Tree-killing Bark Beetles Research Articles

Convergent evolution of semiochemicals across Kingdoms: bark beetles and their fungal symbionts

Convergent evolution of semiochemical use in organisms from different Kingdoms is a rarely described phenomenon. Tree-killing bark beetles vector numerous symbiotic blue-stain fungi that help the beetles colonize healthy trees. Here we show for the first time that some of these fungi are able to biosynthesize bicyclic ketals that are pheromones and other semiochemicals of bark beetles. Volatile emissions of five common bark beetle symbionts were investigated by gas chromatography-mass spectrometry. When grown on fresh Norway spruce bark the fungi emitted three well-known bark beetle aggregation pheromones and semiochemicals (exo-brevicomin, endo-brevicomin and trans-conophthorin) and two structurally related semiochemical candidates (exo-1,3-dimethyl-2,9-dioxabicyclo[3.3.1]nonane and endo-1,3-dimethyl-2,9-dioxabicyclo[3.3.1]nonane) that elicited electroantennogram responses in the spruce bark beetle Ips typographus. When grown on malt agar with 13C d-Glucose, the fungus Grosmannia europhioides incorporated 13C into exo-brevicomin and trans-conophthorin. The enantiomeric compositions of the fungus-produced ketals closely matched those previously reported from bark beetles. The production of structurally complex bark beetle pheromones by symbiotic fungi indicates cross-kingdom convergent evolution of signal use in this system. This signaling is susceptible to disruption, providing potential new targets for pest control in conifer forests and plantations.

Anatomical defences against bark beetles relate to degree of historical exposure between species and are allocated independently of chemical defences within trees.

Conifers possess chemical and anatomical defences against tree-killing bark beetles that feed in their phloem. Resins accumulating at attack sites can delay and entomb beetles while toxins reach lethal levels. Trees with high concentrations of metabolites active against bark beetle-microbial complexes, and more extensive resin ducts, achieve greater survival. It is unknown if and how conifers integrate chemical and anatomical components of defence or how these capabilities vary with historical exposure. We compared linkages between phloem chemistry and tree ring anatomy of two mountain pine beetle hosts. Lodgepole pine, a mid-elevation species, has had extensive, continual contact with this herbivore, whereas high-elevation whitebark pines have historically had intermittent exposure that is increasing with warming climate. Lodgepole pine had more and larger resin ducts. In both species, anatomical defences were positively related to tree growth and nutrients. Within-tree constitutive and induced concentrations of compounds bioactive against bark beetles and symbionts were largely unrelated to resin duct abundance and size. Fewer anatomical defences in the semi-naïve compared with the continually exposed host concurs with directional differences in chemical defences. Partially uncoupling chemical and morphological antiherbivore traits may enable trees to confront beetles with more diverse defence permutations that interact to resist attack.

Fungal associates of the tree-killing bark beetle, Ips typographus, vary in virulence, ability to degrade conifer phenolics and influence bark beetle tunneling behavior

The collaboration between TZ and AH was funded by the Deutsche Forschungsgemeinschaft (Grant HA7617/1-1 awarded to AH). Field work was funded by the Swedish Research Council Formas (Grant 229-2011-890 awarded to TZ) and the Research Council of Norway (Grant 221479/F20 awarded to PK). The chemical analyses were funded by the Max Planck Society.

Temperature affects phenological synchrony in a tree-killing bark beetle.

Phenological synchrony can promote population growth in species with positive density dependence. Variation among life stages in the thermal thresholds for development can foster phenological synchrony under thermal regimes that include frequent occurrence of temperatures between developmental thresholds. The southern pine beetle is an insect with positive density dependence that has recently undergone important shifts in population abundance at the northern extremes of their distribution. We evaluated the hypothesis that cooler winter temperatures in their northern range cause a convergence of the population life stage structure that leads to synchrony in spring flight phenology. We used a combination of approaches. First, in situ laboratory experiments demonstrated a threshold temperature for pupation that was greater than was required for larval development; rearing larvae at lower temperatures increased the pooling of individuals at the end stage of larval development and synchrony in adult emergence. Second, a development rate model showed a similar convergence of the majority of the population at the end stage of larval development when brood experienced the cooler temperatures of the northern region, but not with temperatures from the southern region, or as a null model. Finally, field trapping of wild beetles showed greater synchrony in the pine forests of New Jersey than in the warmer, historically occupied forests of Georgia and Mississippi. Given these results, pine-dominated forests in the northern edge of the southern pine beetle's range may experience more frequent occurrence of outbreaks, due to the positive feedbacks associated with a synchronous spring emergence of this insect.

Defence syndromes in lodgepole - whitebark pine ecosystems relate to degree of historical exposure to mountain pine beetles.

Warming climate is allowing tree-killing bark beetles to expand their ranges and access naïve and semi-naïve conifers. Conifers respond to attack using complex mixtures of chemical defences that can impede beetle success, but beetles exploit some compounds for host location and communication. Outcomes of changing relationships will depend on concentrations and compositions of multiple host compounds, which are largely unknown. We analysed constitutive and induced chemistries of Dendroctonus ponderosae's primary historical host, Pinus contorta, and Pinus albicaulis, a high-elevation species whose encounters with this beetle are transitioning from intermittent to continuous. We quantified multiple classes of terpenes, phenolics, carbohydrates and minerals. Pinus contorta had higher constitutive allocation to, and generally stronger inducibility of, compounds that resist these beetle-fungal complexes. Pinus albicaulis contained higher proportions of specific monoterpenes that enhance pheromone communication, and lower induction of pheromone inhibitors. Induced P. contorta increased insecticidal and fungicidal compounds simultaneously, whereas P. albicaulis responses against these agents were inverse. Induced terpene accumulation was accompanied by decreased non-structural carbohydrates, primarily sugars, in P. contorta, but not P. albicaulis, which contained primarily starches. These results show some host species with continuous exposure to bark beetles have more thoroughly integrated defence syndromes than less-continuously exposed host species.

Cold tolerance of mountain pine beetle among novel eastern pines: A potential for trade-offs in an invaded range?

Novel hosts may have unforeseen impacts on herbivore life history traits. The mountain pine beetle (Dendroctonus ponderosae Hopkins) is a tree-killing bark beetle native to western North America but constrained by cold temperatures in the northern limits of its distribution. In recent years, this insect has spread north and east of its historical range, and continued expansion, or accidental introduction, could result in the mountain pine beetle becoming invasive in eastern North America. The limiting effect of cold temperatures among novel host pines is unknown, yet crucial for understanding the risk posed to northeastern North American forests. We report the susceptibility of mountain pine beetle to cold temperatures while overwintering in six different pine species. Brood developed in two western pine hosts (Pinus contorta Dougl. var. latifolia Engelm. and P. ponderosa Dougl. ex. Laws. var. scopulorum Engelm.) as well as four eastern pines (P. banksiana Lamb., P. resinosa Ait., P. strobus L. and P. sylvestris L.) novel to this insect. The cold tolerance and cold tolerance strategy of the most common overwintering stage varied by host and year. Models describing lower lethal temperatures more accurately predicted observed field mortality of overwintering larvae than models based on temperatures at which larvae froze. Rapid development to less cold tolerant pupal and adult stages by brood in novel hosts prior to winter may constitute a trade-off between increased host suitability and winter mortality. We demonstrate that overwintering survival of mountain pine beetles in novel hosts depends on a match between the climate and ecophysiological effects of pine species. These results have implications for risk assessment models and management planning for eastern forests as mountain pine beetle continues to expand its range.

Colonization behaviors of mountain pine beetle on novel hosts: Implications for range expansion into northeastern North America.

As climates change, thermal limits may no longer constrain some native herbivores within their historical ranges. The mountain pine beetle, Dendroctonus ponderosae Hopkins, is a tree-killing bark beetle native to western North America that is currently expanding its range. Continued eastward expansion through the newly invaded and novel jack pine (Pinus banksiana Lamb.) trees of the Canadian boreal forest could result in exposure of several species of novel potential host pines common in northeastern North America to this oligophagous herbivore. Due to the tightly co-evolved relationship between mountain pine beetle and western pine hosts, in which the insect utilizes the defensive chemistry of the host to stimulate mass attacks, we hypothesized that lack of co-evolutionary association would affect the host attraction and acceptance behaviors of this insect among novel hosts, particularly those with little known historical association with an aggressive stem-infesting insect. We studied how beetle behavior differed among the various stages of colonization on newly cut logs of four novel potential pine host species; jack, red (P. resinosa Ait.), eastern white (P. strobus L.) and Scots (P. sylvestris L.) pines, as well as two historical hosts, ponderosa (P. ponderosa Dougl. ex. Laws. var. scopulorum Engelm.) and lodgepole (P. contorta Dougl. var. latifolia Engelm.) pines. Overall, we found that beetle colonization behaviors at each stage in the colonization process differ between pine hosts, likely due to differing chemical and physical bark traits. Pines without co-evolved constitutive defenses against mountain pine beetle exhibited reduced amounts of defensive monoterpenoid chemicals; however, such patterns also reduced beetle attraction and colonization. Neither chemical nor physical defenses fully defended trees against the various stages of host procurement that can result in tree colonization and death.

Differences in photoperiod-induced diapause plasticity among different populations of the bark beetleIps typographusand its predatorThanasimus formicarius

Photoperiod is a common cue for diapause induction in insects. In a warmer climate, the photoperiod-sensitive life stage can be expected to be reached earlier in the season, when day length is still long, thereby increasing the probability of an additional generation. Populations from four latitudes in Sweden of the tree-killing bark beetle Ips typographus (L.) and its predator Thanasimus formicarius (L.) (Coleoptera, Cleridae) were reared at day lengths from 8 to 23.5 h. Ips typographus adults were classified as being reproductive or in diapause by dissection. Thanasimus formicarius new generation adults were classified as direct developers, whereas last-instar larvae in pupal chambers were classified as in developmental diapause. The frequency of reproductive diapause among new generation I. typographus adults was negatively correlated with day length and positively correlated with latitude of population origin. The two northernmost populations included a considerable proportion of individuals that entered reproductive diapause even at the longest day lengths. By contrast, diapause entry in the predator T. formicarius was generally independent of photoperiod and geographical origin. In a warmer climate, two generations per year may be more common for I. typographus in Sweden. The predator is less likely to increase voltinism.

Modelling collective foraging in endemic bark beetle populations

Tree-killing bark beetles are widely studied at epidemic population densities because of their significant impacts on forests. At endemic levels, these species are restricted to poorly defended resources, such as wind-felled, lightning-struck, or suppressed trees. It is poorly understood how these scattered and unpredictable resources are discovered and exploited. In this prospect, the collective foraging shown by most bark beetles, in the form of independent searching by individual beetles combined with mutual attention to each other’s chemical signals, represents an efficient strategy to increase the probability to discover the resources.In order to explore the role of host distribution and collective foraging on the discovery of and settlement on wind-felled trees by bark beetles at endemic population densities, we developed a spatially explicit foraging model for the major forest pest in Europe, Ips typographus. The model is based on the diffusion of the beetles, the landing of pioneers on wind-felled trees realistically distributed based on field observations in southern Belgium, and the response of flying beetles to pheromones emitted by these pioneers.The results show that social foraging effectively increases the proportion of beetles that landed on wind-felled resources compared to solitary foragers. Furthermore, the model highlights the parameters that influenced the foraging strategy of I. typographus and that need narrower quantification in further experiments. Finally, in the frame of the tested parameters, the model highlights the existence of thresholds in the reproduction rate and in the size of the starting population under which populations collapse. If confirmed, this prediction would shed new light on the understanding of foraging at low, endemic population levels.

A dynamical model for bark beetle outbreaks

Tree-killing bark beetles are major disturbance agents affecting coniferous forest ecosystems. The role of environmental conditions on driving beetle outbreaks is becoming increasingly important as global climatic change alters environmental factors, such as drought stress, that, in turn, govern tree resistance. Furthermore, dynamics between beetles and trees are highly nonlinear, due to complex aggregation behaviors exhibited by beetles attacking trees. Models have a role to play in helping unravel the effects of variable tree resistance and beetle aggregation on bark beetle outbreaks. In this article we develop a new mathematical model for bark beetle outbreaks using an analogy with epidemiological models. Because the model operates on several distinct time scales, singular perturbation methods are used to simplify the model. The result is a dynamical system that tracks populations of uninfested and infested trees. A limiting case of the model is a discontinuous function of state variables, leading to solutions in the Filippov sense. The model assumes an extensive seed-bank so that tree recruitment is possible even if trees go extinct. Two scenarios are considered for immigration of new beetles. The first is a single tree stand with beetles immigrating from outside while the second considers two forest stands with beetle dispersal between them. For the seed-bank driven recruitment rate, when beetle immigration is low, the forest stand recovers to a beetle-free state. At high beetle immigration rates beetle populations approach an endemic equilibrium state. At intermediate immigration rates, the model predicts bistability as the forest can be in either of the two equilibrium states: a healthy forest, or a forest with an endemic beetle population. The model bistability leads to hysteresis. Interactions between two stands show how a less resistant stand of trees may provide an initial toe-hold for the invasion, which later leads to a regional beetle outbreak in the resistant stand.

Volatile and Within-Needle Terpene Changes to Douglas-fir Trees Associated With Douglas-fir Beetle (Coleoptera: Curculionidae) Attack

Mass attack by tree-killing bark beetles (Curculionidae: Scolytinae) brings about large chemical changes in host trees that can have important ecological consequences. For example, mountain pine beetle (Dendroctonus ponderosae Hopkins) attack increases emission of terpenes by lodgepole pine (Pinus contorta Dougl. ex Loud.), affecting foliage flammability with consequences for wildfires. In this study, we measured chemical changes to Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) foliage in response to attack by Douglas-fir beetles (Dendroctonus pseudotsugae Hopkins) as trees die and crowns transitioned from green/healthy, to green-infested (year of attack), to yellow (year after attack), and red (2 yr after attack). We found large differences in volatile and within-needle terpene concentrations among crown classes and variation across a growing season. In general, emissions and concentrations of total and individual terpenes were greater for yellow and red needles than green needles. Douglas-fir beetle attack increased emissions and concentrations of terpene compounds linked to increased tree flammability in other conifer species and compounds known to attract beetles (e.g., [Formula: see text]-pinene, camphene, and D-limonene). There was little relationship between air temperature or within-needle concentrations of terpenes and emission of terpenes, suggesting that passive emission of terpenes (e.g., from dead foliage) does not fully explain changes in volatile emissions. The potential physiological causes and ecological consequences of these bark beetle-associated chemical changes are discussed.

Dispersal variability and associated population-level consequences in tree-killing bark beetles.

BackgroundDispersal is a key process in the response of insect populations to rapidly changing environmental conditions. Variability among individuals, regarding the timing of dispersal initiation and travelled distance from source, is assumed to contribute to increased population success through risk spreading. However, experiments are often limited in studying complex dispersal interactions over space and time. By applying a local-scaled individual-based simulation model we studied dispersal and emerging infestation patterns in a host − bark beetle system (Picea abies – Ips typgraphus). More specifically, we (i) investigated the effect of individual variability in beetle physiology (flight capacity) and environmental heterogeneity (host susceptibility level) on population-level dispersal success, and (ii) elucidated patterns of spatial and/or temporal variability in individual dispersal success, host selectivity, and the resulting beetle density within colonized hosts in differently susceptible environments.ResultsIndividual variability in flight capacity of bark beetles causes predominantly positive effects on population-level dispersal success, yet these effects are strongly environment-dependent: Variability is most beneficial in purely resistant habitats, while positive effects are less pronounced in purely susceptible habitats, and largely absent in habitats where host susceptibility is spatially scattered. Despite success rates being highest in purely susceptible habitats, scattered host susceptibility appeared most suitable for dispersing bark beetle populations as it ensures population spread without drastically reducing success rates. At the individual level, dispersal success generally decreases with distance to source and is lowest in early flight cohorts, while host selectivity increased and colonization density decreased with increasing distance across all environments.ConclusionsOur modelling approach is demonstrated to be a powerful tool for studying movement ecology in bark beetles. Dispersal variability largely contributes to risk spreading among individuals, and facilitates the response of populations to changing environmental conditions. Higher mortality risk suffered by a small part of the dispersing population (long-distance dispersers, pioneers) is likely paid off by reduced deferred costs resulting in fitness benefits for subsequent generations. Both, dispersal variability in space and time, and environmental heterogeneity are characterized as key features which require particular emphasis when investigating dispersal and infestation patterns in tree-killing bark beetles.Electronic supplementary materialThe online version of this article (doi:10.1186/s40462-016-0074-9) contains supplementary material, which is available to authorized users.

The Role of Multimodal Signals in Species Recognition Between Tree-Killing Bark Beetles in a Narrow Sympatric Zone.

When related species coexist, selection pressure should favor evolution of species recognition mechanisms to prevent interspecific pairing and wasteful reproductive encounters. We investigated the potential role of pheromone and acoustic signals in species recognition between two species of tree-killing bark beetles, the southern pine beetle, Dendroctonus frontalis Zimmermann, and the western pine beetle, Dendroctonus brevicomis LeConte, in a narrow zone of sympatry, using reciprocal pairing experiments. Given the choice of adjacent con- or heterospecific female gallery entrance in a log, at least 85% of walking males chose the entrance of the conspecific, and half the males that initially entered heterospecific galleries re-emerged and entered the conspecific gallery within 15 min. Waveform analysis of female acoustic "chirps" indicated interspecific differences in chirp timing. Males may use information from female acoustic signals to decide whether to enter or remain in the gallery. Individuals in forced heterospecific pairings (produced by confinement of a heterospecific male within the female entrance) did not differ in pheromone production from individuals of conspecific pairs. However, due to the absence of the right species of male, galleries with heterospecific pairs released an abnormal pheromone blend that lacked at least one key component of the aggregation pheromone of either species. The complete aggregation pheromone (i.e., the pheromone blend from entrances with pairs) does not appear to deter interspecific encounters or confer premating reproductive isolation per se; however, it may confer selective pressure for the maintenance of other reproductive isolation mechanisms.

Elevational shifts in thermal suitability for mountain pine beetle population growth in a changing climate

Future forests are being shaped by changing climate and disturbances. Climate change is causing large-scale forest declines globally, in addition to distributional shifts of many tree species. Because environmental cues dictate insect seasonality and population success, climate change is also influencing tree-killing bark beetles. The mountain pine beetle, Dendroctonus ponderosae, is a major disturbance in Pinus forests of the western US. Using a mechanistic, phenology-based demographic model driven by downscaled daily temperature data, we describe recent and future spatial and temporal thermal suitability for mountain pine beetle population growth in a topographically complex region. Trends in model-predicted growth rates among Global Climate Models were similar and suggest that, relative to future trends, mountain pine beetle population growth within the past 60 years was most optimal at middle elevations and least optimal at the lowest and highest elevations. This trend aligns with observed mountain pine beetle-caused tree mortality that was greatest at middle elevations between 1997 and 2013, as estimated from Aerial Detection Surveys. However, thermal suitability for optimal phenological synchrony was predicted to shift in recent years, and by the end of the century, the best thermal habitats for mountain pine beetle will be at the lowest and highest elevations. Mechanistic demographic models are valuable tools for modelling future thermal regimes that may be both beneficial and maladaptive for mountain pine beetle population growth and subsequent tree mortality.

Fallen trees’ last stand against bark beetles

The weakened or suppressed hosts targeted by tree-killing conifer bark beetles during endemic phases are considered to be poorly defended and available to several competing species. Hosts face trade-offs in the allocation of resources in growth and reproduction or in defense, and the defense/resources status varies with time after host’s death or weakening event. Tree-killing bark beetles at endemic levels must choose along a spectrum, compromising between encountering minimal defense and facing high competition, or benefiting from high nutritional quality but facing high host defense.The relationships between resources and defenses in dead/dying trees have not been quantitatively assessed to date, because most studies focus on epidemic conditions. We measured the changes in constitutive and induced defenses in the phloem of spruce trees artificially uprooted at different times (up to 17months prior to experiment initiation) and treated with methyl-jasmonate (MeJ) in order to induce chemical defense. We measured the effect of these changes on different steps of the host selection process and brood success of a major forest pest in Eurasia, Ips typographus (Coleoptera, Curculionidae).Our results show that uprooted trees maintain some level of constitutive defenses, in particular monoterpenes, that decrease with time after uprooting. Moreover, the trees maintain some inducibility and can produce monoterpenes in response to MeJ treatment up to 7months after uprooting. These defenses are correlated in part with the energy available in the phloem, but are likely too weak to exert pressure on the beetles’ success and selection, the latter being related to the amount of energy available.Our results confirm that wind-felled trees are poorly defended, consistent with the hypothesis of the evolution of the tree-killing behavior from saprophagy in response to an overwhelming interspecific competition on undefended hosts. Wind-felled trees still represent a useful resource, exposing endemic populations of aggressive beetles to low defense levels while facilitating reproduction, growth and development. The constraint on the beetles’ populations then becomes the ability to locate these scattered and unpredictable resources.

Fungal Symbionts of the Spruce Bark Beetle Synthesize the Beetle Aggregation Pheromone 2-Methyl-3-buten-2-ol.

Tree-killing bark beetles depend on aggregation pheromones to mass-attack their host trees and overwhelm their resistance. The beetles are always associated with phytopathogenic ophiostomatoid fungi that probably assist in breaking down tree resistance, but little is known about if or how much these fungal symbionts contribute to the beetles' aggregation behavior. In this study, we determined the ability of four major fungal symbionts of the spruce bark beetle Ips typographus to produce beetle aggregation pheromones. The fungi were incubated on Norway spruce Picea abies bark, malt agar, or malt agar amended with 0.5% (13)C glucose. Volatiles present in the headspace of each fungus were analyzed for 7 days after incubation using a SPME autosampler coupled to a GC/MS. Two Grosmannia species (G. penicillata and G. europhioides) produced large amounts of 2-methyl-3-buten-2-ol (MB), the major component in the beetles' aggregation pheromone blend, when growing on spruce bark or malt agar. Grosmannia europhioides also incorporated (13)C glucose into MB, demonstrating that the fungi can synthesize MB de novo using glucose as a carbon source. This is the first clear evidence that fungal symbionts of bark beetles can produce components in the aggregation pheromone blend of their beetle vectors. This provides new insight into the possible ecological roles of fungal symbionts in bark beetle systems and may deepen our understanding of species interactions and coevolution in these important biological systems.

Pheromone-Mediated Mate Location and Discrimination by Two Syntopic Sibling Species of Dendroctonus Bark Beetles in Chiapas, Mexico.

Where their geographic and host ranges overlap, sibling species of tree-killing bark beetles may simultaneously attack and reproduce on the same hosts. However, sustainability of these potentially mutually beneficial associations demands effective prezygotic reproductive isolation mechanisms between the interacting species. The pine bark beetle, Dendroctonus frontalis Zimmermann, is syntopic in the Central American region with a recently described sibling species, Dendroctonus mesoamericanus Armendáriz-Toledano and Sullivan, but mechanisms for their reproductive isolation are uncertain. We investigated whether semiochemicals mediate species discrimination by mate-seeking males of both species. In olfactometer bioassays, walking males of both species strongly preferred odors from gallery entrances of conspecific females. Coupled gas chromatography-electroantennographic detection and gas chromatography-mass spectrometry isolated 16 olfactory stimulants for males in these odors, but only two, ipsdienol and endo-brevicomin (both from D. mesoamericanus females), differed in quantity in female-associated odors between the species. In olfactometer bioassays, with 10, 1, or 0.1 female entranceequivalents of synthetic semiochemicals, the combination of ipsdienol and endo-brevicomin inhibited responses of male D. frontalis and enhanced responses of male D. mesoamericanus to two compounds associated with female entrances of both species (the pheromone component frontalin and host odor α-pinene). We conclude that ipsdienol and endo-brevicomin, pheromone components produced by females of just one of the two species (D. mesoamericanus), mediate interspecific mate discrimination by males of both species and provide an apparently symmetrical reproductive isolation mechanism.

Fire-mediated interactions between a tree-killing bark beetle and its competitors

In forest ecosystems, biotic interactions such as competition or predation may be mediated by abiotic disturbances such as fire. Bark and ambrosia beetles are important biotic factors regulating forest ecosystems, especially tree-killing species that can reach outbreak levels. In a recent study we found that although endemic-level Dendroctonus ponderosae, a tree-killing bark beetle, preferentially colonized fire-injured Pinus contorta (lodgepole pine) after prescribed fire, colonization rates decreased consistently over a 3-year period after an initial pulse. This decline may be due in part to an increase in the abundance of the subcortical community, especially competitors such as other bark beetles and ambrosia beetles. Additionally, while bark and ambrosia beetles can be attracted to fire-injured trees, we do not yet fully understand the temporal progression of the insect community in burned forest stands, and whether other species can outbreak after fire. Thus, our main objective was to characterize the bark and ambrosia beetle community and to examine if a disturbance (fire) can mediate the regulation of a tree-killing bark beetle by intensifying competition. We divided beetles into four groups: lower-, main-, and upper-stem bark beetles, and ambrosia beetles. We found that all groups generally increased in abundance after fire, and we observed a time-lag of approximately 1year in the overall abundance of all species, wherein beetles increased in burned stands more rapidly than in non-burned stands. All groups were present in burned pine stands 1year after fire, albeit at varying abundances relative to subsequent years. Finally, we found differences in the bark and ambrosia beetle community between areas with and without D. ponderosae: areas with D. ponderosae contained more bark and ambrosia beetles. Thus, fire likely increases competition, explaining why an initial increase in D. ponderosae colonization did not result in a long-term population increase after prescribed fire.

A Model for Mountain Pine Beetle Outbreaks in an Age-Structured Forest: Predicting Severity and Outbreak-Recovery Cycle Period.

The mountain pine beetle (MPB, Dendroctonus ponderosae), a tree-killing bark beetle, has historically been part of the normal disturbance regime in lodgepole pine (Pinus contorta) forests. In recent years, warm winters and summers have allowed MPB populations to achieve synchronous emergence and successful attacks, resulting in widespread population outbreaks and resultant tree mortality across western North America. We develop an age-structured forest demographic model that incorporates temperature-dependent MPB infestations. Stability of fixed points is analyzed as a function of (thermally controlled) MPB population growth rates and indicates the existence of periodic outbreaks that intensify as growth rates increase. We devise analytical methods to predict outbreak severity and duration as well as outbreak return time. After incorporating a spatial aspect and controlling initial stand demographic variation, the model predicts cycle periods that fall within observed outbreak return time ranges. To assess future MPB impact on forests, we use climate model projected temperatures with our model-based approximation methods to predict potential severity of future outbreaks that reflect the effects of changing climate.

Exploiting fugitive resources: How long-lived is “fugitive”? Fallen trees are a long-lasting reward for Ips typographus (Coleoptera, Curculionidae, Scolytinae)

Because of their significant impact on forests, tree-killing bark beetles have been widely studied at epidemic population densities. However, bark-beetle outbreaks emerge from populations at low, endemic levels, where population maintenance and growth depend on scarce, ephemeral resources such as lightning-struck trees, weakened and suppressed standing trees or wind-felled trees, depending on the species. The longevity of these resources strongly influences population dynamics, but very little is known in this respect.We tested the hypothesis that the driving factor reducing the availability of fallen trees is the decrease of their nutritional quality. We analysed the qualitative changes of the phloem of spruces artificially uprooted at different times (2–17month before the experiment started) and simultaneously exposed during the same flight season to attacks of the major forest pest in Eurasia, Ips typographus. We also measured the impact of resources’ decay on the insects’ fecundity and the quality of their offspring.Our results show a surprisingly slow decrease of phloem quality over time. The beetles could develop on the oldest (17month-old) resources with no significant differences in terms of fecundity, progeny body mass and global energy acquisition.Although they retain their nutritional quality over long periods, transient resources are exposed to other factors that make them rapidly unsuitable for I. typographus. We suggest that interspecific competition is probably the most influential of them, and acts as a major driver in the beetles’ race for new resources.