Spruce Bark Beetle Ips Typographus Research Articles

Fine Structure and Distribution of Antennal Sensilla of the Predatory Clerid Beetle, Thanasimus lewisi (Coleoptera: Cleridae)

The predatory clerid beetle Thanasimus lewisi (Jacobson) (Coleoptera: Cleridae) is an important predator of bark beetles (Scolytidae) and has been used in China for the biological control of the larch bark beetle Ips subelongatus Motschulsky and the spruce bark beetle Ips typographus Linnaeus. In this study, the number, external morphology and distribution of antennal sensilla of T. lewisi were determined using scanning electron microscopy. The filiform antennae of both sexes of both species consist of the scape, pedicel, and nine flagellomeres. Two types of sensilla chaetica (SC1 and SC2), four types of sensilla basiconica (SB1, SB2, SB3 and SB4), two types of sensilla trichodea (ST1 and ST2), and Böhm's bristles (BB) were identified according to the morphology and fine structure of each type of sensilla in both sexes. No difference in shape, structure, sensilla distribution and typology was observed between the sexes. The density of sensilla was greater on the last 3 than on the first 8 segments. SC1, SC2, ST1 and ST2 occurred on all antennal segments in both sexes. SB1, SB2 and SB3 were absent on the first 4 segments of the antennae in both sexes. SB4 were found only on the last 2 segments of the antennae. BB only occurred on the scape and pedicel. The distal antennal sensilla of T. lewisi had notably more sensilla than proximal ones in type and number, indicating this is the main area to detect environmental stimuli for feeding or oviposition. These structures likely have roles in the host locating and habitat searching behavior of adult T. lewisi and suggest future studies on the olfaction and host location behavior of T. lewisi and other coleopteran predators.

Thermal sum requirements for development and flight initiation of new‐generation spruce bark beetles based on seasonal change in cuticular colour of trapped beetles

Abstract The spruce bark beetle Ips typographus is the most important pest on Norway spruce in Europe. To improve predictions of bark beetle phenology in a warmer climate, correct thermal sums representing the development time of the new generation are needed. A standardized method for classifying adults into five different colours was used for describing the seasonal change in colour of beetles in breeding substrate and in weekly trap catches from regions located in southern, central and northern Sweden in 2015–2020. Repeated sampling of I. typographus from breeding substrate demonstrated that the adults get gradually darker, from light brown when newly moulted to dark brown or black in the following summer. In spring, four to five colours co‐occurred among trapped beetles. Over time, the proportions of darker individuals increased until the two lightest colours were absent except for 2 cases out of 21 trapping locations/years. Thereafter, the individuals of the two lightest colours started to occur again, indicating that they belonged to the new generation. The average thermal sum from start of flight of parental generation in spring until onset of new‐generation flight in summer was higher for southern Sweden [lower developmental threshold (LDT) 5°C = 744 degree‐days (dd); LDT 8.3°C = 467 dd] than for northern Sweden (LDT 5°C = 668 dd; LDT 8.3°C = 418 dd). New‐generation flight occurred in every year and region, but generally constituted only a small proportion of total seasonal flight activity.

Effect of mineral dusts alone and in combination with the entomopathogenic fungus Beauveria bassiana (Bals.) Vuill. against the bark beetle Ips typographus L. (Col., Scolytidae) in the laboratory and under field conditions

The objective of our study was to determine the effect of mineral dusts alone or in combination with the entomopathogenic fungus Beauveria bassiana to control the spruce bark beetle Ips typographus. In laboratory experiments with silicon dioxide (SiO2), diatomaceous earth (DE), Otavi-Perlit® (OP) and SilicoSec® (Ssec), 100% of all beetles died within 7 days on spruce bark at a temperature of 25 °C and a relative humidity (RH) of 40%, while the control mortality was 21%. The median survival time (MST) was between 2.0 and 4.1 days compared to the control with 9.8 days. After treatment on artificial diet, only in the SIO2 and DE treatment the mortality and MST were significantly different from the control. At 40% RH all beetles treated with siloxide (SiO) were dead after 3 days, while at 70% and 100% RH no effect of SiO on the beetles was observed, which shows that the effect of SiO on I. typographus is dependent on RH. An aqueous solution of SiO2 (1% and 10%) was as effective against I. typographus as the powder formulation, when the bark was dipped into the solution and dried afterwards. In the combination test, conidia of B. bassiana (1x107 spores/ml) were mixed with a solution of SiO2 at a final concentration of 0.5% and 5%. The highest mortality and lowest MST at 40%, 70% and 100% RH and 25 °C were observed in the 5% SiO2/B. bassiana mixture and in the powder formulation of both. In the field, I. typographus beetles were treated in bore holes or nuptial chambers with the B. bassiana product Boverol® or SiO2 alone and in combination on spruce logs. After 5 days, the highest number of galleries and pupal chambers was recorded in the untreated control. The mortality in all treatments was between 83 and 93% (control 54%). The rate of mycosis was 75–93% in all Boverol® treatments. In a final field experiment, beetles were trapped with pheromones, treated with B. bassiana or Ssec alone and in combination, and then were released on spruce bark in cages. After 7 days, the mortality and MST were significantly higher in all treatments compared to the control. In the B. bassiana treatments the mycosis rate was more than 90%. We suggest testing further mineral dusts alone or in combination with an entomopathogenic fungus (e. g. B. bassiana) against I. typographus in the field and to study the practicability of treatments within forest management strategies.

Habitat preferences and vertical distribution of dormice in the Tatra Mountains

Dormice monitoring was carried out between 2014 and 2019 at 14 areas located in various regions of the Tatra National Park in Poland. The areas were located in various habitats: beech forests and spruce forests in the low mountain zone; spruce forests, windbreaks and mountain pine (Pinus mugo) thickets in the high mountain zone. The choice of the area for monitoring resulted from the habitat type and technical possibilities enabling regular inspections. Each area was equipped with 30 nest boxes dedicated to dormice. The boxes were hung on trees at a height of 1.5 to 2 meters, only in mountain pine thickets. In habitats with a dense cover of European Red Raspberries (Rubus idaeus) the boxes were hung lower, on branches or on specially constructed poles. The use of boxes was determined on the basis of the presence of dormice or their nests. A comparison of nest box use shows that the Hazel Dormouse has specific habitat preferences in mountainous areas. This species was most frequently found in windbreaks, often those with a high proportion of trees destroyed after an outbreak of the spruce bark beetle (Ips typographus). These habitats are characterized by extensive undergrowth with numerous raspberries, Common Rowan (Sorbus aucuparia), Rosebay Willow Herb (Epilobium angustifolium) and spruce undergrowth (Picea abies). Beech forest (Fagus sylvatica) forms a habitat which is not preferred, but is unavoidable. Negative selectivity was demonstrated in relation to the spruce and mountain pine forests that are dominant in the Tatra National Park. The Forest Dormouse (Dryomys nitedula) was found at only one plot in the fourth year of observation. It was located in a small patch of beech in the vicinity of the site where this species was detected in 2011.

Transcriptomic changes during the establishment of long-term methyl jasmonate-induced resistance in Norway spruce.

Norway spruce (Picea abies) is an economically and ecologically important tree species that grows across northern and central Europe. Treating Norway spruce with jasmonate has long‐lasting beneficial effects on tree resistance to damaging pests, such as the European spruce bark beetle Ips typographus and its fungal associates. The (epi)genetic mechanisms involved in such long‐lasting jasmonate induced resistance (IR) have gained much recent interest but remain largely unknown. In this study, we treated 2‐year‐old spruce seedlings with methyl jasmonate (MeJA) and challenged them with the I. typographus vectored necrotrophic fungus Grosmannia penicillata. MeJA treatment reduced the extent of necrotic lesions in the bark 8 weeks after infection and thus elicited long‐term IR against the fungus. The transcriptional response of spruce bark to MeJA treatment was analysed over a 4‐week time course using mRNA‐seq. This analysis provided evidence that MeJA treatment induced a transient upregulation of jasmonic acid, salicylic acid and ethylene biosynthesis genes and downstream signalling genes. Our data also suggests that defence‐related genes are induced while genes related to growth are repressed by methyl jasmonate treatment. These results provide new clues about the potential underpinning mechanisms and costs associated with long‐term MeJA‐IR in Norway spruce.

Biorhythm paralleled release of pheromone by photothermal conversion for Long-term bark beetle control

The spruce bark beetle Ips typographus, as the most notorious pest in Eurasia spruce forest ecosystem, mainly feed on phloem of spruce tree trunk. The concealment of their habit makes regulation of insect population a difficulty to cope with, contributing to spruce forest recession, destruction of environment balance and forest transformation from carbon sinks into carbon source. Based on the cross point between the biorhythm of the I. typographus and photothermal material, description herein was a novel sunlight-triggered biorhythm-paralleled photothermal sponge (BPPS) for continuous control of I. typographus. Three steps including preparation of sugar template based porous polyurethane (PU) sponge, polypyrrole (PPy) modification and pheromone filling were involved in material manufacturing. The attractant and repellant agents were employed into the PU separately to realize a comprehensive regulation on I. typographus behavior. Laboratory simulation and forest field experiments illustrated that both the rapid release within a short period and long-term effectiveness in 21 days were verified. Due to the advantages of designability, cost-effectiveness and environmental friendliness of our sustained-release sponge, this manufacturing strategy offers great promise for generating photothermal responsive sustained-release material, shedding light on long-term control of injurious trunk borers and environmental protection.

Identifying optimal reference genes for gene expression studies in Eurasian spruce bark beetle, Ips typographus (Coleoptera: Curculionidae: Scolytinae)

Eurasian spruce bark beetle (Ips typographus [L.]) causes substantial damage to spruce forests worldwide. Undoubtedly, more aggressive measures are necessary to restrict the enduring loss. Finishing genome sequencing is a landmark achievement for deploying molecular techniques (i.e., RNA interference) to manage this pest. Gene expression studies assist in understanding insect physiology and deployment of molecular approaches for pest management. RT-qPCR is a valuable technique for such studies. However, accuracy and reliability depend on suitable reference genes. With the genome sequence available and the growing requirement of molecular tools for aggressive forest pest management, it is crucial to find suitable reference genes in Ips typographus under different experimental conditions. Hence, we evaluated the stability of twelve candidate reference genes under diverse experimental conditions such as biotic (developmental, sex and tissues) and abiotic factors (i.e., temperature and juvenile hormone treatment) to identify the reference genes. Our results revealed that ribosomal protein 3a (RPS3-a) was the best reference gene across all the experimental conditions, with minor exceptions. However, the stability of the reference gene can differ based on experiments. Nevertheless, present study provides a comprehensive list of reference genes under different experimental conditions for Ips typographus and contributes to “future genomic and functional genomic research”.

Anti-attractant activity of (+)-trans-4-thujanol for Eurasian spruce bark beetle Ips typographus: Novel potency for females.

Eurasian spruce bark beetles (Ips typographus) use both attractant and anti-attractant semiochemicals to find suitable mature host trees. Trans-4-thujanol is abundant in young, unsuitable spruce trees. Electrophysiological studies have demonstrated its high activity levels, but field data are lacking. Enantioselective GC-MS analysis showed that only (1R,4S)-(+)-trans-4-thujanol was present in Norway spruce bark volatiles. In a factorial design field-trapping experiment, trans-4-thujanol alone was not attractive to Ips typographus. Traps baited with I. typographus' aggregation pheromone and trans-4-thujanol or the known anti-attractant 1,8-cineole caught fewer beetles than those baited with the aggregation pheromone alone. Catches for trans-4-thujanol and 1,8-cineole were dose-dependent. Intermediate doses of trans-4-thujanol and 1,8-cineole had a similar effect. Surprisingly, in contrast to 1,8-cineole and other known Ips anti-attractants, which all inhibit males more strongly than females, the addition of trans-4-thujanol to the aggregation pheromone reduced the attraction of females more. The Norway spruce volatile (+)-trans-4-thujanol is a novel I. typographus anti-attractant with potency comparable to the known anti-attractants 1,8-cineole and verbenone, and is more effective for females than for males. Incorporating (+)-trans-4-thujanol into anti-attractant lures could improve protection of trees from mass attack by I. typographus.

Remote Sensing Assessment of the Expansion of Ips typographus Attacks in the Chuprene Reserve, Western Balkan Range

The Chuprene Reserve was created in 1973 to preserve the natural coniferous forests in the Western Balkan Range in Bulgaria. The first infestations by European spruce bark beetle (Ips typographus) were registered in Norway spruce (Picea abies) stands in the mid-1980s. The aim of this study is to assess the damages caused by I. typographus in the Chuprene Reserve using remote sensing techniques – unmanned aerial vehicle (UAV) images, airborne images, and satellite images of European Space Imaging (EUSI), combined with terrestrial verification. High-resolution images in four bands of the electromagnetic spectrum and in a standard RGB channel were taken in 2017 via a multispectral camera ‘Parrot Sequoia’, integrated with a specialized professional UAV system eBee ‘Flying Wing’. The health status of Norway spruce stands in the reserve was assessed with the normalized difference vegetation index, based on the digital mixing of imagery captured in the red and near infrared range. The dynamic of bark beetle attacks was studied in GIS on the basis of maps generated from photographic surveys, airborne images taken in 2011 and 2015, and satellite images from 2020. In the UAV-captured area (314.0 ha), the size of Norway spruce stands attacked by I. typographus increased from 7.6 ha (2.4%) in 2011 to 44.9 ha (14.3%) in 2020. The satellite images showed that on the entire territory of the Chuprene Reserve (1451.9 ha), I. typographus killed spruce trees on 137.4 ha, which is 9.6% of the total area.

Northernmost European spruce bark beetle Ips typographus outbreak: Modelling tree mortality using remote sensing and climate data

Acute or chronic drought stress caused by climate change can contribute to the weakening of forest ecosystems and lead to extensive bark beetle infestations. Siberian spruce (Picea obovata Ledeb.) forests of the Dvinsko-Pinegskiy, a natural reserve in the Arkhangelsk region, Russia, have been subject to unprecedented tree cover loss caused by the Eurasian spruce bark beetle (Ips typographus L.) in the last two decades. This is the first recorded case of such an extensive outbreak of Ips typographus occurring at higher latitudes. We used remote sensing and climate data to model and compute annual tree-loss change due to natural factors, with a focus on bark beetle outbreaks, over a 14-year period (2001–2014). Usinglinear regression models, we found a combination of average annual temperature and precipitation, temperature and precipitation in June, to be the most important drivers of annual tree-loss.

Following the damage: Increasing western barbastelle bat activity in bark beetle infested stands in Białowieża Primeval forest

Natural forest disturbances are increasingly common due to ongoing climate changes but their impact on most of forest organisms is poorly studied. Here we investigate the link between spruce bark beetle Ips typographus outbreak in 2011–2017 in Białowieża Primeval Forest (Poland) and activity of a forest bat of conservation concern: the western barbastelle Barbastella barbastellus. Bats were surveyed by recording ultrasound signals on 8 transects (3 km each) in a mixed coniferous and deciduous forest with a different share of Norway spruce. The activity pattern of bats was found to correlate with the number of bark beetle infested Norway spruces in the following years within a 1 km buffers of the transects (339,354 trees in total) using generalised additive mixed models. We demonstrated that the number of bark beetle infested spruces was a positive predictor of barbastelle activity largely with 1- and 2-year lags: number of spruces infested in a year correlates with bat activity one and two years later. This latency period most likely mirrors the decomposition process in standing dead spruces. We suspect that the massive appearance of dying Norway spruces provides the barbastelle with roosting sites under the bark and may lead barbastelle to colonize spruce stands otherwise rarely occupied by this species. It cannot be excluded, however, that other mechanisms (e.g. increased food biomass or availability) also drive the barbastelle to colonize stands infested by bark beetle. Our findings suggest that salvage logging and dead spruce removal, routinely implemented in stands infested by bark beetle, including Białowieża Primeval Forest, may negatively impact western barbastelle population.

A highly-contiguous genome assembly of the Eurasian spruce bark beetle, Ips typographus, provides insight into a major forest pest

Conifer-feeding bark beetles are important herbivores and decomposers in forest ecosystems. These species complete their life cycle in nutritionally poor substrates and some can kill enormous numbers of trees during population outbreaks. The Eurasian spruce bark beetle (Ips typographus) can destroy >100 million m3 of spruce in a single year. We report a 236.8 Mb I. typographus genome assembly using PacBio long-read sequencing. The final phased assembly has a contig N50 of 6.65 Mb in 272 contigs and is predicted to contain 23,923 protein-coding genes. We reveal expanded gene families associated with plant cell wall degradation, including pectinases, aspartyl proteases, and glycosyl hydrolases. This genome sequence from the genus Ips provides timely resources to address questions about the evolutionary biology of the true weevils (Curculionidae), one of the most species-rich animal families. In forests of today, increasingly stressed by global warming, this draft genome may assist in developing pest control strategies to mitigate outbreaks.

Scientific response to intensifying bark beetle outbreaks in Europe and North America

Tree-killing bark beetles are globally the most destructive forest pests and their impacts have increased in recent decades. Such an increase has been consistently reported from Europe and North America, and it is, with high confidence, driven by climate change. We investigated how the scientific community in both continents responded to this situation by conducting a comprehensive search of the Scopus database from 1970 to 2020. Studies that investigated interactions between climate change and two prominent bark beetles in Europe and North America, the European spruce bark beetle Ips typographus (ESBB) and the mountain pine beetle Dendroctonus ponderosae (MPB), were identified. We used several hierarchical search criteria, starting from general aspects of pest – climate change interactions, to studies with clear implications for management and policies.We found that authors investigating the two bark beetle species mentioned climate change in publications beginning in 1998, and have constituted 8.9 and 13.8 % of all studies on ESBB (N = 987) and MPB (N = 1479) recorded in Scopus. However, only part of these studies addressed climate change as a fundamental or integral part of their research design (59.1 % in ESBB and 38.7 % in MPB). We identified 30 studies on ESBB and 50 studies on MPB which informed efforts towards improving bark beetle management strategies to address climate change-affected ecosystem dynamics. Publications on both insects consistently highlighted the importance of vegetation management aiming to reduce the risk and severity of outbreaks and prevent large-scale population expansion. Only a minor portion of studies placed their findings into the context of relevant policies and legislation, and this connection was particularly lacking in studies on MPB.We conclude that research on bark beetle management under climate change has received inadequate attention and it lags behind observed and foreseen global-scale impacts. We suggest that focused and applied research with clear management implications is needed to develop new climate-adapted and evidence-based management strategies.

Facultative and obligate diapause phenotypes in populations of the European spruce bark beetle Ips typographus

The bark beetle Ips typographus is the most destructive insect pest in Norway spruce-dominated forests. Its potential to establish multiple generations per year (multivoltinism) is one major trait that makes this beetle a severe pest. Ips typographus enters diapause to adjust its life cycle to seasonally changing environments. Diapause is characterized by developmental and reproductive arrest; it prolongs generation time and thus affects voltinism. In I. typographus a facultative, photoperiod-regulated diapause in the adult stage has been described. In addition, the presence of an obligate, photoperiod-independent, diapause has been hypothesized. The diapause phenotype has important implications for I. typographus voltinism, as populations with obligate diapausing individuals would be univoltine. To test for the presence of different I. typographus diapause phenotypes, we exposed Central and Northern European individuals to a set of photoperiodic treatments. We used two ovarian traits (egg number and vitellarium size) that are associated with gonad development, to infer reproductive arrest and thus diapause. We found a distinct effect of photoperiod on ovarian development, with variable responses in Central and Northern European beetles. We observed obligate diapausing (independent of photoperiod) individuals in Northern Europe, and both facultative (photoperiod-regulated) as well as obligate diapausing individuals in Central Europe. Our results show within-species variation for diapause induction, an adaptation to match life cycles with seasonally fluctuating environmental conditions. As the diapause phenotype affects the potential number of generations per season, our data are the basis for assessing the risk of outbreaks of this destructive bark beetle.

Functional Evolution of a Bark Beetle Odorant Receptor Clade Detecting Monoterpenoids of Different Ecological Origins.

Insects detect odors using an array of odorant receptors (ORs), which may expand through gene duplication. How and which new functions may evolve among related ORs within a species remain poorly investigated. We addressed this question by functionally characterizing ORs from the Eurasian spruce bark beetle Ips typographus, in which physiological and behavioral responses to pheromones, volatiles from host and nonhost trees, and fungal symbionts are well described. In contrast, knowledge of OR function is restricted to two receptors detecting the pheromone compounds (S)-(–)-ipsenol (ItypOR46) and (R)-(–)-ipsdienol (ItypOR49). These receptors belong to an Ips-specific OR-lineage comprising seven ItypORs. To gain insight into the functional evolution of related ORs, we characterized the five remaining ORs in this clade using Xenopus oocytes. Two receptors responded primarily to the host tree monoterpenes (+)-3-carene (ItypOR25) and p-cymene (ItypOR27). Two receptors responded to oxygenated monoterpenoids produced in larger relative amounts by the beetle-associated fungi, with ItypOR23 specific for (+)-trans-(1R, 4S)-4-thujanol, and ItypOR29 responding to (+)-isopinocamphone and similar ketones. ItypOR28 responded to the pheromone E-myrcenol from the competitor Ips duplicatus. Overall, the OR responses match well with those of previously characterized olfactory sensory neuron classes except that neurons detecting E-myrcenol have not been identified. The characterized ORs are under strong purifying selection and demonstrate a shared functional property in that they all primarily respond to monoterpenoids. The variation in functional groups among OR ligands and their diverse ecological origins suggest that neofunctionalization has occurred early in the evolution of this OR-lineage following gene duplication.

Identification of male produced compounds in the bark beetle Polygraphus subopacus and establishment of (Z)-2-(3,3-dimethylcyclohexylidene)-ethanol as an aggregation pheromone component

Bark beetles of the genus Polygraphus have recently been involved in large bark beetle outbreaks in central Sweden, together with the European spruce bark beetle Ips typographus. Three species of Polygraphus can be found in this region; Polygraphus poligraphus, Polygraphus punctifrons and Polygraphus subopacus. Efficient pheromone traps would facilitate further investigations of these species and their role in bark beetle outbreaks. Pheromone compounds have previously been identified in P. poligraphus and P. punctifrons, but not in P. subopacus. Thus, we allowed males and females of P. subopacus to bore in the bark of stem sections of Norway spruce (Picea abies) in the laboratory. Volatile organic compounds from boring insects were sampled with SPME and analysed with GC–MS and several male-specific compounds were observed. The male specific compounds were 3-methyl-3-buten-1-ol, 3-methyl-2-buten-1-ol, 3-methyl-2-butenal, grandisol, fragranol, (Z)-2-(3,3-dimethylcyclohexylidene)-ethanol, (E)-2-(3,3-dimethylcyclohexylidene)-ethanol, (Z)-2-(3,3-dimethylcyclohexylidene)-acetaldehyde, (E)-2-(3,3-dimethylcyclohexylidene)-acetaldehyde, geranial and γ-isogeraniol. (Z)-2-(3,3-dimethylcyclohexylidene)-ethanol, [(Z)-DMCHE], was identified from GC–MS analysis to be the major male-specific compound while the (E)-isomer, [(E)-DMCHE], was found as a minor compound. These two compounds gave positive responses in EAG analyses with antennae from males and females of P. subopacus. Thus, (Z)- and (E)-DMCHE were used in a field experiment in central Sweden but only (Z)-DMCHE was found to be attractive to males and females of P. subopacus. Consequently, (Z)-DMCHE was established to be a component of P. subopacus aggregation pheromone.

Seasonal variation in Norway spruce response to inoculation with bark beetle-associated bluestain fungi one year after a severe drought

In 2018, up to 4 million m3 Norway spruce was killed by the spruce bark beetle Ips typographus in Sweden. The event was unique for Sweden, in terms of both affected volume and the fact that it was triggered by severe drought stress, not by ample availability of relatively defenseless storm-felled trees. The outbreak continued in 2019 and 2020, each year with twice as many trees killed as in 2018. The aim of this study was to quantify seasonal variation and potential lag-effects in tree defense capacity the year after a severe drought stress. Inoculation with a bark beetle-associated bluestain fungus, repeated four times with one-month-intervals between May and August 2019, were carried out at three field sites with spruce provenances of Swedish and East European origin representing early and late bud burst, respectively.All sites had experienced moderate to severe drought stress in 2018, and site-specific defense capacity correlated positively with the cumulative precipitation two months before inoculation. Sites with two-month precipitation levels <100 mm had larger necrotic lesions in the phloem following inoculation, an indication of lower tree defense capacity. Lesion size did not differ between provenances, and all trees were able to confine fungal infection successfully.There were some seasonal differences in necrotic lesion size, with the sites Skärsnäs and Norberg having significantly larger lesions in June than in May, and site Lugnet having large lesions also in May. Lesions were generally smaller in July and August than in June.The cross-sectional area and number of traumatic resin ducts was measured in sapwood samples from one site, Lugnet, to quantify an additional aspect of tree defenses. The area of resin ducts produced in May and June were larger than that in July and August. This is in line with a positive correlation between lesion area and resin duct area, indicating that a stronger fungal infection following inoculation in spring triggered a stronger induced defense response. The East European provenances had more resin ducts than Swedish provenances, but the area of resin ducts did not differ significantly between provenances.

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

The spruce bark beetle Ips typographus is the most damaging pest in European spruce forests and has caused great ecological and economic disturbances in recent years. Although native to Eurasia, I. typographus has been intercepted more than 200 times in North America and could establish there as an exotic pest if it can find suitable host trees. Using in vitro bioassays, we compared the preference of I. typographus for its coevolved historical host Norway spruce (Picea abies) and two non-coevolved (naïve) North American hosts: black spruce (Picea mariana) and white spruce (Picea glauca). Additionally, we tested how I. typographus responded to its own fungal associates (conspecific fungi) and to fungi vectored by the North American spruce beetle Dendroctonus rufipennis (allospecific fungi). All tested fungi were grown on both historical and naïve host bark media. In a four-choice Petri dish bioassay, I. typographus readily tunneled into bark medium from each of the three spruce species and showed no preference for the historical host over the naïve hosts. Additionally, the beetles showed a clear preference for bark media colonized by fungi and made longer tunnels in fungus-colonized media compared to fungus-free media. The preference for fungus-colonized media did not depend on whether the medium was colonized by conspecific or allospecific fungi. Furthermore, olfactometer bioassays demonstrated that beetles were strongly attracted toward volatiles emitted by both con- and allospecific fungi. Collectively, these results suggest that I. typographus could thrive in evolutionary naïve spruce hosts if it becomes established in North America. Also, I. typographus could probably form and maintain new associations with local allospecific fungi that might increase beetle fitness in naïve host trees.

Devastating outbreak of bark beetles in the Czech Republic: Drivers, impacts, and management implications

Outbreaks of tree-killing insects are intensifying globally, affecting economies, human well-being, and driving ecosystem transitions. The Czech Republic has recently become Europe’s epicenter of the outbreak of spruce bark beetle Ips typographus, the most aggressive species in Eurasia. We investigated a countrywide outbreak dynamic during the period 2003–2019, with a special focus on the period 2017–2019 when the outbreak reached an unprecedented intensity. In order to identify main outbreak drivers, we investigated annual time series of the volume of trees killed by bark beetles in the Czech districts (n = 77), and a suite of climatic and forest structure-related predictors using Generalized Additive Models. Finally, we reviewed a large body of public materials to understand broader social, ecological, and economic implications of the outbreak.We found that bark beetles were damaging 0.2–1.4% of Norway spruce growing stock annually across the Czech Republic in the period 2003–2016. This level increased to 3.1–5.4% in 2017–2019, causing the total depletion of spruce in some regions. The long-term bark beetle dynamics (2003–2019) was driven by the combination of wind disturbance and climatic drivers, represented in our study by annual temperature anomaly and Standardized Precipitation-Evapotranspiration Index. However, the effect of wind was diminished during the period 2017–2019, whereas the effect of drought dominated. Our findings thus suggest a transition from wind- to drought-driven bark beetle dynamics.The outbreak and subsequent large-scale salvaging and wood transportation affected quality of life of people in a broad vicinity of outbreak areas. Extensive management actions aggravated some of the notorious conflicts between forest management and nature conservation, and highlighted the poor harmonization of respective policies. A decrease in timber price, an excessive workload, and other cascading effects caused severe revenue loss, requiring state interventions amounting to ca 260 million EUR in 2018–2019.We suggest that increasing frequency of climate extremes in combination with the unfavorable forest structure pushed Central European spruce forests to the margin of their ecological space and unfolded large-scale forest transformations. Effective responses will require fundamental structural changes in the regional forest-based sector, particularly aiming at increased social and ecological resilience.

Impacts of mountain forest dieback on aquatic insect communities: A DNA metabarcoding analysis of samples from the Bavarian Forest

Mountain forests are increasingly affected by changes in rainfall and pest outbreaks, and the way forests are managed can have direct consequences for the streams flowing through forests. Aquatic macroinvertebrate communities are great bioindicators and changes to their ecosystem likely translates to changes in their overall composition and abundance. The Bavarian Forest National Park (SE Germany) is dominated by the Norway spruce (Picea abies) which, weakened by storms and other stressors, is susceptible to infestation by the European spruce bark beetle (Ips typographus). The result is large scale forest dieback in some areas, and forest management practices that lead to a predominance of three different forest types (hereafter habitats): Intact forest that is healthy and not impacted by Ips typographus; Disturbed forest that was impacted by Ips typographus, left to regenerate naturally, and from which deadwood was not removed; and Salvaged forest that was heavily impacted by Ips typographus with the same consequences, but from which deadwood was removed, creating a treeless forest meadow. Intact forest that is healthy and not impacted by Ips typographus; Disturbed forest that was impacted by Ips typographus, left to regenerate naturally, and from which deadwood was not removed; and Salvaged forest that was heavily impacted by Ips typographus with the same consequences, but from which deadwood was removed, creating a treeless forest meadow. To analyze the impacts these different forest management strategies have on the aquatic insect communities, 30 samples from 11 different streams were taken using kick-sampling. Operational taxonomic units (OTUs) were identified by bulk metabarcoding of dried, ground samples. A mock community was used to verify the setup and a DNA spike-in with three foreign OTUs was added to each sample to measure the biases introduced by PCR amplification and sequencing. Biases varied across samples, but spike-in OTUs produced a pattern indicating predictable biases which could lead to quantifiable metabarcoding results in the future. In total, 260 macroinvertebrate OTUs were identified. In comparison, a morphological study by Bojková et al. (2018) in the same region with twice the number of sampling sites collected 194 taxa in the same month as our samples. This underlines the potential for metabarcoding in evaluating species richness. Species richness was high across all habitats. A significant difference between the forest conditions was detected: The number of detected Diptera OTUs was lowest in disturbed habitats (55) and highest in salvaged areas (73). A permutational multivariate analysis of variance (PERMANOVA) indicated that habitat (i.e., intact, disturbed, salvaged) had an effect on the observed OTU distribution (9.2%), but that the stream catchment had a much larger effect (39.3%) regardless of the habitat. Our findings indicate that forest management can affect stream macroinvertebrate communities, and that this was most pronounced for the Diptera, a group for which DNA metabarcoding is particularly well suited because of their small size and high diversity.