis is supported by the following facts: beetles stay in dry bark where the freezing risk is very low (Koštál et al., 2011); the risk of infection is minimal in dry conditions (Doležal et al., 2009); bark heated by sun isolation in spring results with earlier nutrients exploitation, development completion (Dworschak et al., 2014) and a prolonged fly period for the new host search (Hrašovec et al., 2011).
The number of beetles overwintering under the bark, which is inversely proportional with elevation, highlights the difficulties in sanitation felling which needs to be adapted to