DIGITALNA ARHIVA ŠUMARSKOG LISTA
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generations per year (Šrot 1976, Michalski and Mazur 1999, Krehan 2004, Knížek 2006, Zúbrik et al. 2008, Grodzki and Kosibowicz 2009), but the number of generations depends on the weather (Krehan 2004) and altitude (Grodzki and Kosibowicz 2009). Some re-emerging beetles of the overwintering generation may fly in June. The offspring generation adults have a maturation feeding period in late summer, either in the branches of younger trees (as observed in the current study) or near the larval galleries if fresh bark is still present (Postner 1974).
Using emergence traps in spring, we collected I. cembrae beetles that evidently overwintered in the forest litter. In the case of I. typographus, 80% of adults overwinter in forest litter directly under or near the infested trunk (Zumr 1985). According to the observed flight activity, overwintering I. cembrae beetles begin to be active in May.
The current report demonstrates the presence of overwintering I. cembrae beetles in logs and their immediate activity after being transferred to the laboratory. Adults overwinter in tunnels resulting from maturation feeding under the thicker bark of trunks lying on the ground or, more commonly, in the forest litter (Schneider 1977). The protracted emergence of adults from study logs indicates that part of the population overwinters as larvae and pupae. This is consistent with the previous report that I. cembrae may, in the case of incomplete development, overwinter as larvae or pupae (Postner 1974). Overwintering survival likely depends on temperature and other aspects of the winter weather.
The proportion of the I. cembrae population overwintering in the imaginal stage and the place of overwintering depend on the weather in autumn and on the possibility of completing development under bark before winter. Part of the early breeding beetles will leave the place of development and overwinter in forest litter or stay at the feeding site. For that reason, some findings document overwintering at the feeding site (Krehan and Cech 2004) and others document overwintering in forest litter (Schneider 1977, Grodzki and Kosibowicz 2009). The tendency to overwinter in forest litter is clear, and its explanation lies in the lower mortality that occurs in the litter than in tree trunks [see Ips typographus (Linné, 1758)] (Wermelinger 2004).
I. cembrae abundance was generally similar in different sections of traps trees but was occasionally higher in the crown than in the lower sections. Relative to lower sections, crown sections had longer maternal galleries with fewer larval galleries (Table 3). In I. subelongatus, the number of egg niches and adult offspring produced per gallery decline exponentially with infestation density (Zhang et al. 1992). These exponential relationships were attributed to intraspecific competition between adult females for breeding space and between larvae for limited food resources. Because I. cembrae abundance per unit area of bark was generally similar among sections in the current study, the beetles in the higher sections would have experienced increased competition for food because the phloem is thinner in higher than in lower sections. To reduce this intraspecific competition and to provide adequate food for larvae, females apparently deposit fewer eggs and at greater spacing in the crown, resulting in longer larval galleries in the crown than in lower sections.
Control of I. cembrae is identical to that for other bark beetles that attack conifers. In European countries, the intensity of control efforts varies depending on the severity of the damage, and control methods include: (i) silviculture management via clear cutting, selective thinning, and silvicultural selection; (ii) chemical treatment of felled trees; (iii) trapping with trap trees, trap logs, pheromone traps, and baited slash; and (iv) monitoring by pheromone attractants, trap trees/logs, visual survey, and questionnaire (Grégoire and Evans 2004).
At Slezské Rudoltice, trapping of adults in pheromone and trap trees decreased substantially in 2008 as a result of implemented protective measures (pheromone traps, trap trees, timely elimination of infested trees) in 2007.
Traps from trees of d1.3 30–45 cm prepared in our study in the second half of March on sunlit places became heavily infested with I. cembrae. Trap trees can remain active until the first pupae occur, because invasion continued even when there was a great abundance of entry holes. In addition, Elsner (1997) showed that timber from the April felling was infested first and more heavily than timber felled later in the season. Under laboratory conditions, however, I. cembrae breeding success in larch timber was greatest on trees felled during February–March (Elsner 1997).
The main problems in the control of I. cembrae are: (i) the beetle can develop on branches (our results, see also Knížek 2006), (ii) a considerable part of the population can overwinter in forest litter (our results), and (iii) trees processed by harvesters are not protected against infestation because they frequently retain their bark (Watzek and Niemeyer 1996).
Regarding the first problem, given that I. cembrae can develop on branches as small as 3 cm in diameter, logging residues and brushwood (burning and chipping) must be disposed of so that this material cannot be used for feeding or reproduction. Unprocessed logging waste from thinning in young stands can also be invaded and should be disposed of.
With respect to problem 2, because I. cembrae can overwinter in forest litter, removal of infested trees may not be sufficiently effective in decreasing the population density. Extension of the vegetation period by favourable temperatures