Bark Beetle Population Dynamics Research Articles

Understanding bark beetle outbreaks: exploring the impact of changing temperature regimes, droughts, forest structure, and prospects for future forest pest management

AbstractClimate change has increased the susceptibility of forest ecosystems, resulting in escalated forest decline globally. As one of the largest forest biomasses in the Northern Hemisphere, the Eurasian boreal forests are subjected to frequent drought, windthrow, and high-temperature disturbances. Over the last century, bark beetle outbreaks have emerged as a major biotic threat to these forests, resulting in extensive tree mortality. Despite implementing various management strategies to mitigate the bark beetle populations and reduce tree mortality, none have been effective. Moreover, altered disturbance regimes due to changing climate have facilitated the success of bark beetle attacks with shorter and multivoltine life cycles, consequently inciting more frequent bark beetle-caused tree mortality. This review explores bark beetle population dynamics in the context of climate change, forest stand dynamics, and various forest management strategies. Additionally, it examines recent advancements like remote sensing and canine detection of infested trees and focuses on cutting-edge molecular approaches including RNAi-nanoparticle complexes, RNAi-symbiotic microbes, sterile insect technique, and CRISPR/Cas9-based methods. These diverse novel strategies have the potential to effectively address the challenges associated with managing bark beetles and improving forest health in response to the changing climate.

Climate-driven tree growth and mortality in the Black Forest, Germany-Long-term observations.

Episodic tree mortality can be caused by various reasons. This study describes climate-driven tree mortality and tree growth in the Black Forest mountain range in Germany. It is based on a 68-year consistent data series describing the annual mortality of all trees growing in a forest area of almost 250 thousand ha. The study excludes mortality caused by storm, snow and ice, and fire. The sequence of the remaining mortality, the so-called "desiccated trees," is analyzed and compared with the sequence of the climatic water balance during the growing season and the annual radial growth of Norway spruce in the Black Forest. The annual radial growth series covers 121 years and the climatic water balance series 140 years. These unique time series enable a quantitative assessment of multidecadal drought and heat impacts on growth and mortality of forest trees on a regional spatial scale. Data compiled here suggest that the mortality of desiccated trees in the Black Forest during the last 68 years is driven by the climatic water balance. Decreasing climatic water balance coincided with an increase in tree mortality and growth decline. Consecutive hot and dry summers enhance mortality and growth decline as a consequence of drought legacies lasting several years. The sensitivity of tree growth and mortality to changes in the climatic water balance increases with the decreasing trend of the climatic water balance. The findings identify the climatic water balance as the main driver of mortality and growth variation during the 68-year observation period on a landscape-scale including a variety of different sites. They suggest that bark beetle population dynamics modify mortality rates. They as well provide evidence that the mortality during the last 140 years never was as high as in the most recent years.

A Mass Reproduction Outbreak and Estimation of the Spreading Risks for the Small Spruce Bark Beetle in Cedar Pine Forests of the Tomsk Region

The results of studying the mass reproduction outbreak of a new invasive pest of the Siberian pine (Pinus s-ibirica Du Tour), the small spruce bark beetle (Ips amitinus (Eichh.)), observed in Siberia for the first time, are presented in this paper. The studies were carried out in the south-eastern part of Western Siberia, within the Tomsk region, in pine forests damaged by an invasive bark beetle. It is shown that the occurrence of the outbreak was provoked by favourable weather conditions of recent decades and the abundance of trees, weakened for various reasons, in the recipient ecosystems of the invasion, among which the outbreak of the Siberian moth in 2016–2018 was of great importance. The small spruce bark beetle population dynamics in new habitats was compared with the native area in Europe, and certain peculiarities were revealed: an increase in the populations’ numbers level, a narrowing of trophic specialisation up to a regional monophagy, a decrease in the role of an interspecific competition due to the displacement of local stem dendrophagous species by the invader, the simultaneous existence of outbreak foci in different development stages and the correlation between their characteristics, the species composition of the forest stands and the weakening factors. An original method for assessing the spreading risk for an outbreak of the small spruce bark beetle breeding in the Tomsk region under the influence of a complex of factors that contribute to an increase in its number has been proposed and implemented at the forestries level.

Warming increased bark beetle-induced tree mortality by 30% during an extreme drought in California.

Quantifying the responses of forest disturbances to climate warming is critical to our understanding of carbon cycles and energy balances of the Earth system. The impact of warming on bark beetle outbreaks is complex as multiple drivers of these events may respond differently to warming. Using a novel model of bark beetle biology and host tree interactions, we assessed how contemporary warming affected western pine beetle (Dendroctonus brevicomis) populations and mortality of its host, ponderosa pine (Pinus ponderosa), during an extreme drought in the Sierra Nevada, California, United States. When compared with the field data, our model captured the western pine beetle flight timing and rates of ponderosa pine mortality observed during the drought. In assessing the influence of temperature on western pine beetles, we found that contemporary warming increased the development rate of the western pine beetle and decreased the overwinter mortality rate of western pine beetle larvae leading to increased population growth during periods of lowered tree defense. We attribute a 29.9% (95% CI: 29.4%-30.2%) increase in ponderosa pine mortality during drought directly to increases in western pine beetle voltinism (i.e., associated with increased development rates of western pine beetle) and, to a much lesser extent, reductions in overwintering mortality. These findings, along with other studies, suggest each degree (°C) increase in temperature may have increased the number of ponderosa pine killed by upwards of 35%-40% °C-1 if the effects of compromised tree defenses (15%-20%) and increased western pine beetle populations (20%) are additive. Due to the warming ability to considerably increase mortality through the mechanism of bark beetle populations, models need to consider climate's influence on both host tree stress and the bark beetle population dynamics when determining future levels of tree mortality.

Landscape-Scale Drivers of Resistance and Resilience to Bark Beetles: A Conceptual Susceptibility Model

Bark beetle (Dendroctonus spp.) outbreaks in the middle latitudes of western North America cause large amounts of tree mortality, outstripping wildfire by an order of magnitude. While temperatures play an important, and direct role in the population dynamics of ectothermic bark beetles, an equally important influence is the nature of the host substrate—the structure and composition of forested communities. For many of the dominant tree species in the western United States, “hazard” indices have been developed for specific bark beetles, which generally include three key variables—host tree size, absolute or relative density of the stand, and percentage of host composition. We provide a conceptual model to apply these three variables across forest ecosystems and bark beetles that shifts the thinking from a species–specific model to a model which focuses on the underlying ecological factors related to bark beetle outbreak susceptibility. We explored the use of our model across multiple scales using the Forest Inventory and Analysis database: Interior West, USA; the states of Colorado and Arizona; and specific national forests within Arizona that are implementing a large-scale restoration effort. We demonstrated that across the Interior West and Colorado, the vast majority of forests have moderate to high susceptibility to bark beetles. Our conceptual model maintains the simplicity of previous “hazard” models but acknowledges the need to consider scale when managing bark beetles. It also shifts the management approach from resistance thinking to the development of “associational resilience”, where the focus is not on any one individual stand or area but the longer-term perspective of forest persistence across the landscape.

Physical and chemical characteristics of blue and Engelmann spruce relative to spruce beetle host selection and colonization

Bark beetles are keystone species that can alter the structure and function of forested ecosystems, yet the mechanisms underlying host selection and successful colonization remain poorly understood for most species. Comparison of closely related tree species that vary in their susceptibility to bark beetles could provide insights into such mechanisms. Here, we compare physical and chemical characteristics of blue (Picea pungens Engelm.) and Engelmann (Picea engelmannii Parry ex Engelm.) spruce, species rarely (blue) and frequently (Engelmann) selected and colonized by the spruce beetle (Dendroctonus rufipennis Kirby) in the western U.S. At three sites (Utah, U.S.) where these species co-occur, 15 trees of each species were selected and traits important for bark beetle survival and population dynamics were measured and compared (bark and phloem thickness, resin flow, phloem and volatile chemistry, beetle landing and colonization success). There were significant differences in bark and phloem thickness and resin flow between species. Bark was thicker and phloem was thinner in blue spruce than Engelmann spruce whereas resin flow was highly variable but greater in blue spruce. Concentrations of within-phloem terpenes in blue spruce were more than double those for Engelmann spruce. Engelmann spruce foliage emitted greater concentrations of volatiles than blue spruce. Spruce beetles landed at higher rates on baited Engelmann spruce than baited blue spruce, and Engelmann spruce was more likely to be colonized. Collectively, these results suggest that blue spruce is a less suitable host for spruce beetle than Engelmann spruce due to a combination of factors including: thicker bark, thinner phloem, higher resin flow, lower concentrations of volatile terpenes, and higher concentrations of constitutive terpenes in phloem tissue, several which are known to be toxic to spruce beetles.

Outstanding scientist-entomologist Z.S. Golovyanko

Golovyanko Zinoviy Stepanovich is an outstanding entomologist well-known in Ukraine and abroad, specialist in insects, which are pests of forest. He is a Doctor of agricultural sciences, Professor. His scientific activity was generally devoted to working out of the measures on creating of pine plantations in the conditions of high number of the roots’ pests, which are largely grubs of May beetle (Melolontha melolontha L., M. hippocastani F.) and July beetle (Polyphillo fullo L.). He revealed that the density of May beetle’s grubs in the soil depends on temperature of this media. Не revealed the capacity of a pine to resist themselves against pests of their roots; this capacity was provided by oleoresin exudation from wounds, which were inflicted by the grubs on roots. He proposed recommendations on pine and deciduous tree species protection from May beetle’s grubs. He developed control measures against scarab beetles grubs for the first time. Z.S. Golovyanko brought the fundamental contribution into solution of the problem on revealing of the factors of population dynamics of bark beetles (Coleopera, Curculionidae: Scolytinae). He had worked out chemical method of forest pests’ control by use of aviation too. The scientific and practical heritage of Z.S. Golovyanko on ecological approach to forest protection from pest organisms is very actual in the present time.

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.

Simulating bark beetle population dynamics in response to windthrow events

The relationship between windthrow disturbance and outbreaks of European spruce bark beetle Ips typographus L. in European Norway spruce forests has been the focus of recent studies. However, the nature in which the spatial characteristics of windthrow events influence bark beetle population dynamics is rarely examined. This represents a significant gap in the literature, as our understanding of how spatial windthrow patterns influence bark beetles can be useful for management efforts to help mitigate large-scale bark beetle disturbance. The objective of this study is to simulate how windthrow events facilitate bark beetle population state transitions from endemic and epidemic levels using a spatially explicit agent-based model. We examined how the spatial extent of windthrow events and the size of tree clusters impacted by windthrow influence this state transition. The results show that the beetle population transition slows with increasing spatial extent of a windthrow event and with larger clusters of windthrown trees, while scattered patterns of windthrown trees accelerate the timing of this transition. This study contributes to our understanding of the role of large-scale wind disturbance in European bark beetle outbreaks. Moreover, it provides a basis for further research to discover the impact of potential forest management applications aiming to mitigate the risk of bark beetle outbreaks.

Population dynamics and epidemiology of four species ofDendroctonus(Coleoptera: Curculionidae): 100 years since J.M. Swaine

AbstractAlmost 100 years have passed since J.M. Swaine, the assistant entomologist in charge of Forest Insect Investigations, wrote, “Canadian bark-beetles: a preliminary classification, with an account of the habits and means of control”. The goal was to “put into the hands of practical foresters information of inestimable practical value… to prevent the continued loss of timber now being destroyed” by “the most insidious enemies of the forest”. In this paper, we celebrate Swaine’s pioneering work by summarising the foundational aspects of his early treatise of 1918: the “general habits” of bark beetles, classifications of their behaviour, causes of population increase, and mitigation tactics. In the founding text, Swaine identified all majorDendroctonusErichson (Coleoptera: Curculionidae) bark beetles found in Canada, although details on life histories were scarce. We summarise current knowledge of the life histories and population dynamics of the spruce beetle,D.rufipennisKirby; the Douglas-fir beetle,D. pseudotsugaeHopkins; the eastern larch beetle,D. simplex; and address the current range expansion of mountain pine beetle,D. ponderosaeHopkins. We review how aspects of global change, such as invasive species, have altered the population dynamics of certain bark beetles. Finally, we conclude with lessons from two of the many past contributors to bark beetle ecology in Canada, J.M. Swaine and H.A. Richmond.

Ponderosa pine resin defenses and growth: metrics matter.

Bark beetles (Coleoptera: Curculionidae, Scolytinae) cause widespread tree mortality in coniferous forests worldwide. Constitutive and induced host defenses are important factors in an individual tree's ability to survive an attack and in bottom-up regulation of bark beetle population dynamics, yet quantifying defense levels is often difficult. For example, in Pinus spp., resin flow is important for resistance to bark beetles but is extremely variable among individuals and within a season. While resin is produced and stored in resin ducts, the specific resin duct metrics that best correlate with resin flow remain unclear. The ability and timing of some pine species to produce induced resin is also not well understood. We investigated (i) the relationships between ponderosa pine (Pinus ponderosa Lawson & C. Lawson) resin flow and axial resin duct characteristics, tree growth and physiological variables, and (ii) if mechanical wounding induces ponderosa pine resin flow and resin ducts in the absence of bark beetles. Resin flow increased later in the growing season under moderate water stress and was highest in faster growing trees. The best predictors of resin flow were nonstandardized measures of resin ducts, resin duct size and total resin duct area, both of which increased with tree growth. However, while faster growing trees tended to produce more resin, models of resin flow using only tree growth were not statistically significant. Further, the standardized measures of resin ducts, density and duct area relative to xylem area, decreased with tree growth rate, indicating that slower growing trees invested more in resin duct defenses per unit area of radial growth, despite a tendency to produce less resin overall. We also found that mechanical wounding induced ponderosa pine defenses, but this response was slow. Resin flow increased after 28 days, and resin duct production did not increase until the following year. These slow induced responses may allow unsuccessfully attacked or wounded trees to resist future bark beetle attacks. Forest management that encourages healthy, vigorously growing trees will also favor larger resin ducts, thereby conferring increased constitutive resistance to bark beetle attacks.

Uticaj ekstremnih klimatskih uslova na populacionu dinamiku potkornjaka u šumama Istočne Srbije

The paper presents the results of a one-year study of population dynamics of bark beetles by the method of bark beetle trapping using pheromone traps with the following pheromone dispensers-aggregation pheromones: IAC Ecolure (for Ips acuminatus and I. sexdentatus), IT Ecolure (for I. typographus) and PC Ecolure (for Pityogenes chalcographus), after the ice storm that hit the region of Eastern Serbia, especially the forest estate Timočke šume Boljevac (State enterprise Srbijašume) in November 2014. The population levels of the monitored bark beetle species in the controlled area in 2015 were generally within the limits of normal conditions.

Choice or constraint: Plasticity in overwintering sites of the European spruce bark beetle

Winter mortality is an important driver in bark beetle population dynamics. Overwintering site – be it in the soil or within the tree – can influence mortality, because temperature dynamics can differ between soil and tree at the same site. For Ips typographus L. (Coleoptera, Curculionidae, Scolytinae), it is not yet definitely resolved which of the two aforementioned is its overwintering site. Both sites, sometimes even mixed within the same population, have been reported in earlier studies. Here we studied the underlying mechanism of soil overwintering in I. typographus. We used soil emergence traps at the base of infested trees with variable amounts of fallen bark during winter. The number of soil overwintering beetles increased with increasing amount of fallen bark and decreased with increasing distance from the stem base of infested trees. Removing the bark beneath soil emergence traps resulted in a smaller number of emerged beetles, indicating that a majority of beetles was located in the fallen bark and not in the litter. Thus, soil hibernation in our study area in Central Europe occurred passively, mediated through falling bark.Furthermore, we studied post-diapause fitness in terms of body weight and lipid reserves in dependence of overwintering site (tree vs. soil). Fresh and dry masses of beetles were higher before than after winter, but not different after winter between overwintering sites. Lipid reserves were more depleted in tree overwintering beetles. Thus, the soil seems to be more advantageous as overwintering site from an energetic point of view. However, both possible overwintering sites have different other advantages and mortality risks regarding onset of flight in spring, inoculative freezing, fungal infection or predation risk.The mixed overwintering sites, caused by individuals’ active movement to the drier part of the bark and remaining in the stem or a passive dropping to the soil via fallen bark, spread winter mortality risks within populations and thus may contribute to local population stability. This phenomenon complicates forest management measures because a considerable number of beetles may be hibernating within fallen bark on the ground. Thus, when trees infested during the summer swarming period are not removed as long as bark is still in close contact with the stem, a great number of beetles can remain in the forest within fallen bark, even when trees are removed at a later point.

Population dynamics of the spruce bark beetle: a long‐term study

Bark beetle population dynamics is thought to be primarily driven by bottom‐up forces affecting insect performance and host tree resistance. Although there are theoretical predictions and empirical evidences that predation and parasitism may play an important role in driving bark beetle population fluctuations, long‐term studies testing the role of both biotic and abiotic controls on population dynamics are still rare. The aim of the study was to quantify the relative importance of predation, negative density feedback and abiotic factors in driving Ips typographus population dynamics. We analyzed a unique time series of population density of I. typographus and its main predator Thanasimus formicarius over almost two decades in four regions across Sweden. We used a discrete population model and a multi‐model inference approach to evaluate the importance of both bottom up and top down factors. We found that availability of breeding substrates in the form of storm‐felled trees was the main outbreak trigger, while strong intra‐specific competition for host trees was the main endogenous regulating factor. Although temperature‐related metrics are known to have strong individual effect on I. typographus development and number of generations, they did not emerge as important drivers of population dynamics. A positive effect of low summer rainfall was evident only in the region located in the southernmost and warmest part of the spruce distribution range in Sweden. Predator density did not emerge as an important prey regulating factor. As the reported damage from storms seems to have increased across whole Europe, spruce forests are expected to be increasingly susceptible to large outbreaks of I. typographus with important economic and ecological consequences for boreal ecosystems. However, the observed negative density feedback seems to be a natural regulating mechanism that impedes a strong long‐term propagation of the outbreaks.

Population dynamics of Ips typographus in the Bohemian Forest (Czech Republic): Validation of the phenology model PHENIPS and impacts of climate change

Phenology models play an important role in insect ecology and pest management. For bark beetle species prone to frequent outbreaks, they help predict swarming periods, discern sister generations hard to recognize in the field yet indispensable in studies of bark beetle population dynamics, predict the extent to which the generations entering diapause develop, as well as help address the omnipresent issue of impacts of climate change. As a prerequisite to developing a simulation model of landscape-level forest disturbances in the Bohemian Forest, including those due to Ips typographus, we validate the phenology model PHENIPS, published in the literature and simulating seasonal development of this species, by data coming from this region. We find a reasonable agreement between the modeled and actually observed bark beetle dynamics, thus strengthening the potential role of PHENIPS in any future attempt to model dynamics of Central European populations of I. typographus. In addition, we use PHENIPS to assess impacts of climate change on temperature-regulated phenology of I. typographus. In contrast to previous studies which used regional climate models to predict future temperature development, we independently account for changes in the mean air temperature and in the frequency of extreme weather events – for the latter, we assume that the inter-annual air temperature variance will also increase. Since bark beetle development is driven by bark temperatures and both low and high bark temperatures inhibit the development, we find that the effects of increases in the air temperature mean and variance are compensatory. Hence, if climate is to change so that both these characteristics grow, we expect only a relatively minor change in generation development time. Moreover, initiation of the third filial generation of I. typographus in the Bohemian Forest is predicted to occur only for relatively large shifts in the mean air temperature.

Climate and weather influences on spatial temporal patterns of mountain pine beetle populations in Washington and Oregon

Widespread outbreaks of mountain pine beetle in North America have drawn the attention of scientists, forest managers, and the public. There is strong evidence that climate change has contributed to the extent and severity of recent outbreaks. Scientists are interested in quantifying relationships between bark beetle population dynamics and trends in climate. Process models that simulate climate suitability for mountain pine beetle outbreaks have advanced our understanding of beetle population dynamics; however, there are few studies that have assessed their accuracy across multiple outbreaks or at larger spatial scales. This study used the observed number of trees killed by mountain pine beetles per square kilometer in Oregon and Washington, USA, over the past three decades to quantify and assess the influence of climate and weather variables on beetle activity over longer time periods and larger scales than previously studied. Influences of temperature and precipitation in addition to process model output variables were assessed at annual and climatological time scales. The statistical analysis showed that new attacks are more likely to occur at locations with climatological mean August temperatures >15 degrees C. After controlling for beetle pressure, the variables with the largest effect on the odds of an outbreak exceeding a certain size were minimum winter temperature (positive relationship) and drought conditions in current and previous years. Precipitation levels in the year prior to the outbreak had a positive effect, possibly an indication of the influence of this driver on brood size. Two-year cumulative precipitation had a negative effect, a possible indication of the influence of drought on tree stress. Among the process model variables, cold tolerance was the strongest indicator of an outbreak increasing to epidemic size. A weather suitability index developed from the regression analysis indicated a 2.5x increase in the odds of outbreak at locations with highly suitable weather vs. locations with low suitability. The models were useful for estimating expected amounts of damage (total area with outbreaks) and for quantifying the contribution of climate to total damage. Overall, the results confirm the importance of climate and weather on the spatial expansion of bark beetle outbreaks over time.

Olfactory Experience Modifies Semiochemical Responses in a Bark Beetle Predator

A typical feature of forest insect pests is their tendency to undergo large fluctuations in abundance, which can jeopardize the persistence of their predaceous natural enemies. One strategy that these predators may adopt to cope with these fluctuations would be to respond to sensory cues for multiple prey species. Another possible adaptation to temporal variation in the prey community could involve the learning of prey cues and switching behavior. We conducted three experiments to investigate the ability of the generalist bark beetle predator Thanasimus dubius (F.) (Coleoptera: Cleridae) to respond to different prey signals and to investigate the effect of olfactory experience. We first conducted a field choice test and a wind tunnel experiment to examine the kairomonal response of individual predators toward prey pheromone components (frontalin, ipsenol, ipsdienol, sulcatol) along with the pine monoterpene α-pinene, which is a volatile compound from the host of the prey. We also presented semiochemically naive predators with two prey pheromone components, frontalin and ipsenol, alone or associated with a reward. Our results showed that T. dubius populations are composed of generalists that can respond to a broad range of kairomonal signals. Naive T. dubius also were more attracted to ipsenol following its association with a reward. This work constitutes the first evidence that the behavior of a predatory insect involved in bark beetle population dynamics is influenced by previous olfactory experience, and provides a potential explanation for the pattern of prey switching observed in field studies.

Factors Influencing Bark Beetle Outbreaks After Forest Fires on the Iberian Peninsula

Fires are among the most globally important disturbances in forest ecosystems. Forest fires can be followed by bark beetle outbreaks. Therefore, the dynamic interactions between bark beetle outbreaks and fire appear to be of general importance in coniferous forests throughout the world. We tested three hypotheses of how forest fires in pine ecosystems (Pinus pinaster Alton and P. radiata D. Don) in Spain could alter the population dynamics of bark beetles and influence the probability of further disturbance from beetle outbreaks: fire could affect the antiherbivore resin defenses of trees, change their nutritional suitability, or affect top-down controls on herbivore populations. P. radiata defenses decreased immediately after fire, but trees with little crown damage soon recovered with defenses higher than before. Fire either reduced or did not affect nutritional quality of phloem and either reduced or had no effect on the abundance, diversity, and relative biomass of natural enemies. After fire, bark beetle abundance increased via rapid aggregation of reproductive adults on scorched trees. However, our results indicate that for populations to increase to an outbreak situation, colonizing beetles must initiate attacks before tree resin defenses recover, host trees must retain enough undamaged phloem to facilitate larval development, and natural enemies should be sufficiently rare to permit high beetle recruitment into the next generation. Coincidence of these circumstances may promote the possibility of beetle populations escaping to outbreak levels.

Population dynamics in changing environments: the case of an eruptive forest pest species

In recent decades we have seen rapid and co-occurring changes in landscape structure, species distributions and even climate as consequences of human activity. Such changes affect the dynamics of the interaction between major forest pest species, such as bark beetles (Coleoptera: Curculionidae, Scolytinae), and their host trees. Normally breeding mostly in broken or severely stressed spruce; at high population densities some bark beetle species can colonise and kill healthy trees on scales ranging from single trees in a stand to multi-annual landscape-wide outbreaks. In Eurasia, the largest outbreaks are caused by the spruce bark beetle, Ips typographus (Linnaeus), which is common and shares a wide distribution with its main host, Norway spruce (Picea abies Karst.). A large literature is now available, from which this review aims to synthesize research relevant for the population dynamics of I. typographus and co-occurring species under changing conditions. We find that spruce bark beetle population dynamics tend to be metastable, but that mixed-species and age-heterogeneous forests with good site-matching tend to be less susceptible to large-scale outbreaks. While large accumulations of logs should be removed and/or debarked before the next swarming period, intensive removal of all coarse dead wood may be counterproductive, as it reduces the diversity of predators that in some areas may play a role in keeping I. typographus populations below the outbreak threshold, and sanitary logging frequently causes edge effects and root damage, reducing the resistance of remaining trees. It is very hard to predict the outcome of interspecific interactions due to invading beetle species or I. typographus establishing outside its current range, as they can be of varying sign and strength and may fluctuate depending on environmental factors and population phase. Most research indicates that beetle outbreaks will increase in frequency and magnitude as temperature, wind speed and precipitation variability increases, and that mitigating forestry practices should be adopted as soon as possible considering the time lags involved.

Bark beetles in the Tatra Mountains. International research 1998– 2005 – an overview

This paper is a review of fundamental information on bark beetles and their interactions with several predisposing factors (air pollution, drought/temperature interactions, windthrows, management activities) that are thought to contribute to the outbreaks in the High Tatra Mountains. The findings of many research projects indicate that the impact of air pollution on bark beetle populations is indirect and complex and that the disturbances in the physiology and natural resistance of trees may be of crucial importance to bark beetle population dynamics. An active forest protection approach is needed to be applied to the secondary Norway spruce forests affected in the past by human activity. Bark beetle populations in natural and near-natural forests (mainly in the upper montane zone) are regulated by natural mechanisms; bark beetles are therefore a natural factor contributing to forest development, including the transition of future generations of spruce.