Forest entomology and forest protection are forestry-related research fields linking fundamental science with practical forest management and integrating various aspects of other disciplines, such as forest ecology, nature conservation and silviculture. European forests are subjected to a long-standing tradition of forest management aiming at the provision of timber products and forest functions (ecosystem services such as protection of drinking water, protection from natural hazards, welfare) that are important for human society (Maroschek et al., 2009). European forest management concepts are manifold and vary with the geographic location of the forests and their size and ownership. The management strategies and goals range from plantation to close-to-nature forestry (Jactel et al., 2012) and also include non-intervention strategies to preserve specific forest habitats, species communities and ecosystem processes. The complex dynamics of regeneration and succession are commonly driven by abiotic and biotic disturbance regimes, shaping the structure and species composition in the forest ecosystems largely unaffected by human intervention (Turner, 2010). Catastrophic events such as storm throw and insect pest outbreaks, on the other hand, pose a threat to the values connected with forests and the achievement of monetary and non-monetary (ideell) management objectives (Hanewinkel et al., 2011). The sustainable provision of forest ecosystem services is increasingly impeded due to the establishment of invasive flora and fauna (neobiota) and alterations in local and regional climate regimes under global change (Robinson et al., 2020). In particular, temperature increase and rising frequencies of extreme weather events such as storm, heat waves and drought episodes impact the growing conditions, site adaptation and distribution of tree and other plant species, and the population dynamics of forest insect herbivores.
Climatic stress of forests and improved environmental conditions for the development and performance of important forest pest insects increase the risk of pest outbreaks and forest mortality (Seidl et al., 2014). The multitrophic relationships between host tree nutritional quality and resistance to attack, potentially damaging biotic agents as part of complex species communities and environmental factors, are in large part insufficiently understood. A fundamental understanding of the causalities of biotic disturbances is needed, as well as a transfer of scientific knowledge and new findings into practical forest management. Risk/predisposition assessment systems and models predicting insect phenology or climate-related forest stress can support decisions about prophylactic and protective silvicultural measures and the development of adaptation and mitigation strategies in the context of climate change (Seidl et al., 2019). Furthermore, a comprehensive investigation of the biology and population dynamics of important forest insect species and host tree–insect herbivore relationships requires a combination of diverse methods, ranging from literature research, over field, semi-field and laboratory experiments to modelling approaches.
This overview focuses on four core research topics in the field of forest entomology and forest protection: (1) the concepts of disturbance, predisposition and risk; (2) the role of climate in the risk of forest pest insect outbreaks; (3) modelling bark beetle development and outbreak risks and (4) interactions among Norway spruce, the bark beetle
Forestry practices can strongly affect the occurrence of biotic and abiotic hazards and stand susceptibility to these damaging agents (Jactel et al., 2009); yet, climatic parameters are the main triggers of abiotic disturbance and forest pest insect outbreaks by directly and indirectly influencing resistance and resilience of forest ecosystems and insect life cycles (section 2.2, topic 2). It is important to understand the effects of global climate change and how increased temperatures and frequency of extreme weather events such as storm and drought impact on the sensitivity of European forest ecosystems, that is, the degree to which forests might be affected in an adverse or beneficial way (Lindner et al., 2010). Some examples for direct effects of climate change on forest insect herbivores and indirect effects via host plants and natural enemies are presented in this section, with particular focus on two insects strongly profiting from temperature increase, the pine processionary moth (PPM),
Our understanding of how bark beetle infestations and other biotic disturbance events are triggered by environmental conditions can be strongly improved by models. A core subject of section 2.3 (topic 3) is the prediction of phenology and development of
The main questions and results published by the author regarding the research topics (1)–(4) are summarised and discussed in the following sections with respect to the current state of knowledge and future perspectives.
Forests are dynamic ecosystems subjected to regenerative, competitive and successional processes over space and time. Catastrophic events, such as storm throw, landslides, avalanches, drought episodes, fungal diseases and large-scale insect outbreaks, are the major forces interrupting these processes and drastically disturbing forest development, composition, structure and function (Attiwil, 1994). Resilient forest ecosystems are able to return to initial or equilibrium states after disturbances (Holling, 1973). While resilience relates to the rate of ecosystem recovery from a distinct disturbance event, resistance is indicated by the degree of disturbance necessary for destabilising an equilibrium state (Attiwil, 1994).
Long-term stability and diversity of forest ecosystems can be positively linked to disturbance events. For example, the recurring infestations of North American mixed hardwood and conifer forests by defoliating insects have led to high compositional and age structural diversity resulting from gap dynamics promoted by herbivore-induced mortality (Reinikainen et al., 2012). Mass outbreaks of mountain pine beetle,
In anthropogenically strongly altered environments, as characteristic for most parts of Europe, forest management interfering in ecosystem processes might be judged as just another type of (human) disturbance (Attiwil, 1994). However, sustainable, multipurpose forest management is aimed at maintaining the important functions of forests, including a large variety of ecosystem services such as timber production, protection from natural hazards and conservation of species and biodiversity (Bončina et al., 2019; Irauschek et al., 2017). Depending on the geographic location and management objectives, management strategies can, therefore, range from non-intervention or close-to-nature forestry, over combined monetary and ideell objectives, to intensive, even-aged plantation forestry and short-rotation wood biomass production (Jactel et al., 2012).
From a socioeconomic point of view, natural disturbance represents the risk of losing essential goods and services provided by vital forest ecosystems (Hanewinkel et al., 2011; Maroschek et al., 2009). Forestry-related risk can be defined as the probability of disturbance events triggered by specific hazardous factors and leading to damage resulting in monetary and/or ideell costs from a single tree and forest stand to the regional and landscape level (Seidl et al., 2019; Hanewinkel et al., 2011; Shore et al., 2000). The evaluation of all hazards potentially relevant for predicting risk requires a sufficient understanding of the underlying ecological mechanisms and community interactions (Nelson et al., 2008). With respect to biotic risk (specifically, tree disease), Manion (1991) classified potential hazards into predisposing, inciting and contributing factors in a disease spiral, eventually leading to tree death (Figure 1).
Analogous to the definition of disturbance, tree/forest disease can be seen as ‘any deviation in the normal functioning of plants caused by a persistent abiotic or biotic agent’, acting either as short-term, triggering (inciting) factor or as contributing factor (Manion, 1991). Forest trees/stands become particularly susceptible to contributing stressing agents, such as wood and bark boring insects, when already weakened by predisposing and inciting factors which reduce the overall resistance to diseases and insect infestations. Inciting and contributing factors might not only act together, but also have the potential to cause tree mortality directly on their own. Endogenous genotypic disposition is further modified into predisposition of single trees or forests to abiotic and biotic damage by external factors such as climate, site and soil conditions as well as the structural characteristics of the stand (Manion, 1991; Schimitschek, 1969). Predisposing site- and stand-related variables are commonly used as predictors in applied hazard assessment systems for evaluating the current and future probabilities of disturbance events (Berryman, 1986).
The so-called
Netherer and Nopp-Mayr (2005) provide an example for PAS application and verification in an area where predisposing factors and actual occurrence of the addressed biotic disturbance factor, an erupting conifer bark beetle species, were concurring. Predisposition to the Eurasian spruce bark beetle,
Although the possibilities of protective forest management and control measures are limited in national parks, PAS can support the identification of high-risk zones, establishment of buffer zones and deduction of management priorities (Netherer and Nopp-Mayr, 2005). In managed stands, forest protection and silvicultural measures will not fully eliminate the risks of abiotic and biotic disturbance, but are suited to balance risks and benefits expected from potential damaging agents and forest products (Jactel et al., 2009). By directly affecting the likelihood of occurrence of damaging agents and forest predisposition to disturbing events, specific silvicultural operations, such as matching of site conditions and tree species composition, site preparation, stand composition, regeneration methods, cleaning and weed control, thinning and pruning, as well as harvesting, significantly modify the risk levels at various points in time. For example, tree species are differently adapted to particular soil structural and nutrient conditions. A mismatch between the choice of tree species composition and site conditions might result in higher risks of fungal diseases and windthrow due to damaged roots and impaired anchorage of trees (Woodward et al., 1998; Nicoll et al., 2006). Soil texture and quality play an important role in survival of herbivore insect species spending parts of their life cycles in the upper soil or humus layer. Considering the various effects of optional silvicultural operations on the ecosystem processes, management measures which strongly influence species composition and stand structure seem to affect abiotic and biotic risks most significantly (Jactel et al., 2009).
Predisposition (susceptibility) of forests to disturbance, the population dynamics of forest insects and the epidemiology of forest diseases are clearly driven by the local microclimate, provision of fuel and resources to biotic and abiotic damage factors, biological control of biotic agents by natural enemies and tree physiology and development (Jactel et al., 2009). These elements are directly linked to global climate and its drastic changes being observed over the last decades, triggering tree mortality (Allen et al., 2009). Based on data for the period 2001–2014, Sommerfeld et al. (2018) demonstrated a strong relationship between warmer and drier than average conditions and increased frequencies and intensities of disturbances in temperate forest ecosystems worldwide. These more recent empirical results substantiate earlier assumptions that forests in European temperate oceanic and continental regions are specifically susceptible to extreme events, such as drought periods and heat waves (Lindner et al., 2010).
Lindner et al. (2010) synthesised existing knowledge about the potential effects of climate change on European forest ecosystems, with particular emphasis on regional differences in the degree to which systems might be affected (sensitivity) and their ability to adapt to these changes (adaptive capacity). The study, supported by the European Commission, was thought to provide the scientific basis for the development of options to encourage and support adaptation measures in European forestry in response to global change (Kolström et al., 2011). Such measures are highly needed in view of the manifold goods and services European forests provide to human society (Maroschek et al., 2009) and concern both adaptation of the forest sector itself and the inherent adaptive capacity of the forest ecosystems. The adjustment of tree species to altered environmental conditions by natural selection and gene flow mechanisms may be severely constrained by increased pressure through natural disturbance agents, especially at the margins of species’ distribution. In the European south, forest growth and productivity are increasingly limited by the negative impacts of drought (Ogaya et al., 2003). In regions with harsh climate, such as the boreal zone and mountainous areas, rising temperatures and prolonged growing seasons let expect positive effects such as increased plant growth rates, but also negative consequences such as northward/altitudinal shifts and increased frequency of mass outbreaks of forest insect pests (Veteli et al., 2005). Important examples discussed by Lindner et al. (2010) are the conifer bark beetle species
Insects are ectothermic organisms and are thus strongly influenced by environmental parameters in their flight and dispersal behaviour, reproduction, development, performance and mortality. For example, lower mortality rates were observed in warmer winters for the winter moth,
The interactions between climate parameters, such as increased concentrations of tropospheric CO2 and temperature as well as drought, and host quality and resistance against insect herbivores are diverse and do not follow clear trends among species and feeding guilds (Netherer and Schopf, 2010). While leaf-mining and chewing insects seem to be mainly negatively affected by drier and tougher foliage, elevated levels of defensive compounds and reduced nitrogen contents, some defoliating species such as the PPM,
Adapted to a Mediterranean climate, PPM spends the cold months as third or fourth instar larvae actively feeding on the needles of their (mainly) pine host trees. Distributional range of PPM is limited by those winter temperatures which do not allow the gregarious larvae to leave their protecting silk nests for feeding. Battisti et al. (2005) investigated the causes of range expansion to the north of the Paris Basin and to higher altitudes of the Venosta/Vinschgau Valley in Italy occurring since the 1970s. The observed altitudinal range shift by almost 250 m from 1975 to 2005 in the Vinschgau valley is largely due to increasing winter temperatures allowing for more frequent feeding, reducing the duration of starvation periods and enhancing larval survival. The altitudinal increase was higher on the south-faced slopes of the valley due to the effects of insolation on nest temperatures. Colony survival in the field, as additionally observed by Battisti et al. (2005), was higher at warmer and sunnier site conditions that favour larval growth and larger nest construction and allow for a higher number of feeding hours. There is strong evidence for an ongoing range expansion of PPM. An outbreak of PPM initiated in 2015 (or earlier) was detected in Carinthia in a scattered
In mountain areas, occurrence of and damage by forest insect pests are expected to be substantially affected by climate change (Lindner et al., 2010). The Eurasian spruce bark beetle,
Models can be particularly useful for analysing the causalities of disturbance in forest ecosystems, evaluating insect population dynamics or transferring scientific knowledge into practical forest management. The review by Seidl et al. (2011) addresses different modelling concepts and approaches with regard to the abiotic and biotic disturbance agents wind, drought, fire, ungulate browsing and insect pests. Science-based expert models for practical decision support in forestry provide a more generalised picture of causalities, which is gained from existing knowledge, expert experience and case studies. Although geographically limited, empirical studies at a local level can yield essential insights into the causal relationships of disturbance, commonly derived from statistical analyses such as tree-based classification, logistic regression models to predict the likelihood of attack and multiple regression.
Mechanistic models parametrised with experimental and/or literature data are of great value for the investigation of insect–host plant interactions and their potential effects at a population level (e.g. mass outbreaks). An example presented by Seidl et al. (2011) is the scenario analysis regarding possible responses of the European pine sawfly
A more comprehensive and advanced modelling concept is to take up simulations of how the defence or physiological status of trees might influence outbreak risks in an ecosystem modelling approach. Jönsson et al. (2012) ‘guessed’ the impact of
In Austria, efforts to simulate
PHENIPS predicts the potential number of generations per season depending on temperature and photoperiodic limits. Simulations, therefore, consider facultative diapause induction at a certain daylength (Baier et al., 2007) or obligate diapause for predominantly univoltine populations (Annila, 1969; Schebeck et al., 2022). Furthermore, re-emergence of parental beetles from the initially established breeding systems and initiation of sister broods are included in model procedures. Parental beetles commonly leave the breeding systems at the time of advanced larval stage to establish further (sister) broods at less-densely colonised parts of the trunk or at new host trees (Schopf et al., 2019; Anderbrant, 1989).
PHENIPS was repeatedly validated for Central European spruce bark beetle populations (Fleischer et al., 2016; Berec et al., 2013). Moreover, the phenology model was used for investigating the causalities behind extensive tree mortality incited by bark beetle infestation in the forest stands of the High Tatra National Park in Slovakia (Mezei et al., 2017). The outbreaks extended to the timberline in the protected, core areas of the national park (Figure 2) and peaked in the 1990s and again, from 2005 onwards, triggered by a large windthrow event in 2004. The subalpine climate conditions in the area commonly restrict bark beetle development to one generation per year, which raised the question whether attack pressure on Norway spruce stands was enhanced in years warm enough to allow for the development of sister broods. Sister broods can significantly increase the population growth of
The timing and completion of filial and sister generations and onset of spring swarming was calculated retrospectively with PHENIPS for each year of the period 1987–2012 for the forest sites of Tatranská Javorina district of the High Tatra National Park (Mezei et al., 2017). Effective seasonal temperature sums (air and bark temperature above the developmental zero of 8.3°C necessary for complete generation development) (Baier et al., 2007; Wermelinger and Seifert, 1998) were included as predictive variables in nonlinear regression models to explain changes in tree mortality. Annual tree mortality was expressed as timber volume of spruce trees killed by bark beetles. Except for 2012, in no year of the study period, the temperature requirements for the termination of a second bark beetle generation were met, which emphasised the predominantly univoltine character of the Tatra beetle population (Netherer and Pennerstorfer, 2003). However, yearly maximum air temperature sums were continuously increasing over the study period and
Rainfall deficit, increased summer temperatures and availability of wind-thrown trees were identified as the main parameters explaining cross-regional temporal fluctuations in the volumes of standing trees killed by
While model outcomes have strongly corroborated the relevance of increased temperature and precipitation deficits for
Heat and drought stress cause the closure of stomata in the needles of Norway spruce, which limits photosynthesis and initiates a cascade of biochemical reactions, involving the synthesis of antioxidants and specialised metabolites for quenching reactive oxygen species and enhancing tree defence (Tausz et al., 2001). On the other hand, the concentrations of secondary metabolites in tree bark, volatile fractions of monoterpenes and other defence compounds, and pheromone components produced in the gut of beetles from specific monoterpenes and
More research is, therefore, needed to uncover the impact of stress intensity on tree resistance, the synergisms and tradeoffs among environmental stress factors, and final effects on the performance and outbreak risks of biotic invaders. At the same time, the wealth of knowledge available demonstrates for how long and how intensively this model system has already been studied (Netherer and Hammerbacher, 2021). From dendrochronological and palaeoecological records, we know that spruce bark beetle outbreaks are not at all a new phenomenon but have been recurring on a regular basis since centuries (Kuosmanen et al., 2020). Norway spruce forest ecosystems are, in fact, well adapted to bark beetle infestation and show high regenerative capacity and resilience to disturbance events, as observed after large-scale
Among the most intriguing questions are whether pioneer beetles are attracted by susceptible Norway spruce trees and if yes, how beetles recognise and deliberately choose stressed trees with weakened defences (Netherer et al., 2021; Netherer and Hammerbacher, 2021). It is more precisely stated thus: Is drought-stressed Norway spruce more susceptible to spruce bark beetle attack and if yes, due to which physiological and biochemical processes taking place in the host tree? These were the central research questions tackled in the Rosalia Roof Project, a comprehensive rainfall exclusion experiment setup in a mature Norway spruce stand in the Forest Demonstration Centre (Lehrforst) of BOKU. The idea behind this interdisciplinary,
The study setup comprised an area of about 1400 m2 polyester roofs beneath canopy, 1.20 m above the ground level, to install three different treatments – two fully covered (roofed) and two partially covered plots with an average size of 20 × 20 m, as well as two uncovered control plots (Figure 3). The roof treatment caused strong decreases in the volumetric soil water content and soil water potential, and in the osmotic potential at full saturation of the phloem, pre-dawn twig water potential, sap flow, and resin flow rates of the target trees (Netherer et al., 2015; Matthews et al., 2018) in the course of two study seasons. A distinct period of heat and drought in July of season 2 (2013) significantly stressed target trees of all treatments (Netherer et al., 2015). Roofed trees were most drought stressed and less able to defend attacks by resin flow over the whole season (spring to late summer) in 2013; yet, treatment alone did not fully explain tree individual stress states over time.
To relate drought stress of the study trees to the incidence and success of spruce bark beetle attack, a novel experimental approach, the ‘attack box’ method, was applied (Netherer et al., 2015). Such boxes allow bark beetles to move around freely without being forced to attack the tree. Instead, an attack box offers opportunities for the beetles of staying in the entrance bottle, enter and move around in the main body of the box, crawl on the bark, taste the bark tissue or attack the tree, or leave the box into an exit device. Temperature conditions inside the boxes did not differ significantly from those of the forest environment, as tested by use of dataloggers. However, maximum temperature conditions during the attack experiments positively affected the proportion of beetles leaving the entrance bottles and entering the exit device (Netherer et al., 2015), as well as the number of attack attempts (unpublished data). Most significantly, individual stress states of trees played a role in bark beetle attack success, especially in spring (Netherer et al., 2015; Matthews et al., 2018). While more than 90% of attacks were successful at fully covered trees about 1 year after the establishment of roofs, the well water supplied control trees were able to defend more than 70% of attacks by repelling beetles with resin. This relationship became less clear 2 months later, in the middle of the hot and dry summer 2013, when control trees were similarly stressed and successfully attacked as roofed trees.
The overall proportion of beetles trying to attack the trees indicating host attractiveness was comparably high for all study trees any time in the study season. As concluded by Netherer and Hammerbacher (2021), both vital and stressed Norway spruce trees appear to be highly attractive to
Still, knowledge is lacking about the intrinsic causalities of mass outbreaks on biochemical, chemo-ecological and behavioural, and molecular level of multipartite relationships between Norway spruce,
Experimental inoculations of ophiostomatoid (blue-stain) fungi, such as
The study was repeated at new test trees of the Rosalia Roof Project II in summer 2020 (Morgante, 2021). At that time, wound reaction zones in response to
The prediction of disturbance-related risks requires knowledge about relevant predisposing, triggering and contributing factors and an understanding of the underlying ecological causalities. Disturbance risks increase with abundance and frequency of damaging agents, predisposition of forests to disturbance and appreciation of values and goods provided by forests to the society (Jactel et al., 2012). PAS proved as a helpful tool for indicating important site- and stand-related parameters that influence forest susceptibility to damaging agents. The implementation of silvicultural operations significantly contributes to a modification of forest predisposition and overall risk levels over space and time (Jactel et al., 2009). However, more research is needed to elucidate the causalities of disturbance regimes in European forest ecosystems and the population dynamics of important forest pest insects in a changing climate in order to develop effective adaptation and mitigation strategies for forest management (Kolström et al., 2011).
Field and laboratory studies can provide essential knowledge about temperature requirements and limits in key phases of insect life cycles, such as development rates, feeding activity in winter and frost hardiness. In this respect, one of the most important and best studied forest insect pest species is the Eurasian spruce bark beetle. Yet, despite ample knowledge about its life cycle, we still need to improve our understanding of host tree–bark beetle interactions for a better evaluation of outbreak risks in a warmer and drier climate (Biedermann et al., 2019). Important fields of future research involve studies on Norway spruce stress and resistance to bark beetle and ophiostomatoid fungal attack under the consideration of stress history and stress-related biochemical processes in trees. In particular, the impact of drought stress on the induction of tree defences in response to biotic attack, particularly with respect to bark contents and accumulation of secondary metabolites such as monoterpenes and phenolic compounds, requires further examination (Netherer et al., 2021). Field and laboratory bioassays such as Petri-dish arena experiments and artificial inoculation of trees with ophiostomatoid fungi can help to unravel the role of olfactory and gustatory signals in tree attractiveness, host choice and host acceptance of
In summary, the knowledge gained in studying important forest insect pests such as the