Overwintering of Eight Toothed Spruce Bark Beetle (<i>Ips typographus</i>) in Spruce Forests of North Velebit

• Main Authors: Milivoj Franjević, Luka Kasumović, Boris Hrašovec
• Format: Article
• Language: English
• Published: University of Zagreb, Faculty of Forestry 2011-01-01
• Series: Croatian Journal of Forest Engineering
• Online Access: https://hrcak.srce.hr/file/101760

The eight toothed spruce bark beetle (Ips typographus) plays an important role in spruce forests of Croatia, in the same way as it does in the rest of Europe wherever spruce grows. The size and intensity of its outbreaks is considerably lower though, and compared with the majority of countries scoring huge timber losses, Croatian forestry in general is much less affected, both economically and ecologically, by this bark beetle. Mixed tree species composition, natural origin and uneven age structure of spruce stands surely adds to the less pronounced and sporadic outbreaks in the Croatian territory. The recent rise of bark beetle populations in general, and above than average attacks of this bark beetle in the central parts of Dinaric mountain chain, initiated some new, targeted research projects. In the northern part of the Velebit Mountain, in the spring of 2008, a field trial was set up aimed at investigating the overwintering strategy of the eight toothed spruce bark beetle on two experimental plots in different elevation zones. The core idea was to test the already described phenomenon of different hibernation niche choice within the northern dwelling populations (predominantly choosing forest litter) and southern populations (staying within their growing niche, under the spruce bark). The general objective of the research and the expected outcomes fall within the area of sanitation felling and mechanical procedures in a wider concept of integrated bark beetle suppression tactics that could be used by Croatian foresters. Each experimental plot was characterized by the freshly attacked group of spruce trees (last summer – second generation of spruce bark beetle). The aerial distance between the lower »Štirovača« plot (1080 meters a.s.l.) and the higher »Zavižan» plot (1404 meters a.s.l.) was 17.5 kilometers ensuring that the two were well separated in space. The altitudinal difference of 324 meters might not seem striking but climatic conditions in the area of experimental plots are quite different, notably in terms of temperature profile and height and duration of snow cover. On each experimental plot a set of forest floor eclectors was set up within the vicinity of standing spruces hit by the last summer bark beetle attack. Also, on each of the plots, one tree–mounted cylindrical eclector was wrapped around the lower tree trunk of a standing spruce, collecting beetles emerging from the bark (details in Table 1, Fig. 2). In the close vicinity of the plots a pheromone trap was installed (commonly used Theysohn® type trap baited with Pheroprax® aggregation lure). The total area under the eclectors varied on the plots and the results of the collected beetles were interpreted accordingly, per unit of covered area. Beetles emerging in the eclectors and those trapped in the pheromone traps were collected on a weekly basis and counted in the laboratory. The trial started on April 19 and ended on June 9 covering fully the flight of the first, spring generation of spruce bark beetle. This statement is backed up by the parallel pheromone monitoring network data, which clearly confirm the onset and decline of the beetles spring swarm. The obtained results confirmed that the eight toothed spruce bark beetle does overwinter in the forest litter. The ratio between adults staying in their developmental sites (spruce bark) and those that entered forest litter varied in the two plots. In the higher plot the ratio was 50:50. The lower plot revealed a ratio of 60:40 in favor of beetles using bark instead of litter as an overwintering niche (Fig. 4). Assuming the altitudinal to zonal conversion concept, at least on a trend level, the obtained results correspond with the described tendency of south-north preference shift towards the forest litter as an overwintering niche. More importantly, the mere fact that after their full development in late summer/early fall, tenure adults of eight toothed spruce bark beetle enter litter, as confirmed by this research in a typical Croatian forest where spruce grows naturally, sheds new light on the efficiency of sanitation felling. The guidance tactics derived by this research dictate early logging operations, before the end of summer and in the period when beetles are still under the bark. This is especially important when stands are at higher elevations or grow in very harsh winter conditions in terms of temperature. Lower elevations, on the other hand, leave more time to sanitation logging, provided that the operations (removal of attacked trees) are finished before the onset of the beetle spring swarming. The efficiency of sanitation logging, as a part of the integrated bark beetle management, therefore, strongly depends on the proportion of the beetles overwintering in the forest litter. This might help to better understand the various efficiency and uneven impact of sanitation felling on bark beetle outbreak foci in the past in the Croatian spruce forests. Phenological data derived from the temporal analysis of bark beetle appearances in eclectors and pheromone lures (Fig. 3) also give some interesting and useful biological and forestry data. The recorded time lag of roughly two weeks between the start of emergence and peak pheromone trapped beetles on the two plots fits within the concept of the known zonal/altitudinal phenological shift. The practical value lies in the fact that it has been firstly recorded in more detail in one of the Croatian most important spruce growing region. Also, the documented time lag between the first emergers (both bark and forest litter) and first pheromone trapped beetles illustrates the known phenomenon of the first flight phase of the spruce bark beetle – the dispersal flight. The reason why this time lag was longer in the lower plot and shorter in the higher plot is explained by the possible immigration of earlier emerged beetles in the lower elevations lured to the pheromone trap in the higher elevated plot. Also, the duration and dynamics of beetle emergence on both plots varies significantly and is related to climatic regime in the plot area, which was measured (temperature, dew point, absolute and relative humidity) but not presented in this paper. What is evident from the pheromone trap catches (Fig. 3) is the narrower curve of the flying period of the first generation beetles in the higher plot versus lengthier swarming period in the lower plot (two weeks versus two weeks). In our opinion the most important outcome of this research for the Croatian forestry practice is the confirmation that forest litter is an overwintering niche of the eight toothed spruce bark beetle. In spite of the geographical position, i.e. the south part of the common spruce and spruce bark beetle distribution, the Croatian forestry must take into account that a significant part of beetle population can be missed by early fall or spring during sanitation operations, consequently enabling the beetle outbreaks to continue at an unacceptable pace. From the entomological point of view, it would be interesting to continue the research deeper in terms of within–the–species variability of Croatian spruce bark beetle populations. Assuming the fact that these populations are genetically old and wide in ecological plasticity, it is reasonable to expect that they can easily adapt to changing climatic conditions, escaping the winter mortal subzero temperatures by shift in ratio of cohorts of beetles choosing bark or litter as their overwintering niche.