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Introduction: In recent years, Norway spruce (Picea abies L.) forests in Central Europe have faced escalating threats from bark beetles, primarily Ips typographus (L.), and other species, such as I. duplicatus (Sahlberg). Outbreaks are partially attributed to weakened tree defense resulting from drought periods induced by climate change. This study examines Norway spruce's physiological and metabolic reactions to acute drought stress during the growing season and evaluates its susceptibility to I. typographus and I. duplicatus. Methodology: In order to induce drought stress, mature Norway spruces had their roots covered with a roof in April 2021, depriving them of water. Control trees were left with free access to natural rainwater. Over 5 months of the growing season, soil water potential, bark temperature, tree trunk circumference, and sap flow were monitored. Roofed trees and controls were sampled in July, August, and September and analyzed for non-structural carbohydrates and the two classes of defensive compounds, phenolics and terpenes. Furthermore, two different bioassays in tubes and boxes were performed using adult I. typographus and I. duplicatus beetles to assess host choice and acceptance. Results: Roofed trees exhibited signs of stress as early as July, resulting in decreased tree trunk and a consequent increase in non-structural carbohydrate content. Defensive metabolites remained largely unaffected except for an increase in diterpenes in September. In bioassays, I. typographus preferred boring into the bark of roofed trees in August in tubes and in September in boxes. This increased tree acceptance correlated with increased levels of soluble carbohydrates in the phloem. I. typographus and I. duplicatus beetles showed higher mobility in boxes in August and September on roofed trees but not in July, even though bark surface temperatures were elevated in roof-covered trees during all three bioassay periods. Conclusion: The study revealed rapid physiological responses of trees to acute drought stress, although not many changes were observed in defense traits. Despite the absence of natural bark beetle attacks, drought trees were more accepted by I. typographus than naturally watered trees. This response may indicate the beetles' preference for trees with phloem of higher nutritional quality induced by the acute drought stress conditions. [ABSTRACT FROM AUTHOR]

Eurasian spruce bark beetle, Ips typographus is a destructive pest of the Norway spruce (Picea abies). Recent outbreaks in Europe have been attributed to global warming and other anthropogenic impacts. Bark beetles are guided by multiple complex olfactory cues throughout their life cycle. Maleproduced aggregation pheromones, comprising 2-methyl-3-buten-2-ol and cis-verbenol, have been identified as the most powerful attractants for dispersing conspecifics. In addition to host trees, bark beetles interact with multiple organisms, including symbiotic ophiostomatoid fungi, which may promote beetle colonization success and offspring development. Previously, in a short-distance laboratory assay, we demonstrated that I. typographus adults are attracted to the volatile organic compounds (VOCs) produced by three symbiotic fungi: Grosmannia penicillata, Endoconidiophora polonica, and Leptographium europhioides. Furthermore, the abundant fusel alcohols and their acetates were found to be the most attractive odorants in the fungal VOC profile. In this study, using a long-distance field-trapping experiment, we analyzed the role of fungal VOCs as attractants for dispersing I. typographus. Two types of fungal lures were tested in combination with pheromones in traps: (1) live cultures of fungi grown on potato dextrose agar (PDA) and (2) dispensers containing synthetic fusel alcohols and their acetates in equal proportions. Subsequently, the composition of VOCs emitted from live fungal lures were analyzed. We found that the symbiotic fungi synergistically increased the attraction of beetles to pheromones in field traps and the attractiveness of live fungal lures depended on the fungal load. While one Petri dish with E. polonica, when combined with pheromones synergistically increased trapping efficiency, three Petri dishes with L. europhioides were required to achieve the same. The synthetic mix of fungal fusel alcohols and acetates improved the catch efficiency of pheromones only at a low tested dose. VOC analysis of fungal cultures revealed that all the three fungi produced fusel alcohols and acetates but in variable composition and amounts. Collectively, the results of this study show that, in addition to pheromones, bark beetles might also use volatile cues from their symbiotic fungi to improve tree colonization and reproductive success in their breeding and feeding sites. [ABSTRACT FROM AUTHOR]

Ips duplicatus (Coleoptera: Curculionidae) is a bark beetle that recently invaded southern Europe. It produces intense outbreaks in the spruce stands installed outside their natural area. Pheromone dispensers with different designs are used for monitoring this species. Compounds are released either by wick (ID Ecolure) or polyethene foil (ID RO and Pheagr IDU). We performed a series of laboratory and field experiments to compare the efficiency and sustainability of these dispensers in terms of pheromone release. The first experiment compared the Ips duplicatus capture dynamics between the ID Ecolure and ID RO in pheromones traps in three localities for 60 days. Release rates of used dispensers were evaluated in the same period. The second experiment analyzed headspace chemical composition in three dispensers (ID Ecolure, ID RO, Pheagr IDU) and tested their activity in pheromone traps. Active compounds content released by tested dispensers varied. The wick dispensers' release rates (ID Ecolure) were strongly reduced from 365 to 50 mg/day within two weeks and then steadily to 16 mg/day after 60 days. Release rates of the polyethene dispensers (ID RO) remained constant over 60 days (20–24 mg/day). The highest initial emission was in the ID Ecolure. The ratio of pheromone components (ipsdienol/E-myrcenol) was lower for ID Ecolure (5) and ID RO (10) and higher for Pheagr IDU (565). In the field experiments, significantly more Ips duplicatus were caught with ID Ecolure than ID RO and Pheagr IDU in the first two weeks. Later, the attractivity of ID RO remained high, but captures by ID Ecolure were significantly reduced. The captures of Ips typograhus were also considered. We propose that the wick-type dispensers are a promising tool for controlling the hibernating beetle generation. The foil release dispensers can be used to monitor Ips duplicatus flight activity. [ABSTRACT FROM AUTHOR]

• Tree bark temperatures were higher at forest edges than in inner forest. • Resin flow and VOC emissions of trees increased in response to edge creation. • Bioassays performed at edge trees showed high spruce bark beetle activity in spring. • Experimental forest edges were not attacked by the natural bark beetle population. • Bioassays show edge and inner forest trees were mainly attacked in August. Mass outbreaks of the European spruce bark beetle, Ips typographus , have caused devastating damage to Norway spruce-dominated forests in Central Europe. Biotic and abiotic natural disturbances as well as logging activities promote the fragmentation of forest stands, further increasing their susceptibility to damaging events. The creation of forest edges abruptly alters microclimatic conditions, such as trees suddenly being exposed to the sun. A common hypothesis is that trees remaining at the margins of clear cuts are initiation spots for spruce bark beetle infestation. This study comprehensively investigates the direct effects of forest edge establishment in spring on air and bark temperatures, soil water potential, tree sap flow, resin flow, bark defence compounds, and olfactometric cues for bark beetles provided by volatile organic compounds (VOCs) in proximity to trees. Ips typographus host acceptance was tested in limited-choice (attack box) and non-choice (Eppendorf tubes) field bioassays. Bark surface temperatures and resin flow clearly increased in trees of the freshly cut forest edges in spring, while sap flow rates did not differ from those of shaded trees in the inner forest. Soil water potential was high throughout the study season owing to sufficient precipitation and decreased only in August. Contrary to similar amounts of monoterpenes in the phloem of inner forest and edge trees, VOC concentrations were enhanced in May and June at the forest margin, also due to the presence of fresh logging residuals. The increased bark surface temperatures and VOC concentrations near forest edge trees were observed simultaneously with enhanced moving activity of I. typographus in the attack box bioassays soon after edge establishment in May. The effectiveness of increased resin flow in spring as a contributor to defence of edge trees could not be evaluated in the attack box bioassays, as boring holes at both edge and inner forest trees were observed, mainly later in the season. We conclude that fresh forest edges are not more susceptible to I. typographus attack shortly after establishment than closed forest stands but that microclimatic conditions of forest margins and olfactory signals from standing edge trees and logging residuals favour beetle swarming. Spruce bark beetle infestation patterns in fragmented forests are likely dependent on population densities and timing of predisposing abiotic disturbance and harvesting events. [ABSTRACT FROM AUTHOR]

Eurasian spruce bark beetle, Ips typographus , is a destructive pest in spruce forests. The ability of I. typographus to colonise host trees depends on its massive aggregation behaviour mediated by aggregation pheromones, consisting of 2-methyl-3-buten-2-ol and cis -verbenol. Other biologically active compounds such as ipsdienol and verbenone have also been detected in the beetle. Biosynthesis of 2-methyl-3-buten-2-ol and ipsdienol de novo from mevalonate and that of cis -verbenol from α -pinene sequestrated from the host have been reported in preliminary studies. However, knowledge on the molecular mechanisms underlying pheromone biosynthesis in this pest is currently limited. In this study, we performed metabolomic and differential gene expression (DGE) analysis for the pheromone-producing life stages of I. typographus. The highest amounts of 2-methyl-3-buten-2-ol (238 ng/gut) and cis -verbenol (23 ng/gut) were found in the fed male gut (colonisation stage) and the immature male gut (early stage), respectively. We also determined the amount of verbenyl oleate (the possible storage form of cis -verbenol), a monoterpenyl fatty acid ester, to be approximately 1604 ng/mg in the immature stage in the beetle body. DGE analysis revealed possible candidate genes involved in the biosynthesis of the quantified pheromones and related compounds. A novel hemiterpene-synthesising candidate isoprenyl-di-phosphate synthase Ityp09271 gene proposed for 2-methyl-3-buten-2-ol synthesis was found to be highly expressed only in the fed male beetle gut. Putative cytochrome P450 genes involved in c is/trans -verbenol synthesis and an esterase gene Ityp11977, which could regulate verbenyl oleate synthesis, were identified in the immature male gut. Our findings from the molecular analysis of pheromone-producing gene families are the first such results reported for I. typographus. With further characterisation of the identified genes, we can develop novel strategies to disrupt the aggregation behaviour of I. typographus and thereby prevent vegetation loss. [Display omitted] • Analysis of aggregation pheromone biosynthesis at various stages of Ips typographus. • Possible gene candidates for Ips typographus pheromone biosynthesis were identified. • Mevalonate pathway with novel IPDS involved in 2-methyl-3-buten-2-ol formation. • Putative CyP450 gene candidates identified for cis / trans -verbenol biosynthesis. • Esterase gene from immature stage gut for verbenyl oleate biosynthesis. [ABSTRACT FROM AUTHOR]