Interactions among Norway spruce, the bark beetle Ips typographus and its fungal symbionts in times of drought

Resilience and functionality of European Norway spruce forests are increasingly threatened by mass outbreaks of the bark beetle Ips typographus promoted by heat, wind throw and drought. Here, we review current knowledge on Norway spruce and I. typographus interactions from the perspective of drought-stressed trees, host selection, colonisation behaviour of beetles, with multi-level effects of symbiotic ophiostomatoid fungi. By including chemo-ecological, molecular and behavioural perspectives, we provide a comprehensive picture on this complex, multitrophic system in the light of climate change. Trees invest carbon into specialised metabolism to produce defence compounds against biotic invaders; processes that are strongly affected by physiological stress such as drought. Spruce bark contains numerous terpenoid and phenolic substances, which are important for bark beetle aggregation and attack success. Abiotic stressors such as increased temperatures and drought affect composition, amounts and emission rates of volatile compounds. Thus, drought events may influence olfactory responses of I. typographus, and further the pheromone communication enabling mass attack. In addition, I. typographus is associated with numerous ophiostomatoid fungal symbionts with multiple effects on beetle life history. Symbiotic fungi degrade spruce toxins, help to exhaust tree defences, produce beetle semiochemicals, and possibly provide nutrition. As the various fungal associates have different temperature optima, they can influence the performance of I. typographus differently under changing environmental conditions. Finally, we discuss why effects of drought on tree-killing by bark beetles are still poorly understood and provide an outlook on future research on this eruptive species using both, field and laboratory experiments

Testing the Efficiency of the Push-and-Pull Strategy during Severe Ips typographus Outbreak and Extreme Drought in Norway Spruce Stands

Protection of Norway spruce stands using anti-attractants was tested during an outbreak of bark beetles (Ips typographus) in their spring flight. The aims of this study were as follows: (1) to test the proposed experimental design for tree protection; (2) to evaluate height-specific alternatives for dispenser installation on trees; and (3) to evaluate the efficiency of tree protection measures using anti-attractants under bark beetle infestation and drought stress. The experiment was conducted at the forest edges adjacent to recent clearcuts on 10 blocks in the eastern Czech Republic. Each block had three adjacent experimental areas, with 20 trees growing in two rows at the recently cut forest edge (10 trees per row). In front of a block in each of the three areas, four pheromone traps were installed. The treatment area was protected by anti-attractants. The second area served as a so-called switch area, where beetles from the treatment area, as the outflux redirected from the anti-attractant, would start new attacks if not caught in nearby pheromone traps. The third area was a control. We attached anti-attractant tube dispensers on each tree trunk of the treated area at two heights. The results suggest a redirecting effect of anti-attractants, pushing beetles into the switch area and causing subsequent attacks, which was greater than in areas containing treated trees. There was no difference between two dispensers placed at 1 and 8 m height and both at 1 m. A switching effect of beetle attacks occurring outside of the treated areas was observed. Mounting anti-attractant dispensers on tree trunks at one low position above the ground can be substantially less labour-intensive and as efficient as positioning them at two different heights. For areas affected by severe drought and extremely dense bark beetle populations, the use of anti-attractants did not prove effective