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ŠUMARSKI LIST 1-2/2019 str. 22     <-- 22 -->        PDF

to 2001 more than 31 million m3 of spruce wood in Europe was killed (Grégoire and Evans, 2004). At low population densities, I. typographus usually breeds in trees with low or no defense, or in those which are physiologically stressed, particularly in warm spring weather, long drought periods during summer (Christiansen and Bakke, 1997) or after severe storms (Lindelöw and Weslien, 1986; Weslien et al., 1989; Weslien and Lindelöw, 1990; Wermelinger, 2004; Gutowski and Krzysztofiak, 2005; Schroeder, 2010). This species is capable to respond to changes quickly by increasing its population density if there is suitable material in stands (Schroeder and Lindelöw, 2002) which happened in the mountain region of Croatia after the ice storm in the late winter of 2014 (Vuletić et al., 2014). Effective aggregation pheromones (Bakke et al., 1977) and symbiosis with blue stain fungi (Viiri, 1997; Krokene and Solheim, 1998; Kirisits, 2010) enable I. typographus to colonize and kill stressed or healthy growing trees (Botterweg, 1982; Weslien et al., 1989) depending on population density. The species is univoltine in northern Europe (Annila, 1969; Andebrant, 1986; Schroeder, 2013), while in southern and central Europe, it reproduces one to three times a year (Wermelinger, 2004; Faccoli and Stergulc, 2006; Jurc et al., 2006; Zúbrik et al., 2008; Wermelinger et al., 2012), depending on climatic conditions, an elevation and a geographic position (Zúbrik et al., 2008; Faccoli, 2009; Wermelinger et al., 2012; Kasumović, 2016).
I. typographus overwinters in the adult stage (Austara et al., 1977; Coeln et al., 1996; Faccoli, 2002; Baier et al., 2007) either in the bark of attacked trees (Annila, 1971; Hrašovec et al., 2011; Dworschak et al., 2014) or in the litter (Botterweg, 1982; Christiansen and Bakke, 1988; Hrašovec et al., 2011). Beetle in subadult stages can survive the winter when it is mild (Zumr, 1982; Wermelinger and Seifert, 1999; Dworschak et al., 2014; Štefková et al., 2017) but in more cases, they do not make it to spring (Austarå et al., 1977; Coeln et al., 1996) since winter temperatures often fall below lethal thresholds, which are - 13 ˚C and - 17˚C for subadult (Annila, 1969). More rarely, winter temperatures drop below - 20 ˚C or - 22 ˚C, which seem to be the lethal temperatures for callow or fully maturated beetles (Koštál et al., 2007; 2011), thought this is not exception in the research area - Žitnik, northern Europe and higher elevations elsewhere. E.g. only callow beetles under the bark of attacked trees in spring have been detect (Faccoli, 2002).
Similar to Ips grandicollis E. in North America (Lombandero et al., 2000), the majority of northern European I. typographus populations overwinter in the litter (Annila, 1971; Botterweg, 1982; Weslien and Lindelöw, 1989; Weslien, 1992), while in central and southern Europe most bark beetles stay under the bark during the winter period (Zumr, 1982; Faccoli, 2002; Hrasovec et al., 2011).
Norway spruce (Picea abies L. (Karst.)) trees growing in Dinaric mountain range, usually in mixed stands with silver fir (Abies alba Mill.) or beech (Fagus sylvatica L.), or in monocultures where early or late frosts are more frequent. Good knowledge of the overwintering behavior of this pest related to elevation can be helpful for foresters dealing with sanitation felling. This paper aimed at investigating the proportion of I. typographus populations and their natural enemies which overwinter under the bark of attacked trees in a spruce culture at 550 m a.s.l. in south-western Croatia.
The study was conducted in a 50-year-old spruce stand at 550 m a.s.l. in SW Croatia (44˚36’49.41’’ N; 15˚19’13.89’’ E). In mid-July 2014. five spruce trees similar in size (d (min-max) = 27-32 cm) were first felled and then left in the stand to be colonized by I. typographus. In January 2015, trees were cut in 4-m-long longs, and transported to a storehouse. Logs were stored in cold, shadow place where temperature did not exceed 5 °C.
During the first three weeks in February, all the logs were analyzed. Before debarking, one-meter-long sections (bark samples) were marked with spray and for the purpose of calculation of the bark surface, their diameter in the middle of each section had to be measured. The bark was removed with an axe after the exit holes had been counted. All the bark samples were carefully pulled apart in small pieces, which was followed by counting filial beetles, pupae, larvae and predator larvae (Thanasimus (Coleoptera: Cleridae), Medetera (Diptera: Dolichopodidae), predatory gall midges (Diptera: Cecidomyiidae)). Adults, pupae or larvae were considered alive if they showed any sign of movements at room temperature (Faccoli, 2002).
In terms of calculations, one exit hole represents one emerged adult beetle (Schlyter et al., 1984; Komonen et al., 2011). The number of adult beetles which remained under the bark and number of adult beetles which had left the bark were first calculated per m2 and then compared between trees using a nonparametric Kruskal-Wallis test. Spearman’s Rank correlation between the number of beetles which had left the logs actively (number of exit holes) and the predator abundance under the bark was calculated in Statsoft® Statistica 8.
In total, 55 samples (43.5 m2 of the bark) were analyzed. The number of samples differs between trees because wood peckers destroyed a part of the bark in some trunk sections and those samples were not included in analysis.