Repellent Effects of Host Essential Oils and Limited Effects of Host Length on Tree‐Killing Bark Beetle Attacks

Climate change is estimated to increase the risk of the bark beetle (Ips typographus L.) mass outbreaks in Norway Spruce (Picea abies (L.) Karst) forests. Habitats that are thermally suitable for bark beetles may expand, and an increase in the frequency and intensity of droughts can promote drought stress on host trees. Drought affects tree vigor and in unison with environmental features it influences the local predisposition risk of forest stands to bark beetle attacks. We aimed to study how various environmental features influence the risk of bark beetle attacks during a drought year and the following years with more normal weather conditions but with higher bark beetle populations. We included features representing local forest stand attributes, topography, soil type and wetness, the proximity of clear-cuts and previous bark beetle attacks, and a machine learning algorithm (random forest) was applied to study the variation of predisposition risk across a 48,600 km2 study area in SE Sweden.Forest stands with increased risk of bark beetle attack were distinguished with high accuracy both during drought and in normal weather conditions. The results show that during both study periods, spruce and mixed coniferous forests had elevated risk of attack, while forests with a mix of deciduous and coniferous trees had a lower risk. Forests with high average canopy height were strongly predisposed to bark beetle attacks. However, during the drought year risk was more similar between stands with lower and higher canopy height, suggesting that during drought periods younger trees can be predisposed to bark beetle attacks. The importance of soil moisture and position within the local landscape were highlighted as important features during the drought year.Identifying areas with increased risk, supported by information on how environmental features control the predisposition risk during drought, could aid adaptation strategies and forest management intervention efforts. We conclude that geospatial data and machine learning have the potential to further support the digitalization of the forest industry, facilitating development of methods capable to quantify importance and dynamics of environmental features controlling the risk in local context. Corresponding methods could help to direct management actions more effectively and offer information for decision-making in changing climate.

Protection of spruce from colonization by the bark beetle, Ips perturbatus, in Alaska

Bark beetle infestation is a major driver of tree mortality that may be critical for forest persistence under climate change and the forecasted increase of extreme heat and drought episodes. Under this context, the environmental position of host tree populations within the species' climatic niche (central vs. marginal populations) is expected to be a determinant in the dynamics of insect-host systems. Here, we analyzed the recent patterns of bark beetle disturbance and forest resistance across European coniferous forests during the 2010-2018 period. We obtained bark beetle attack and tree mortality data from successive continental-scale forest condition surveys on 130 plots including five host trees and five bark beetle species, and characterized the climatic niche of each species. Then, we analyzed the overall forest resistance and species-specific responses, in terms of bark beetle attack and induced tree mortality, in relation to the distance to the niche optimum of both host tree and beetle species, previous drought events, and plot characteristics. Regional patterns of recent disturbance revealed that forests in central, north, and east of Europe could be at risk under the attack of multivoltine bark beetle species. We found that overall forest resistance to beetle attack was determined by several driving factors, which varied among species responses. Particularly, the environmental position of the affected forest within the host and beetle species' climatic niche and plot characteristics mediated the influence of drought on the resistance to beetle attack. In turn, forest resistance to induced tree mortality was determined exclusively by the maximum intensity and duration of drought events. Our findings highlight the importance of disturbance interactions and suggest that the joint influence of drought events and bark beetle disturbance will threaten the persistence of European coniferous forests, even in those tree populations close to their species' climatic optimum.

Previous studies have suggested that bark beetles and fires can be interacting disturbances, whereby bark beetle-caused tree mortality can alter the risk and severity of subsequent wildland fires. However, there remains considerable uncertainty around the type and magnitude of the interaction between fires following bark beetle attacks, especially in drier forest types such as those dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson). We used a full factorial design across a range of factors thought to control bark beetle-fire interactions, including the temporal phase of the outbreak, level of mortality, and wind speed. We used a three-dimensional physics-based model, HIGRAD/FIRETEC, to simulate fire behavior in fuel beds representative of 60 field plots across five national forests in northern Arizona, USA. The plots were dominated by ponderosa pine, and encompassed a gradient of bark beetle-caused mortality due to a mixture of both Ips and Dendroctonus species. Non-host species included two sprouting species, Gambel oak (Quercus gambelii Nutt.) and alligator juniper (Juniperus deppeana Steud.), as well as other junipers and pinyon pine (Pinus edulis Engelm.). The simulations explicitly accounted for the modifications of fuel mass and moisture distribution caused by bark beetle-caused mortality. We first analyzed the influence of the outbreak phase, level of mortality, and wind speed on the severity of a subsequent fire, expressed as a function of live and dead canopy fuel consumption. We then computed a metric based on canopy fuel loss to characterize whether bark beetles and fire are linked disturbances and, if they are, if the linkage is antagonistic (net bark beetle and fire severity being less than if the two disturbances occurred independently) or synergistic (greater combined effects than independent disturbances). Both the severity of a subsequent fire and whether bark beetles and fire are linked disturbances depended on the outbreak phase of the bark beetle mortality and attack severity, as well as the fire weather (here, wind). Greater fire severity and synergistic interactions were generally associated with the “red phase” (when dead needles remain on trees). In contrast, during the “gray phase” (when dead needles had fallen to the ground), fire severity was either similar to, or less than, green-phase fires and interactions were generally antagonistic, but included both synergistic and neutral interactions. The simulations also revealed that the magnitude of the linkage between these two disturbances was smaller for fires occurring during high wind conditions, especially in the red phase. This complexity might be a reason for the contrasted or controversial perception of bark beetle-fire interactions reported in the literature, since both fire severity and the type and magnitude of the linkage can vary strongly among studies. These results suggest that, for fires burning in the gray phase following moderate levels of mortality, bark beetle-caused mortality may buffer rather than exacerbate fire severity. However, for fires burning under high wind speeds, regardless of the outbreak phase or level of mortality, the near complete loss of canopy fuels may push this ecosystem into an alternative state dominated by sprouting species.

Intensive forest afforestation with native pine species was developed in the 1960s on degraded and deforested lands in the region of the Eastern Rhodopes (south-eastern Bulgaria). Severe damage by wet snow was registered in the coniferous forests of the Rhodopes in March 2015. In the following years, bark beetle attacks were registered on the broken and felled fresh wood. As a result, bark beetle infestation spots appeared in the pine plantations. In the period 2019–2021, damage caused by bark beetles was assessed in the region of State Forestry Kirkovo (the Eastern Rhodopes, south-eastern Bulgaria). An integrated approach using the data of the information system of the Executive Forest Agency (ISEFA), remote sensing data obtained by an “eBee SQ” unmanned aerial vehicle (UAV) equipped with a “Parrot Sequoia” multispectral camera, and subsequent terrestrial observations, was applied. ISEFA data showed that there was no serious damage caused by abiotic and biotic factors in the pine forests of SF Kirkovo until 2014. Snow damage in 2015 affected 513 ha of pine plantations, and bark beetle infestations reached up to 1316 ha in 2016. In 2019, a total of 226.87 ha of pine plantations were captured in three localities—Fotinovo, Kirkovo, and Kremen. The relative share of damage caused by bark beetles was greater in P. sylvestris plantations (15.3–23.0%), compared to damage in P. nigra (2.3%). Four different categories of normalised difference vegetation index (NDVI) were separated in bark beetle infestation spots—living trees, dead trees, grass and shrub vegetation, stones and rocks. The NDVI values in locations with living trees varied between 0.500 (spaces between tree crowns) and 0.700 (central part of the crown projection) (an average of 0.617). In the locations with dead trees, the average values of NDVI of lying trees was 0.273, and in standing trees, NDVI varied between 0.275 (central part of crown projections) and 0.424 (spaces between tree crowns). In the locations with grass and shrub vegetation, stones and rocks, the average NDVI was 0.436 and 0.329, respectively. In the field study, average defoliation of 31.2–32.3% was registered in P. sylvestris plantations, and 47.4% in P. nigra plantations. Defoliations mainly were caused by pine processionary moth (Thaumetopoea pityocampa) and fungal pathogens (Dothistroma septosporum and Lecanosticta acicola). The damage was caused by Ips acuminatus (in P. sylvestris only), and I. sexdentatus, Tomicus piniperda and T. minor (in P. sylvestris and P. nigra). Infestations by other xylophages, such as Phaenops cyanea, Rhagium inquisitor, and Pissodes spp., were also found on pine stems.

A review of semiochemicals associated with bark beetle (Coleoptera: Curculionidae: Scolytinae) pests of coniferous trees: A focus on beetle interactions with other pests and their associates

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

Abstract1. Better understanding of wood‐boring beetle distribution patterns and underlying mechanisms is a central issue for forest protection and management. The primary aim of this study was to detect how these pest guilds are distributed in forests and optimize elimination and control strategies.2. We placed flight intercept traps in the canopy and understory areas of three climatically different forest types across a large elevation gradient and focussed on two important wood‐boring beetles, longhorn and bark beetles, in Yunnan, Southwest China.3. The results showed that species richness and abundance of two selected wood‐boring beetles between the canopy and understory areas had different patterns across the climatically different forest vegetation types. Bark and longhorn beetles tended to stay in understory and canopy areas, respectively, based on the recorded abundance data. However, richness data showed different responses to the climatically different forest types for both longhorn and bark beetles assemblages. Bark beetles species richness did not significantly differ between the canopy and understory areas across all three forest types in the present study. Longhorn beetles had significantly higher richness values in the canopy areas of tropical and subtropical forests but lower in temperate forest.4. The community compositions of both the longhorn and bark beetles varied greatly among forest vegetation types but relatively little between the canopy and understory areas across the vegetation types. Thus, considering different heights and positions in different forests during trap deployment is necessary and needed to optimize wood‐boring pest control strategies when various forest types and pests are targeted.

Temperate forests provide ecosystem services such as protecting water environment, timber and fuel production, carbon sequestration, and reduction of nutrient loss. During the last decades, large forest areas were decimated world-wide by bark beetle attacks. Under climate change, drought and higher temperatures increase the risk of infestation. Future forest ecosystem services are therefore at risk of deterioration and it is essential to understand how bark beetle attacks and following management practices influence nutrient cycling, nitrate (NO3) and dissolved organic carbon (DOC) fluxes in seepage water. After dieback, accelerated mobilization of nutrients can be expected due to an increase in mineralization rates and the lack of plant nutrient uptake, whereas the lack of litter input may reduce nutrient leaching. The significantly reduced interception and evapotranspiration might furthermore increase soil water contents and seepage fluxes. Regeneration strategies (e.g. site clearance vs. keeping the dead trees, natural vs. artificial regeneration, regeneration with nurse crops) are decisive for the extent and persistence of the impact of calamities on water quality and quantity. We use a meta-data-analysis to gather knowledge out of approx. 60 studies around the world, to assess the expected behaviours of DOC and NO3 concentrations in seepage water and streams after bark beetle outbreaks in temperate forests and to identify gap of knowledge. Most studies focussed on nitrate leaching and only few on DOC. Overall, DOC concentrations increase in seepage water and streams directly after dieback, reaching a peak 2 to 3 years after disturbance. In the opposite, the first evidences of increased NO3 concentrations are visible approximately one year after disturbance and peak is reached within 3 to 10 years (on average after 5 years), when DOC decreases. NO3 maxima never exceeded drinking water limit. In all studies, DOC and NO3 concentrations recovered to pre-event or, in some cases, were even below the pre-dieback conditions only few years after the peak. Forest ecosystems seem therefore to be resilient to disturbances showing overall rapid recovery of ecosystem functions. However, the timing and duration of the concentration peaks largely differed among the studies, which might be explained by the extent and velocity of tree dieback in the studied areas, the harvest management practices and the type of vegetation re-growth after disturbance, but also by the local climatic conditions and the catchment size. Only few studies specifically analysed these effects on nutrient fluxes and their results differ considerably. More research is needed for assessing the influence of different regeneration strategies after calamities on water quality risks in forested catchments. A bark beetle attack currently decimating the Norway Spruce forest in the well-monitored Lange Bramke catchment (Harz, Germany), offers a unique opportunity to answer this question. With long-term datasets of NO3 and DOC concentrations in stream and the recent installation of a network of lysimeters at three soil depths in a) a healthy forest area, and infested areas b) with dead trees standing, and c) with site clearance, we will be able to better understand the effect of regeneration strategies on nutrient fluxes.    

Effect of bark beetle (Ips typographus L.) attack on bark VOC emissions of Norway spruce (Picea abies Karst.) trees

На території Ботанічного саду Харківського національного університету (ХНУ) ім. В. М. Каразіна на хвойних породах визначено 4 види короїдів: короїда-типографа (Ips typographus L.), гравера звичайного (Pityogenes chalcographus L.), соснового короїда-крихітку (Crypturgus cinereus Herb.) та тайгового короїда-крихітку (C. subcribrosus Eg.). Відзначено, що з роду Abies лише п’ять видів не мали ознак заселення, а саме: A. balsamia, A. cephalonica, А. concolor, A. grandis, А. violacea. Picea koraiensis та P. alba не були заселені короїдами, але на стовбурі помічено спроби заселення. Встановлено, що форму P. obovata «Glauca» заселяли всі визначені види короїдів. Найбільш поширеним на ялині серед інших видів короїдів виявився Ips typographus. Серед 198 обстежених дерев визначено: живих із відсутністю ознак заселення короїдами – 27,3 %, зі спробами заселення – 9,1 %, заселених короїдами – 63,6 %.

The success of tree colonization by bark beetles depends on their ability to overcome host tree defenses, including resin exudation and toxic chemicals, which deter bark beetle colonization. Resin defenses during insect outbreaks are challenging to study in situ, as outbreaks are stochastic events that progress quickly and thus preclude the establishment of baseline observations of non-infested controls. We use synthetic aggregation pheromones to demonstrate that confined Ips bark beetle herbivory can be successfully initiated to provide opportunities for studying interactions between bark beetles and their hosts, including the dynamics of constitutive and induced resin exudation. In Pinus taeda L. plantations between 12 and 19 years old in North and South Carolina, U.S., trees were affixed with pheromone lures, monitored for evidence of bark beetle attacks, and resin samples were collected throughout the growing season. Baiting increased beetle herbivory to an extent sufficient to produce an induced resin response. Attacked trees exuded about three times more resin at some time than control trees. This supports previous work that demonstrated that information on constitutive resin dynamics alone provides an incomplete view of a host tree’s resistance to bark beetle attack.

The effects of a large-scale ice storm event on the drivers of bark beetle outbreaks and associated management practices

Scots pine (Pinus sylvestris L.) mortality is explained by the climatic suitability of both host tree and bark beetle populations

Host and habitat selection in flight by conifer bark beetles is governed by inhibition of attraction by non‐host volatiles (NHV), in addition to the well‐known attraction to kairomones from host plants and to aggregation pheromones from conspecifics. Antennally‐active NHV from angiosperm birch and aspen trees were field tested on the European spruce bark beetle, Ips typographus (L.), by pheromone baited traps in Sweden using release rates of NHV comparable to those from a non‐host tree. Trap catches were significantly reduced by 50‐70% by individual NHV: trans‐conophthorin, 3‐octanol and 1‐octen‐3‐ol from non‐host bark, 1‐hexanol and (Z)‐3‐hexen‐1‐ol from both leaves and bark and (E)‐2‐hexen‐1‐ol from leaves. Combined NHV signals in binary and ternary blends with the same overall release rates (ca 5 mg/day) showed both redundancy and synergism in their inhibitory effects. The redundancy occurred among individual green leaf alcohols (GLV‐alcohols or C6‐alcohols), C8‐alcohols, and between these two groups, while trans‐conophthorin and verbenone (Vn, from unsuitable host trees) showed significant synergistic effects between each other and with GLV‐alcohols or C8‐alcohols. The coexistence of redundancy and synergism in non‐host chemical signals may indicate different functional levels (non‐host habitats, species, and unsuitable host trees) of these negative volatile signals in the host selection process of conifer bark beetles. We showed an active inhibitory range (AIR) of a NHV‐blend plus Vn on a standard pheromone bait to be at least 2 m, by using a central pheromone trap and surrounding concentric barrier trap rings with radii of 1, 2, and 4 m. The influx of bark beetles flying across concentric rings within the AIR was reduced by 55–99%, depending on the distance to the central pipe trap. Our findings support the hypothesis that as mixed forests have greater semiochemical diversity than pure host stands; they disturb olfactory guided host choice, and may reduce the possibility of outbreaks of conifer‐infesting bark beetles. The synergistic effects as well as the active inhibitory range found in this study also suggest that optimal combinations of these NHV and verbenone may have potential in protecting forests against I. typographus by reducing or stopping attacks on suitable host trees. Mixed stands with higher semio‐chemical diversity disturb olfactory guided host choice and reduce the risk for outbreaks of specialist herbivores.. This ‘semiochemical‐diversity hypothesis’ provides new support to the general ‘stability‐diversity hypothesis’.