DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 9-10/2018 str. 26     <-- 26 -->        PDF

The colonization density varied significantly between analysed sections [K-W H(DF = 4, N = 55) = 26.84310, p = 0.0001]. The lowest gallery density in infested trees amounted to 1.1 galleries per m2, and the highest to 195.4 galleries per m2. It also come to a significant difference in the reproductive success between analysed sections [K-W H(DF = 4, N = 55) = 1.25745, p = 0.0003]. In some sections, the reproduction success reached 20 ♀/♀, but it was accompanied with a very low colonization density, only 9 maternal galleries per m2. There was some sections with as many as 1284 filial beetles, but this figure also varied considerably between infested sections [K-W H(DF = 4, N = 55) = 13.11153, p = 0.0107]. The same thing can be said for the number of predator larvae [K-W H(DF = 4, N = 55) = 17.05745, p = 0.0019], which positively correlates with the colonization density (r = 0.896156, N = 55, p < 0.05). In total, 4622 predator larvae were counted (Medetera – 4012, Cecidomyidae – 604, Thanasimus – 6). The average density ranged from 53 to 135 predator larvae per m2.
The maximum winter mortality of callow beetles amounted 42 %, while the peak winter mortality in the pupal stage was 85 %. As far as the biggest larvae are concerned, some sections were even featured by total mortality.
Systems with 2 maternal galleries prevailed (74.2 %). They were followed by systems with 3 maternal galleries which were found in almost one quarter of analysed samples (23.4 %), whereas the 4 maternal gallery systems were very rare (2.4 %).
DISCUSSION
Rasprava
The dominant overwintering development stage of I. typographus under the bark was the callow beetle. It was followed by pupae while the biggest larvae were the least common. The reproductive success at low attack densities was very low, which is contrary to previous findings (Inouye, 1962; Furuta, 1989). The biological potential of I. typographus is extremely high (Christiansen & Bakke, 1988) up to 20 ♀/♀, and it is very likely that most bark beetle females do not have such high biological potential, either on wind felled (Furuta, 1989; Eriksson et al., 2008; Komonen et al., 2011), alive and standing (Hedgren and Schroeder, 2004; Faccoli and Bernandinelli, 2011; Komonen et al., 2011) or felled trees (Hedgren and Schroeder, 2004; Ericsson et al., 2008).
High winter mortality rate of pupae, larvae and callow beetles and high abundance of predator larvae probably influence the reproductive success in I. typographus.
Overwintering of pre-adult stages of I. typographus is not possible (Austarå et al., 1977; Coeln et al., 1996; Baier et al., 2007) since larvae and pupae are extremely sensitive to low winter temperature (Andebrant et al., 1985; Andebrant, 1988; 1990). This is supported by Faccoli’s research (2002) who detected only callow beetles under the bark of attacked trees in spring. The freezing point for larvae and pupae is -13 C˚ and -17 ˚C, respectively, and concerning callow beetles, this figure ranges from -20 ˚C to -30 ˚C (Annila, 1969), though, more recent studies (Koštal et al., 2007; 2011) narrow this range between -20 ˚C and -22 ˚C. Winter temperatures in the area where the research was conducted commonly drop below the above thresholds relating to larvae and pupae, but lethal temperatures for the callow of fully mature beetles are rarely measured. The attack time is mainly triggered by favourable weather conditions while delayed attacks in summer could result in high winter mortality rates (Wermelinger and Seifert, 1999). Adults with low lipid content heavily survive winter (Botterweg, 1982).