Beetle Attack Research Articles

Relationships between conifer constitutive and inducible defenses against bark beetles change across levels of biological and ecological scale

Plants resist herbivores using both constitutive and induced defenses. Limited resources and biosynthesis costs of anti‐herbivore compounds may impose tradeoffs between these modalities, but the evidence is conflicting. We postulated that biological and ecological scale may influence potential tradeoffs and analyzed constitutive‐induced relationships from genotypic to interspecific levels. We focused on one system, conifer phloem monoterpenes that resist bark beetle attack, to minimize variation from feeding guild or phylogeny. We examined 45 half‐siblings in common gardens (n = 763), seven populations of five species with tree‐level data (n = 436) and 29 studies of nine species with population‐mean data in forests. All studies evaluated mature trees to correspond with beetle behavior in nature and employed common methods of induction and analysis. Rather than a global tradeoff between constitutive and induced defenses, relationships varied with level of biological organization. In common gardens, constitutive concentrations and inducibility (I–C) were inversely related across families and individuals. These relationships did not extend to forest populations, however, where constitutive concentrations and inducibility were positively related. Across populations and species, constitutive concentrations and inducibility were likewise positively related. There was no evidence for inducibility tradeoffs between monoterpene quantity and composition: trees showed no or positive relationships between constitutive concentrations and induced compositional changes. These results suggest environmental variability can supersede underlying physiological tradeoffs in some systems, allowing plants with greater resource pools to excel in multiple defense components. This is further supported by microsite effects on inducibility exceeding family effects in the common gardens, and by relationships between constitutive and absolute induced concentrations being positive at all levels. We present a general model for understanding how underlying physiological tradeoffs are variably manifested across scales. These results can contribute to the protection and resilience of agronomic and native ecosystems and help contextualize organismal‐level allocations to ecosystem‐level patterns across heterogeneous landscapes.

ИССЛЕДОВАНИЕ УЧАСТКОВ ЛЕСАПОДВЕРЖЕННЫХ ВЛИЯНИЮ КОРОЕДА-ТИПОГРАФА (IPS TYPOGRAPHUS) В ЗАПОВЕДНИКЕ «КУРИЛЬСКИЙ» (о. КУНАШИР)

Climate change has a major impact on forest pest outbreaks. In recent years, there have been several major climatic events in the South Kuril Islands that have cascaded impacts on forest pests. One of these effects was exerted by the typhoon of 2014, after which the coniferous forests began to dry out in the Kurilskiy Nature Reserve, as well as in the protected zone. Drying of forests on the island of Kunashir is one of the main problems. The aim of this study was to detect and study the sites of attack of eight-dentated bark beetle (Ips typographus) in the reserve, laying test areas in the distribution foci. This study showed that spruce aged 50-70 are mainly susceptible to the attacks of eight-dentated bark beetle. However, it is worth noting that the attack of the bark beetle was seen more on Sakhalin spruce (Piceaglehnii), less - on Yezo spruce (Piceajezoensis) in the places where the test plots were laid. In addition, the study has identified significant differences between healthy, weakened trees and neighboring dead trees attacked by Ipstypographus. These differences were most pronounced in: individual shading, collective shading, and distance and tier factors. Forest pests have not been relatively studied previously. This study will give a new understanding of its ecology, as well as practical opportunities for its management

Snagfall the first decade after severe bark beetle infestation of high-elevation forests in Colorado, USA.

The persistence and fall rate of snags (standing dead trees) generated during bark beetle outbreaks have consequences for the behavior, effects, and suppression of potential wildfires, hazard tree and timber salvage operations, wildlife habitat, and numerous ecosystem processes. However, post-beetle snagfall dynamics are poorly understood in most forest types. We tagged standing live and dead lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii), including beetle-killed pine snags following the peak of a recent mountain pine bark beetle outbreak in watersheds at the Fraser Experimental Forest in northcentral Colorado and sampled snagfall 10 and 12 years later. Bark beetle attacks began in 2003, peaked by 2006, and killed 78% of overstory lodgepole pine in 133 plots distributed across a range of stand and site conditions. Of those snags, only 17% fell between 2007 and 2018. Most snags broke at ground level, due to butt rot, and were oriented downhill. In contrast, snags that tipped up or snapped off above the ground were oriented with the prevailing winds. Equal numbers of snags fell singly and in multiple-tree groups, and equal numbers remained elevated rather than in contact with the ground. Lodgepole pine snagfall was 1.6-times higher on steep slopes (>40%) where dead pine density was higher, compared to flatter sites. Based on our findings and previous research, we estimate that one-half the beetle-killed lodgepole pine in high-elevation forests such as those at Fraser may fall within 15-20yr of beetle infestation, but that some pine snags are likely to persist for decades longer. Post-outbreak snagfall dynamics create a multiple-decade legacy of bark beetle outbreaks that will persist longer in high-elevation compared to lower-elevation forests.

Longitudinal study of Caribbean pine elucidates the role of 4-allylanisole in patterns of chemical resistance to bark beetle attack

AbstractSouthern pine beetles (Dendroctonus frontalis Zimmermann) and symbiotic fungi are associated with mass mortality in stands of Caribbean pine (Pinus caribaea Morelet). This study provides a 12.7-year assessment of semiochemical mediation between southern pine beetle and Caribbean pine in relation to concentrations of 4-allylanisole (estragole, methyl chavicol) and monoterpenes measured by gas chromatography–mass spectrometry in different seasons in premontane and coastal pine stands of Belize and Guatemala. Individual trees and stands with >2.5% (relative mass %) of 4-allylanisole in the xylem oleoresin exhibited significantly less beetle-induced mortality than those with <2.5%. Changes in relative levels of 4-allylanisole and monoterpenes during this study are consistent with seasonal temperature and cumulative water deficit effects and suggest bark beetle attack of P. caribaea may intensify in the future.

Long-Lasting Insecticide Netting for Protecting Tree Stems from Attack by Ambrosia Beetles (Coleoptera: Curculionidae: Scolytinae).

Ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) are destructive wood-boring insects of horticultural trees. We evaluated long-lasting insecticide netting for protecting stems against ambrosia beetles. Container-grown eastern redbud, Cercis canadensis, trees were flood-stressed to induce ambrosia beetle attacks, and deltamethrin-treated netting was wrapped from the base of the stem vertically to the branch junction. Trees were deployed under field conditions in Ohio, Virginia, Tennessee, and Mississippi with the following treatments: (1) flooded tree; (2) flooded tree with untreated netting; (3) flooded tree with treated ‘standard mesh’ netting of 24 holes/cm2; (4) flooded tree with treated ‘fine mesh’ netting of 28 holes/cm2; and/or (5) non-flooded tree. Treated netting reduced attacks compared to untreated netting and/or unprotected trees in Mississippi in 2017, Ohio and Tennessee in 2018, and Virginia in 2017–2018. Inconsistent effects occurred in Mississippi in 2018. Fewer Anisandrus maiche, Xylosandrus germanus, and Xyleborinus saxesenii were dissected from trees deployed in Ohio protected with treated netting compared to untreated netting; trees deployed in other locations were not dissected. These results indicate long-lasting insecticide netting can provide some protection of trees from ambrosia beetle attacks.

Larger Resin Ducts Are Linked to the Survival of Lodgepole Pine Trees During Mountain Pine Beetle Outbreak.

Periodic mountain pine beetle outbreaks have killed millions of hectares of lodgepole pine forests in western North America. Within these forests some pine trees often remain alive. It has been rarely documented whether anatomical defenses differ between beetle-killed and remaining live pine trees, especially at the northern latitudinal range of beetles in North America. In this study, we compared the resin duct-based anatomical defenses and radial growth between beetle-killed and live residual lodgepole pine trees, and we characterized the resin ducts and the growth of the residual trees before and after outbreak. We found that tree radial growth was not associated with tree survival. The best two predictors of tree survival were resin duct size and production (number per year). Trees having larger but fewer resin ducts showed higher survival probability compared to those with smaller but more abundant resin ducts annually. Residual trees had larger resin ducts prior to the outbreak and continued having so after the outbreak. We further categorized residual trees as healthy (having no signs or symptoms of insect or pathogen attacks), declining (with signs or symptoms of biotic attacks), and survived (from mountain pine beetle attacks during the outbreak) to investigate resin duct-based anatomical defenses among them. Healthy trees had consistently larger resin ducts than declining trees in the past 20 years in post-outbreak stands. Survival trees ranked between healthy and declining trees. Overall, these results demonstrate that resin duct size of lodgepole pine trees can be an important component of tree defenses against mountain pine beetle attacks and suggest that lodgepole pine trees with large resin ducts are likely to show resistance to future bark beetle attacks.

Зміни вмісту монотерпенів живиці сосни у звʼязку з ураженням кореневою губкою (Heterobasidion annosum (Fr.) Bref.)

За допомогою газорідинної хроматографії виявлено тенденції змін вмісту монотерпенів у складі живиці дерев сосни IV класу віку, уражених кореневою губкою у різному ступені. Виявлено, що з розвитком хвороби вміст α-пінену і камфену знижується. Найменший їхній уміст, якщо порівняти зі здоровими деревами, виявлено у дерев, що знаходяться в активній зоні осередку всихання, ступінь ураження коріння яких становить 11–30 %. Уміст ∆3-карену в складі живиці цих дерев проти неуражених є, навпаки, вищим на 15–16 %. Уміст β-пінену в уражених кореневою губкою дерев був в 1,8–2,0 разу вищим, ніж у неуражених. Відзначено, що зміни в складі ефірних олій дерев в активній зоні осередку хвороби є сигналом щодо ослаблення дерева та можливості його заселення лубоїдами. Зміни в складі ефірних олій живиці в осередках кореневої губки провокують як патологічні процеси, так і зміни екології дерев унаслідок розвитку хвороби.

Priming of inducible defenses protects Norway spruce against tree-killing bark beetles.

Plants can form an immunological memory known as defense priming, whereby exposure to a priming stimulus enables quicker or stronger response to subsequent attack by pests and pathogens. Such priming of inducible defenses provides increased protection and reduces allocation costs of defense. Defense priming has been widely studied for short-lived model plants such as Arabidopsis, but little is known about this phenomenon in long-lived plants like spruce. We compared the effects of pretreatment with sublethal fungal inoculations or application of the phytohormone methyl jasmonate (MeJA) on the resistance of 48-year-old Norway spruce (Picea abies) trees to mass attack by a tree-killing bark beetle beginning 35days later. Bark beetles heavily infested and killed untreated trees but largely avoided fungus-inoculated trees and MeJA-treated trees. Quantification of defensive terpenes at the time of bark beetle attack showed fungal inoculation induced 91-fold higher terpene concentrations compared with untreated trees, whereas application of MeJA did not significantly increase terpenes. These results indicate that resistance in fungus-inoculated trees is a result of direct induction of defenses, whereas resistance in MeJA-treated trees is due to defense priming. This work extends our knowledge of defense priming from model plants to an ecologically important tree species.

Plant Hydraulic Stress Explained Tree Mortality and Tree Size Explained Beetle Attack in a Mixed Conifer Forest

AbstractDrought predisposes conifer forests to bark beetle attacks and mortality. Although plant hydraulic stress mechanistically links to tree mortality, its capacity to predict trees' susceptibility to beetle attacks has not been evaluated. Further, both tree size and water supply could influence plant hydraulic stress, but their relative importance remained unknown. In this study, we modeled plant hydraulic stress of individual trees in a mixed forest of Lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and Subalpine fir (Abies lasiocarpa) in southern Wyoming, using an integrated model of plant hydraulics and hydrology, ground surveys of tree size as well as physiological and geophysical measurements. Based on the established link between plant hydraulic stress and tree mortality, we found interspecific differences in the relative importance of water availability and tree size. Pine mortality was best explained by the combination of tree size and water supply, and fir mortality was best explained by variations in water supply. We next compared the prediction of beetle attack by modeled plant hydraulic stress versus tree size and found tree size best explained beetle attack consistently for all three species. Taken together, our results suggested beetle attack was primarily influenced by beetle preference for large trees, potentially as food sources, rather than more hydraulically stressed trees. These findings highlighted the importance of integrated understanding of biotic/abiotic factors and their mechanistic pathways in order to accurately predict the sustainability of forests susceptible to drought and beetle outbreaks.

Evaluation of botanical extracts against <i>Callosobruchus maculatus</i> F. (Coleoptera: Chrysomelidae) on different host grains

As the damage of stored legume grains by <em>Callosobruchus maculatusis</em> severe in Sri Lanka, the present study was conducted to find the remedial solution for this existing problem. The experiment was designed to select the most effective botanical insecticides; Azadirachta indica 10 %, Vitex negundo10 %, Eucalyptus globulus10 %, Annona squamosal 10 % and Allium sativum 10 % against the insect pest, <em>Callosobruchus maculatus</em> (bruchid beetle) on stored grains; Vigna unguiculata var. Waruni (red cowpea), Vigna unguiculata var. Dhawala (cowpea with black eye), Vigna radiate (green gram) and Cicer arietinum (chickpea). Among these treatments, A. indica 10 % and A. sativum 10 % were found to be the significantly best along with 80 % of mortality in adult weevils and zero damaged seeds in all treated grains whereas V. negundo 10 % and E. globulus 10 % had average mortality effect (40-50 %) at the beginning of the study. Apart from this zero seed, damage was observed in chickpea treated with all treatments including untreated control. The study showed the non-preference of chickpea by <em>C. maculatus</em> as the seed coating was resistant to the beetle attack.

Tratamientos de preservación de Bambusa vulgaris vittata contra el ataque de Dinoderus minutus

Las cañas de bambú son un material de construcción único para diversos tipos de estructuras y cada vez hay una mayor aceptación de los bambúes para usos estructurales. Sin embargo, tienen como inconveniente su propensión al ataque por insectos. En este trabajo se hizo un estudio de la eficacia de tres diferentes tipos de preservadores: 1) químicos tradicionales, 2) naturales de bajo riesgo para el ser humano y 3) térmicos a diferentes temperaturas, para prevenir el ataque del insecto escarabajo Dinoderus minutus al bambú de la especie Bambusa vulgaris vittata. Los dos primeros tratamientos se realizaron por inmersión y el tipo térmico fue hecho por el proceso de secado. Cada dos semanas, se evaluó el ataque a tres especímenes de cada tratamiento, incluyendo los correspondientes a un testigo sin elemento preservador alguno. Los resultados mostraron que la mayoría de los tratamientos por inmersión fueron eficaces, reduciendo considerablemente el ataque de los insectos en comparación con el testigo.En cuanto al tratamiento térmico, las temperaturas menores o iguales a 130 °C no mostraron ser eficaces; las temperaturas de 190 °C y 210 °C revelaron eficacias aceptables.

Whitebark Pine in Crater Lake and Lassen Volcanic National Parks: Assessment of Stand Structure and Condition in a Management and Conservation Perspective

Whitebark pine (Pinus albicaulis. Engelm.) is vulnerable to a number of threats including an introduced pathogen (Cronartium ribicola J.C. Fisch.), epidemic levels of native mountain pine beetle (Dendroctonus ponderosae Hopkins), fire suppression, and climate change. To describe the structure of whitebark pine populations in two national parks in the southern Cascades (Crater Lake, Oregon, USA (CRLA) and Lassen Volcanic, California, USA (LAVO) National Parks), we surveyed trees in 30 × 50 × 50 m plots in both parks. We used these plots to describe the extent of white pine blister rust (the disease caused by Cronartium ribicola), mountain pine beetle occurrence, and to elucidate factors influencing the presence of pests and pathogens, cone production, and canopy kill. In each plot, we recorded data related to tree health, including symptoms of blister rust and mountain pine beetle, and reproductive vigor (cone production). In both parks, encroachment from other species, particularly mountain hemlock (Tsuga mertensiana (Bong.) Carrière), was negatively associated with cone production. In CRLA, water stress was a good predictor of blister rust infection and cone production. For CRLA and LAVO, the presence of mountain pine beetle and blister rust was associated with higher canopy kill for whitebark pine. Lastly, we found evidence for a pest-pathogen interaction, mountain pine beetle attack was greater for trees that showed symptoms of blister rust infection in CRLA. Our results indicate that whitebark pine populations in the southern Cascade Range are experiencing moderate levels of blister rust infection compared with other sites across the species range, and that competition from shade-tolerant species may result in an additional threat to whitebark pine in both parks. We present our findings in the context of park management and situate them in range-wide and regional conservation strategies aimed at the protection and restoration of a declining species.

DEAD WOOD DETECTION BASED ON SEMANTIC SEGMENTATION OF VHR AERIAL CIR IMAGERY USING OPTIMIZED FCN-DENSENET

Abstract. The assessment of the forests’ health conditions is an important task for biodiversity, forest management, global environment monitoring, and carbon dynamics. Several research works were proposed to evaluate the state condition of a forest based on remote sensing technology. Concerning existing technologies, employing traditional machine learning approaches to detect the dead wood in aerial colour-infrared (CIR) imagery is one of the major trends due to its spectral capability to explicitly capturing vegetation health conditions. However, the complicated scene with background noise restricted the accuracy of existing approaches as those detectors normally utilized hand-crafted features. Currently, deep neural networks are widely used in computer vision tasks and prove that features learnt by the model itself perform much better than the hand-crafted features. The semantic image segmentation is a pixel-level classification task, which is best suitable to dead wood detection in very high resolution (VHR) mode because it enables the model to identify and classify very dense and detailed components on the tree objects. In this paper, an optimized FCN-DenseNet is proposed to detect dead wood (i.e. standing dead tree and fallen tree) in a complicated temperate forest environment. Since the appearance of dead trees generally occupies greatly different scales and sizes; several pooling procedures are employed to extract multi-scale features and dense connection is employed to enhance the inline connection among the scales. Our proposed deep neural network is evaluated over VHR CIR imagery (GSD-10cm) captured in a natural temperate forest in Bavarian national forest park, Germany, which has undergone on-site bark beetle attack. The results show that the boundary of dead trees can be accurately segmented, and the classification are performed with a high accuracy, even though only one labelled image with moderate size is used for training the deep neural network.

Drought-Mediated Changes in Tree Physiological Processes Weaken Tree Defenses to Bark Beetle Attack.

Interactions between water stress and induced defenses and their role in tree mortality due to bark beetles are poorly understood. We performed a factorial experiment on 48 mature ponderosa pines (Pinus ponderosa) in northern Arizona over three years that manipulated a) tree water stress by cutting roots and removing snow; b) bark beetle attacks by using pheromone lures; and c) phloem exposure to biota vectored by bark beetles by inoculating with dead beetles. Tree responses included resin flow from stem wounds, phloem composition of mono- and sesqui-terpenes, xylem water potential, leaf gas exchange, and survival. Phloem contained 21 mono- and sesqui-terpenes, which were dominated by (+)-α-pinene, (-)-limonene, and δ-3-carene. Bark beetle attacks (mostly Dendroctonus brevicomis) and biota carried by beetles induced a general increase in concentration of phloem mono- and sesqui-terpenes, whereas water stress did not. Bark beetle attacks induced an increase in resin flow for unstressed trees but not water-stressed trees. Mortality was highest for beetle-attacked water-stressed trees. Death of beetle-attacked trees was preceded by low resin flow, symptoms of water stress (low xylem water potential, leaf gas exchange), and an ephemeral increase in concentrations of mono- and sesqui-terpenes compared to surviving trees. These results show a) that ponderosa pine can undergo induction of both resin flow and phloem terpenes in response to bark beetle attack, and that the former is more constrained by water stress; b) experimental evidence that water stress predisposes ponderosa pines to mortality from bark beetles.

Short-term forecasting of bark beetle outbreaks on two economically important conifer tree species

In recent years bark beetles have been shown to be an important risk factor in European forests. An early warning system is needed to mitigate bark beetle damage, and short-term forecasting models that assist efforts to identify attacked trees comprise an important part of such a system. The aim of this study was to develop short-term forecasting models of the probability of bark beetle outbreaks on two important conifer tree species: Norway spruce (Picea abies) and silver fir (Abies alba). For the development of the models, we used a time series of 20 years of sanitary felling because of bark beetles and relief data (altitude, slope and exposition), several soil variables, climate data (temperature and SPI), sanitary felling because of bark beetles, sanitary felling due to harmful abiotic factors, and amount of weakened trees due to bark beetles. The forecasting variable was sanitary felling because of bark beetles in the current year. The models were developed with a general linear model with binomial error distribution. For the probability of bark beetle outbreaks on silver fir, the amount of fir, soil base saturation percentage, sanitary felling of attacked fir, weakened fir, and sanitary felling because of abiotic factors increased the probability of sanitary felling because of fir bark beetles. Altitude, exposition, slope, phosphorus, soil depth, soil cation exchange capacity, SPI and temperature decreased the probability of sanitary felling because of fir bark beetles. For Norway spruce, the amount of Norway spruce, soil base saturation percentage, SPI, temperature, amount of sanitary felling in the previous year, amount of weakened trees in the previous year, and amount of sanitary felling because of abiotic factors in the previous year increased the probability of sanitary felling of Norway spruce because of bark beetles in the current year. Slope, soil cation exchange capacity, and precipitation decreased the probability of sanitary felling because of bark beetles in the current year. The performance of the bark beetle risk model for Norway spruce was very good. The performance of the model for silver fir was also good, but not on par with that for Norway spruce. Therefore, additional research on fir bark beetles is needed to further improve the risk model for bark beetle attacks on silver fir.

Forest Decline Triggered by Phloem Parasitism-Related Biotic Factors in Aleppo Pine (Pinus halepensis)

Climate models predict increasing mean temperatures and reduced precipitation for Mediterranean ecosystems already subjected to major hydrological fluctuations. Forest decline phenomena relate extreme droughts or heat waves with other organisms, e.g., insects or microorganisms acting as pests, but their role needs to be elucidated. A biotic factor responsible for forest diseases is Candidatus Phytoplasma pini which is a phloem-parasitism that negatively affects Spanish pine forests in drought-prone areas. In several healthy and declining Aleppo pine stands, we monitored pine infection by PCR (Polimerase Chain Reation), determined the tree phloem tissue terpene composition, carbohydrate content, measured several relevant morpho-physiological variables and examined trees affected by bark beetles. PCR confirmed C. P. pini infection was widespread in all stands, regardless of to the presence of symptomatically affected trees. However, visible symptomatic decline only occurred in trees living under more stressful conditions. The terpene composition of pines in declining stands differed from those in healthy ones, and could be related with bark beetle attacks when pines were previously weakened by the phytoplasma disease. Our results indicate that biotic factors, such as C. P. pini, affecting phloem tissue may be triggering factors for drought-mediated forest decline and suggest that phloem diseases can play a key role in forest declining processes during extreme drought.

Changes in Content of Polyphenols and Ascorbic Acid in Leaves of White Cabbage after Pest Infestation.

Crops, such as white cabbage (Brassica oleracea L. var. capitata (L.) f. alba), are often infested by herbivorous insects that consume the leaves directly or lay eggs with subsequent injury by caterpillars. The plants can produce various defensive metabolites or free radicals that repel the insects to avert further damage. To study the production and effects of these compounds, large white cabbage butterflies, Pieris brassicae and flea beetles, Phyllotreta nemorum, were captured in a cabbage field and applied to plants cultivated in the lab. After insect infestation, leaves were collected and UV/Vis spectrophotometry and HPLC used to determine the content of stress molecules (superoxide), primary metabolites (amino acids), and secondary metabolites (phenolic acids and flavonoids). The highest level of superoxide was measured in plants exposed to fifty flea beetles. These plants also manifested a higher content of phenylalanine, a substrate for the synthesis of phenolic compounds, and in activation of total phenolics and flavonoid production. The levels of specific phenolic acids and flavonoids had higher variability when the dominant increase was in the flavonoid, quercetin. The leaves after flea beetle attack also showed an increase in ascorbic acid which is an important nutrient of cabbage.

Acute Drought Is an Important Driver of Bark Beetle Infestation in Austrian Norway Spruce Stands

Infestations of Norway spruce by the Eurasian spruce bark beetle Ips typographus have recently caused peaks in salvaged timber in Central European forests. Apart from extensive breeding material due to abiotic disturbance, increased spring and summer temperatures as well as precipitation deficits are presumably key drivers of bark beetle population dynamics. In this study, we investigated the influence of effective temperature sums, chronic and acute drought stress, and stand characteristics for explaining salvage logging due to bark beetle attack in Norway spruce stands of Austrian Federal Forests. We retrospectively simulated temperature and transpiration deficit as drought proxy by use of the monitoring tool PHENIPS-TDEF, a well-proven bark beetle phenology model, combined with an additional hydrological module developed for simulating transpiration deficits (TDEF) in forest stands. Overall, we found that bark beetle infestation occurred more frequently in stands with high share of Norway spruce, increased stand age and stand density. The probability of ongoing attacks was significantly higher in stands subject to salvage logging in the previous year. Chronically dry forest stands described in the forest inventory database as growing on shallow, xeric, and low moisture soil conditions, were significantly less prone to bark beetle attack across all forest enterprises. However, acute drought indicated by stand transpiration deficits was a significant predictor of bark beetle attacks, especially in stands geographically located in warmer and drier areas of the country. Although importance of TDEF parameters differed with geographic location and observation period, the clear increase in attacks across all forest stands in 2015 was principally linked to previous year’s and actual summer transpiration deficits. We conclude from our results that in the absence of severe abiotic disturbance a combination of ample host availability, favorable temperature conditions for bark beetle development, and acute disposition of trees to attack caused by drought stress can intensify population growth and very likely lead to bark beetle mass outbreaks.

Timing of red-edge and shortwave infrared reflectance critical for early stress detection induced by bark beetle (Ips typographus, L.) attack

Forest disturbance in Europe, induced by European spruce bark beetle Ips typographus, L., results in regional-scale dieback. Early stress detection in Norway spruce stands caused by bark beetle infestation at the green attack stage (when trees are yet to show distinct symptoms observable by the human eye) is crucial and can lead to improved forest management and reduced economic losses. This study aims to investigate and understand the dynamics of leaf traits and reflectance of Norway spruce (Picea abies) trees during bark beetle attack. Using high-resolution temporal images from RapidEye and SPOT-5 in parallel with the collection of field data, we examined which spectral regions and leaf traits are affected by infestation over time and how they help the discrimination between healthy and infested plots at the early stage of the attack. To achieve this aim, we used a novel approach by targeting both leaf and canopy level. We measured leaf reflectance spectra and six leaf traits (water content, nitrogen, chlorophyll fluorescence, chlorophyll and stomatal conductance) from 66 (30 plots) healthy and 54 (8 plots) infested trees at three consecutive time measurements in the summer of 2015 in the Bavarian Forest National Park. Concurrently, canopy reflectance and spectral vegetation indices (SVIs) were extracted from the selected plots (30 healthy plots) using seven RapidEye images and six SPOT-5 images. For the infested plots, in addition to the field measured plots (8), canopy spectral reflectance were extracted from the reference infestation data (291 plots) obtained through visual interpretation of high-resolution aerial photographs. Results demonstrated significant differences (p < 0.05) in the studied leaf traits between healthy and infested samples, and these differences increased with the progression of infestation. We found that leaf and canopy reflectance were significantly higher (p ≤ 0.05) for the infested trees by bark beetle than the healthy ones in the ‘red edge’ (680–790 nm) and ‘shortwave infrared’ (1110–1490 nm) spectrum throughout the infestation event. Our results further demonstrated that the spectral vegetation indices calculated from the red-edge and SWIR spectral bands, such as NDRE, DSWI, LWCI and NDWI, were able to differentiate between healthy and infested trees earlier than the other SVIs. The new insight offered by these results is that the red-edge and SWIR spectral information from multispectral satellites has the potential to considerably improve monitoring and detection of forest stress and has important implications for European field bark beetle management and future studies.

Advances in Semiochemical Repellents to Mitigate Host Mortality From the Spruce Beetle (Coleoptera: Curculionidae).

We tested 3-methyl-2-cyclohexen-1-one (MCH) and novel semiochemicals as potential spruce beetle (Dendroctonus rufipennis Kirby) (Coleoptera: Curculionidae, Scolytinae) repellents over multiple years in Utah and Colorado trapping bioassays. MCH is a known spruce beetle repellent and our testing revealed Acer kairomone blend (AKB) and isophorone plus sulcatone as repellents. We subsequently tested these semiochemicals for area and single tree protection to prevent spruce beetle attacks at locations in Utah, Colorado, Wyoming, New Mexico, and Alaska. Individual tree protection trials found MCH-AKB provided significant protection against spruce beetle attacks in the southern Rocky Mountains but not in Alaska. Adding sulcatone or doubling MCH-AKB pouches did not further enhance protection. A degree of protection was extended to spruce at least 10 m distant from the repellents, including in Alaska. Tree diameter was not a significant covariate among treated trees but was positively correlated with the probability of infestation for surrounding spruce. In area protection trials, spruce in control plots were 2.4 times more likely to be in a higher severity attack class compared with spruce in plots treated with MCH-AKB pouches deployed at 30 sets per hectare. Tree diameter had a significant, positive relationship to the probability of infestation. We found MCH-AKB to offer a high degree of protection against beetle attack in Engelmann spruce (Picea engelmannii Parry ex Engelm.) (Pinales: Pinaceae) (Picea engelmannii Parry ex Engelm.) (Pinales: Pinaceae), especially for single tree protection (66% of control trees were strip- or mass-attacked compared with 6% of repellent-treated trees). AKB requires registration and labeling, however, before this economical and environmentally benign semiochemical can be used operationally.