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

Abstract

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.

Fungal Interactions and Host Tree Preferences in the Spruce Bark Beetle Ips typographus

“…However, the decision to accept or reject a particular tree depends on short-range olfactory, tactile, and gustatory cues. Non-volatile phenolic compounds have been suggested to play key roles in host acceptance ( Faccoli and Schlyter, 2007 ; Huang et al, 2020 ; Netherer et al, 2021 ). In the absence of fungal colonization, I. typographus did not distinguish between different spruce species in our multi-species bioassays and entered medium made from all three spruce species to the same degree.…”

“…Strong attraction and preference of beetles to tunnel in fungus-colonized medium indicates the importance of fungal symbionts to I. typographus . Fungal symbionts play multiple roles in bark beetle colonization and development by overwhelming tree defenses, regulating attack densities, metabolizing host defenses, and translocating and providing essential nutrients ( Schlyter et al, 1987 ; Lieutier et al, 2009 ; Wang et al, 2014 ; Wadke et al, 2016 ; Six and Elser, 2019 ; Netherer et al, 2021 ). Larvae or immature adults of many bark beetles can fulfill their nutritional requirements by feeding on fungi alone or on fungus-colonized phloem ( Six, 2012 , 2013 ).…”

Defoliation, Recovery and Increasing Mortality in Italian Forests: Levels, Patterns and Possible Consequences for Forest Multifunctionality

“…Alpine conifers (P. abies and L. decidua) have low defoliation levels (Def > 25%). In P. abies, there is a significant trend to increasing defoliation (Def > 60%) and mortality, with a high ratio between dead and extremely defoliated trees, probably in relation to frequent biotic [64] and abiotic disturbances (windstorms [30]) causing relevant damage to conifer forests.…”

Evaluating Recent and Future Climatic Suitability for the Cultivation of Norway Spruce in the Czech Republic in Comparison with Observed Tree Cover Loss between 2001 and 2020

“…The stand transpiration deficit was identified as an important driver of bark beetle attacks in warmer and drier areas by Netherer et al [26]. The bark beetles have profited from warmer spring and summer, enabling more generations during the growing season, and at the same time, the precipitation deficits have led to a weakening of the spruce defense system (resin flow decreases or even stops during severe water deficiency) [26,50,51]. Especially in situations when the extensive breeding material provided by storm damage is also available in stands, the risk of a bark beetle outbreak is very high [50].…”