Phytochemicals as mediators for host range expansion of a native invasive forest insect herbivore

Feeding Preferences Shift from Protein to Carbohydrates Across Life Stages in a Phloeophagus Bark Beetle Species

Understanding stage-specific nutritional requirements is essential for determining insect feeding strategies and developing targeted pest management approaches. We examined the feeding preferences, developmental duration, survival rates, and digestive efficiency of the mountain pine beetle across different life stages under different nutritional conditions. We tested three artificial diets with varying protein-to-carbohydrate ratios, including high-protein-low-carbohydrate (HP-LC), medium-protein-medium-carbohydrate (MP-MC), and low-protein-high-carbohydrate (LP-HC). The results showed stage-specific differences in feeding preference among beetle larvae. Early-instar larvae preferred HP-LC and MP-MC diets, whereas late-instar larvae preferred LP-HC diets. Adults of both sexes strongly favoured LP-HC diets. Larvae on MP-MC diets exhibited the fastest development and highest digestive efficiency, indicating optimal protein-carbohydrate balance for growth. Survival was highest on the HP-LC and MP-MC diets but was lower on the LP-HC diets, especially in early instars. Poor digestive efficiency in LP-HC diets suggests that excessive carbohydrates hinder nutrient assimilation. These findings show that mountain pine beetle developmental stages have distinct nutritional needs, with early instar larvae requiring higher protein for survival and development. The observed dietary shifts may be linked to seasonal changes in the nutrient composition of host trees and fungal symbionts of the mountain pine beetles. These stage-specific nutritional preferences further suggest opportunities to disrupt beetle growth through targeted, nutrition-based pest management strategies.

Food nutrition and facultative endosymbiont modulate dietary breadth of a polyphagous aphid

While host plants and endosymbionts have been implicated in influencing dietary breadth in polyphagous herbivores, the underlying mechanism remains vague. In this study, we focused on the food nutrition and nutrition provision of endosymbionts to elucidate the determination of dietary breadth in a polyphagous aphid Aphis gossypii. Our findings demonstrated that high sugar and riboflavin presence in food decreased aphid fitness, while Arsenophonus infections improved aphid performance. Aphids collected from cotton could not use cucumber whether they were infected with Arsenophonus or not, signifying a distinct specialization toward cotton. Further, both the Arsenophonus-infected and free aphids fed on artificial diet varying in sugar titer failed to utilize cucumber. However, Arsenophonus-free aphids attained the ability to utilize cucumber after feeding on the riboflavin-free diet, but not on the riboflavin-containing diet, indicating riboflavin and Arsenophonus-absent expansion in dietary breadth. Notably, up-regulated expression of riboflavin synthase genes of the obligated symbiont Buchnera aphidicola was detected in the Arsenophonus-infected aphids which may provide more riboflavin. Arsenophonus promoting riboflavin synthesis in the obligated symbiont B. aphidicola and riboflavin ingestion enhancing host specialization of aphids to cotton modulate dietary breadth of A. gossypii.

Alliance Between Conifer Trees and Endophytic Fungi Against Insect Defoliators

Fungal endophytes can alter plant resistance against herbivores by indirectly influencing plant secondary metabolism or through direct effects of their own metabolism. However, the role of fungal endophytes in conifer defences to insect herbivores remains largely unknown. We characterised the endophytic fungal communities and terpene concentrations of 30 white spruce families across two sites. We determined the effects of fungal endophytes on a defoliating insect, eastern spruce budworm, by testing the budworm responses to media amended with fungal endophytes or exposing them to their volatile organic compounds. We further examined whether the changes in the endophytic fungal communities and abundance alter the terpene concentrations of white spruce by inoculating seedlings with endophytic fungi. Terpene and fungal community compositions in mature trees varied among families and sites. The bioassays showed fungal endophytes can kill budworms or reduce their fitness due to the toxicity of fungal mycelium or volatile compounds. The inoculation experiments demonstrated that the changes in fungal communities and abundance can alter the terpene concentrations in seedlings. We developed a "Plant Partnership Hypothesis" to reflect the role of fungal endophytes in plant resistance to insect herbivores, demonstrating a co-evolutionary relationship among fungal endophytes, tree defences, and insect herbivores.

Divergent Response of Two Bark Beetle-Fungal Symbiotic Systems to Host Monoterpenes Reflects Niche Partitioning Strategies

The successful establishment of bark beetle-fungus symbionts on plants is required to overcome host defenses. However, little is known about how different bark beetle-fungus symbionts adapt to different niches on the same host plant. Here, we investigated the niche partitioning mechanism of two co-occurring bark beetle-fungus symbiotic systems, Ips nitidus-Ophiostoma bicolor and Dendroctonus micans-Endoconidiophora laricicola, on Qinghai spruce (Picea crassifolia) tree. The lower niche of the spruce trunk inhabited by D. micans showed a higher content of monoterpenes than the upper niche of the trunk inhabited by I. nitidus. Dendroctonus micans showed greater tolerance and higher metabolic efficiency toward monoterpenes than I. nitidus. However, both beetle species showed a similar metabolic profile toward α-pinene, albeit with different levels of metabolites. Additionally, O. bicolor, transmitted by I. nitidus, showed a significantly higher tolerance to monoterpenes and pathogenicity to spruce trees than E. laricicola, transmitted by D. micans. In particular, monoterpenoid metabolites were observed to attenuate the inhibitory effect of high-dose α-pinene on E. laricicola, thus increasing its fitness in a high-dose monoterpene microhabitat. These results show that these two bark beetle-fungus symbionts have adapted to different niches, leading to fitness differences in niche distribution that are at least partly related to the different distribution of monoterpene concentration in the spruce trunk. This research provides a novel perspective for understanding the coevolution between bark beetle-fungus symbionts and their host plants.

Gut bacteria facilitate leaf beetles in adapting to dietary specialization by enhancing larval fitness

Dietary specialization between insect stages can reduce intraspecific food competition. The involvement of gut bacteria and the mechanisms underlying this phenomenon received limited attention. Plagiodera versicolora is a pest harming Salicaceae trees. Here, we confirmed dietary specialization in P. versicolora, wherein adults prefer new leaves, while larvae predominantly consume mature leaves when both types are available. We demonstrated the larval preference for mature leaves confers ecological advantages by promoting growth, development and immunity and this advantage is contingent upon the presence of gut bacteria. Gut microbiota in larvae revealed a significant enrichment of Pantoea when feeding new leaves, with P. anthophila exhibiting the most pronounced inhibitory effect on larval development. Further exploration identified specific metabolites, such as Tyrosyl-valine, with higher content in new leaves, which serve as substrates for the entomopathogenic gut bacterium to facilitate its proliferation. This study provides a fresh perspective on the ecological role of gut bacteria.

Biogenic secondary organic aerosol participates in plant interactions and herbivory defense

Biogenic secondary organic aerosols (SOAs) can be formed from the oxidation of plant volatiles in the atmosphere. Herbivore-induced plant volatiles (HIPVs) can elicit plant defenses, but whether such ecological functions persist after they form SOAs was previously unknown. Here we show that Scots pine seedlings damaged by large pine weevils feeding on their roots release HIPVs that trigger defenses in neighboring conspecific plants. The biological activity persisted after HIPVs had been oxidized to form SOAs, which was indicated by receivers displaying enhanced photosynthesis, primed volatile defenses, and reduced weevil damage. The elemental composition and quantity of SOAs likely determines their biological functions. This work demonstrates that plant-derived SOAs can mediate interactions between plants, highlighting their ecological significance in ecosystems.

The Legacy Effect of Mountain Pine Beetle Outbreaks on the Chemical and Anatomical Defences of Surviving Lodgepole Pine Trees

The recent mountain pine beetle outbreaks have caused widespread mortality among lodgepole pine trees in western North America, resulting in a reduced population of surviving trees. While previous studies have focused on the cascading impacts of these outbreaks on the physiology and growth of the surviving trees, there remains a need for a comprehensive study into the interactions among various physiological traits and the growth in post-outbreak stands. Specifically, the relationship between chemical (primarily terpenes) and anatomical (mainly resin ducts) defences, as well as the allocation of non-structural carbohydrates (NSCs) to support these defence modalities, is poorly understood. To address these gaps, we conducted a field survey of surviving lodgepole pine trees in post-mountain pine beetle outbreak stands in western Canada. Our retrospective analysis aimed at determining correlations between the post-outbreak concentrations of monoterpenes, diterpenes, and NSCs in the phloem and the historical resin duct characteristics and growth traits before and after the outbreak. We detected strong correlations between the post-outbreak concentrations of monoterpenes and historical resin duct characteristics, suggesting a possible link between these two defence modalities. Additionally, we found a positive relationship between the NSCs and the total concentrations of monoterpenes and diterpenes, suggesting that NSCs likely influence the production of these terpenes in lodgepole pine. Furthermore, historical tree growth patterns showed strong positive correlations with many individual monoterpenes and diterpenes. Interestingly, while surviving trees had enhanced anatomical defences after the outbreak, their growth patterns did not vary before and after the outbreak conditions. The complexity of these relationships emphasizes the dynamics of post-outbreak stand dynamics and resource allocations in lodgepole pine forests, highlighting the need for further research. These findings contribute to a broader understanding of conifer defences and their coordinated responses to forest insect outbreaks, with implications for forest management and conservation strategies.

Exploring behavioural and physiological adaptations in mountain pine beetle in response to elevated ozone concentrations

Elevated ozone (eO3) concentrations pose a threat to insect populations by potentially altering their behaviour and physiology. This study investigates the effects of eO3 concentrations on the mountain pine beetle which is a major tree-killing species of conifers in northwestern North America. We are particularly interested in understanding the effects of eO3 concentrations on beetle behaviour and physiology and possible transgenerational impacts on bark beetle broods. We conducted O3-enrichment experiments in a controlled laboratory setting using different O3 concentrations (100-200 ppb; projected for 2050-2100) and assessed various beetle responses, including CO2 respiration, mating behaviour, survival probability, locomotion, and attraction behaviour. Transgenerational impacts on the first and second generations were also analyzed by studying brood morphology, mating behaviour, survival, and pheromone production. We found that beetles exposed to eO3 concentrations had shorter oviposition galleries and reduced brood production. Beetle pheromones were also degraded by eO3 exposure. However, exposure to eO3 also prompted various adaptive responses in beetles. Despite reduced respiration, eO3 improved locomotor activity and the olfactory response of beetles. Surprisingly, beetle survival probability was also improved both in the parents and their broods. We also observed transgenerational plasticity in the broods of eO3-exposed parents, suggesting potential stress resistance mechanisms. This was evident by similar mating success, oviposition gallery length, and brood numbers produced in both control and eO3 concentration treatments. This study demonstrates the sensitivity of mountain pine beetles to increased O3 concentrations, contributing crucial insights into the ecological implications of eO3 concentrations on their populations. Overall, the outcome of this study contributes to informed climate change mitigation strategies and adaptive management practices for the development of resilient forests in response to emerging forest insect pests worldwide.

Pathophysiology and transcriptomic responses of Pinus armandii defenses to ophiostomatoid fungi

Pinus armandii Franch. is an ecologically and economically important evergreen tree species native to western China. Dendroctonus armandi Tsai and Li and pathogenic ophiostomatoid fungi pose substantial threats to P. armandii. With the interplay between species, the defense mechanisms of P. armandii have evolved to withstand external biotic stressors. However, the interactions between P. armandii and pathogenic ophiostomatoid fungal species/strains remain poorly understood. We aimed to analyze the pathophysiological and molecular changes in P. armandii following artificial inoculation with four ophiostomatoid species (Graphilbum parakesiyea, Leptographium qinlingense, Ophiostoma shennongense and Ophiostoma sp. 1). The study revealed that L. qinlingense produced the longest necrotic lesions, and G. parakesiyea produced the shortest. All strains induced monoterpenoid release, and monoterpene levels of P. armandii were positively correlated with fungal virulence (R2 = 0.93, P < 0.01). Co-inoculation of two dominant highly (L. qinlingense) and weakly virulent (O. shennongense) pathogens reduced the pathogenicity of the highly virulent fungi. Transcriptomic analysis of P. armandii (LQ: L. qinlingense treatments, QS: co-inoculation treatments and OS: O. shennongense treatments) showed that the expression pattern of differentially expressed genes (DEGs) between QS and OS was similar, but different from that of LQ. The DEGs (LQ vs QS) involved in flavonoid biosynthesis and phenylpropanoid biosynthesis were downregulated. Notably, compared with LQ, QS significantly decreased the expression of host defense-related genes. This study provides a valuable theoretical basis for managing infestations of D. armandi and associated ophiostomatoid fungi.

Terpenoids are involved in the expression of systemic-induced resistance in Austrian pine

Terpenoids are defense metabolites that are induced upon infection or wounding. However, their role in systemic-induced resistance (SIR) is not known. Here, we explored the role of terpenoids in this phenomenon at a very early stage in the interaction between Austrian pine and the tip blight and canker pathogen Diplodia pinea. We induced Austrian pine saplings by either wounding or inoculating the lower stems with D. pinea. The seedlings were then challenged after 12 h, 72 h, or 10 days with D. pinea on the stem 15 cm above the induction. Lesion lengths and terpenoids were quantified at both induction and challenge locations. Key terpenoids were assayed for antifungal activity in in vitro bioassays. SIR increased with time and was correlated with the inducibility of several compounds. α-Pinene and a cluster of β-pinene, limonene, benzaldehyde, dodecanol, and n-dodecyl acrylate were positively correlated with SIR and were fungistatic in vitro, while other compounds were negatively correlated with SIR and appeared to serve as a carbon source for D. pinea. This study shows that, overall, terpenoids are involved in SIR in this system, but their role is nuanced, depending on the type of induction and time of incubation. We hypothesize that some, such as α-pinene, could serve in SIR signaling.

Transcriptome profiles reveal response mechanisms and key role of PsNAC1 in Pinus sylvestris var. mongolica to drought stress

BACKGROUND

Drought stress severely impedes plant growth, and only a limited number of species exhibit long-term resistance to such conditions. Pinus sylvestris var. mongolica, a dominant tree species in arid and semi-arid regions of China, exhibits strong drought resistance and plays a crucial role in the local ecosystem. However, the molecular mechanisms underlying this resistance remain poorly understood.

RESULTS

Here, we conducted transcriptome sequence and physiological indicators analysis of needle samples during drought treatment and rehydration stages. De-novo assembly yielded approximately 114,152 unigenes with an N50 length of 1,363 bp. We identified 6,506 differentially expressed genes (DEGs), with the majority being concentrated in the heavy drought stage (4,529 DEGs). Functional annotation revealed enrichment of drought-related GO terms such as response to water (GO:0009415: enriched 108 genes) and response to water deprivation (GO:0009414: enriched 106 genes), as well as KEGG categories including MAPK signaling pathway (K04733: enriched 35 genes) and monoterpenoid biosynthesis (K21374: enriched 27 genes). Multiple transcription factor families and functional protein families were differentially expressed during drought treatment. Co-expression network analysis identified a potential drought regulatory network between cytochrome P450 genes (Unigene4122_c1_g1) and a core regulatory transcription factor Unigene9098_c3_g1 (PsNAC1) with highly significant expression differences. We validated PsNAC1 overexpression in Arabidopsis and demonstrated enhanced drought resistance.

CONCLUSIONS

These findings provide insight into the molecular basis of drought resistance in P. sylvestris var. mongolica and lay the foundation for further exploration of its regulatory network.

Complementary roles of two classes of defense chemicals in white spruce against spruce budworm

MAIN CONCLUSION

Monoterpenes and phenolics play distinct roles in defending white spruce trees from insect defoliators. Monoterpenes contribute to the toxicity of the foliage, deterring herbivory, whereas phenolics impede budworm growth. This study demonstrates the complex interplay between monoterpenes and phenolics and their collective influence on the defense strategy of white spruce trees against a common insect defoliator. Long-lived coniferous trees display considerable variations in their defensive chemistry. The impact of these defense phenotype variations on insect herbivores of the same species remains to be thoroughly studied, mainly due to challenges in replicating the comprehensive defense profiles of trees under controlled conditions. This study methodically examined the defensive properties of foliar monoterpenes and phenolics across 80 distinct white spruce families. These families were subsequently grouped into two chemotypes based on their foliar monoterpene concentrations. To understand the separate and combined effects of these classes on tree defenses to the eastern spruce budworm, we conducted feeding experiments using actual defense profiles from representative families. Specifically, we assessed budworm response when exposed to substrates amended with phenolics alone or monoterpenes. Our findings indicate that the ratios and amounts of monoterpenes and phenolics present in the white spruce foliage influence the survival of spruce budworms. Phenotypes associated with complete larval mortality exhibited elevated ratios (ranging from 0.4 to 0.6) and concentrations (ranging from 1143 to 1796 ng mg-1) of monoterpenes. Conversely, families characterized by higher phenolic ratios (ranging from 0.62 to 0.77) and lower monoterpene concentrations (ranging from 419 to 985 ng mg-1) were less lethal to the spruce budworm. Both classes of defense compounds contribute significantly to the overall defensive capabilities of white spruce trees. Monoterpenes appear critical in determining the general toxicity of foliage, while phenolics play a role in slowing budworm development, thereby underscoring their collective importance in white spruce defenses.

Gene expression plasticity facilitates different host feeding in Ips sexdentatus (Coleoptera: Curculionidae: Scolytinae)

Host shift is ecologically advantageous and a crucial driver for herbivore insect speciation. Insects on the non-native host obtain enemy-free space and confront reduced competition, but they must adapt to survive. Such signatures of adaptations can often be detected at the gene expression level. It is astonishing how bark beetles cope with distinct chemical environments while feeding on various conifers. Hence, we aim to disentangle the six-toothed bark beetle (Ips sexdentatus) response against two different conifer defences upon host shift (Scots pine to Norway spruce). We conducted bioassay and metabolomic analysis followed by RNA-seq experiments to comprehend the beetle's ability to surpass two different terpene-based conifer defence systems. Beetle growth rate and fecundity were increased when reared exclusively on spruce logs (alternative host) compared to pine logs (native host). Comparative gene expression analysis identified differentially expressed genes (DEGs) related to digestion, detoxification, transporter activity, growth, signalling, and stress response in the spruce-feeding beetle gut. Transporter genes were highly abundant during spruce feeding, suggesting they could play a role in pumping a wide variety of endogenous and xenobiotic compounds or allelochemicals out. Trehalose transporter (TRET) is also up-regulated in the spruce-fed beetle gut to maintain homeostasis and stress tolerance. RT-qPCR and enzymatic assays further corroborated some of our findings. Taken together, the transcriptional plasticity of key physiological genes plays a crucial role after the host shift and provides vital clues for the adaptive potential of bark beetles on different conifer hosts.

Host Plant Effects on Sexual Selection Dynamics in Phytophagous Insects

Natural selection is notoriously dynamic in nature, and so, too, is sexual selection. The interactions between phytophagous insects and their host plants have provided valuable insights into the many ways in which ecological factors can influence sexual selection. In this review, we highlight recent discoveries and provide guidance for future work in this area. Importantly, host plants can affect both the agents of sexual selection (e.g., mate choice and male-male competition) and the traits under selection (e.g., ornaments and weapons). Furthermore, in our rapidly changing world, insects now routinely encounter new potential host plants. The process of adaptation to a new host may be hindered or accelerated by sexual selection, and the unexplored evolutionary trajectories that emerge from these dynamics are relevant to pest management and insect conservation strategies. Examining the effects of host plants on sexual selection has the potential to advance our fundamental understanding of sexual conflict, host range evolution, and speciation, with relevance across taxa.

A Pine in Distress: How Infection by Different Pathogenic Fungi Affect Lodgepole Pine Chemical Defenses

In North America, lodgepole pine is frequently subjected to attacks by various biotic agents that compromise its ability to defend against subsequent attacks by insect herbivores. We investigated whether infections of lodgepole pine by different pathogenic fungal species have varying effects on its defense chemistry. We selected two common pathogens, Atropellis canker, Atropellis piniphila, and western gall rust, Endocronartium harknessii, affecting mature lodgepole pine trees in western Canada. We also included three ophiostomatoid fungi Grosmannia clavigera, Ophiostoma montium, and Leptographium longiclavatum associated with the mountain pine beetle (Dendroctonus ponderosae), because they are commonly used to investigate induced defenses of host trees of bark beetles. We collected phloem samples from lodgepole pines infected with the rust or the canker and healthy lodgepole pines in the same stand. We also inoculated mature lodgepole pines with the three fungal symbionts and collected phloem samples 2 weeks later when the defense chemistry was at its highest level. Different fungal species differentially altered the terpene chemistry of lodgepole pine trees. E. harknessii and the fungal symbionts altered the terpene chemistry in a similar pattern while trees responded to the infection by the A. piniphila differently. Our study highlights the importance of considering specific biotic stress agents in tree susceptibility or resistance to the subsequent attacks by insect herbivores, such as mountain pine beetle.

Genomic virulence features of Beauveria bassiana as a biocontrol agent for the mountain pine beetle population

BACKGROUND

The mountain pine beetle, Dendroctonus ponderosae, is an irruptive bark beetle that causes extensive mortality to many pine species within the forests of western North America. Driven by climate change and wildfire suppression, a recent mountain pine beetle (MPB) outbreak has spread across more than 18 million hectares, including areas to the east of the Rocky Mountains that comprise populations and species of pines not previously affected. Despite its impacts, there are few tactics available to control MPB populations. Beauveria bassiana is an entomopathogenic fungus used as a biological agent in agriculture and forestry and has potential as a management tactic for the mountain pine beetle population. This work investigates the phenotypic and genomic variation between B. bassiana strains to identify optimal strains against a specific insect.

RESULTS

Using comparative genome and transcriptome analyses of eight B. bassiana isolates, we have identified the genetic basis of virulence, which includes oosporein production. Genes unique to the more virulent strains included functions in biosynthesis of mycotoxins, membrane transporters, and transcription factors. Significant differential expression of genes related to virulence, transmembrane transport, and stress response was identified between the different strains, as well as up to nine-fold upregulation of genes involved in the biosynthesis of oosporein. Differential correlation analysis revealed transcription factors that may be involved in regulating oosporein production.

CONCLUSION

This study provides a foundation for the selection and/or engineering of the most effective strain of B. bassiana for the biological control of mountain pine beetle and other insect pests populations.

Bark Beetles Utilize Ophiostomatoid Fungi to Circumvent Host Tree Defenses

Bark beetles maintain symbiotic associations with a diversity of microbial organisms, including ophiostomatoid fungi. Studies have frequently reported the role of ophiostomatoid fungi in bark beetle biology, but how fungal symbionts interact with host chemical defenses over time is needed. We first investigated how inoculations by three fungal symbionts of mountain pine beetle affect the terpene chemistry of live lodgepole pine trees. We then conducted a complimentary laboratory experiment specifically measuring the host metabolite degradation by fungi and collected the fungal organic volatiles following inoculations with the same fungal species on lodgepole pine logs. In both experiments, we analyzed the infected tissues for their terpene chemistry. Additionally, we conducted an olfactometer assay to determine whether adult beetles respond to the volatile organic chemicals emitted from each of the three fungal species. We found that all fungi upregulated terpenes as early as two weeks after inoculations. Similarly, oxygenated monoterpene concentrations also increased by several folds (only in logs). A large majority of beetles tested showed a strong attraction to two fungal species, whereas the other fungus repelled the beetles. Together this study shows that fungal symbionts can alter host defense chemistry, assist beetles in overcoming metabolite toxicity, and provide possible chemical cues for bark beetle attraction.

Multiple-trait analyses improved the accuracy of genomic prediction and the power of genome-wide association of productivity and climate change-adaptive traits in lodgepole pine

Background

Genomic prediction (GP) and genome-wide association (GWA) analyses are currently being employed to accelerate breeding cycles and to identify alleles or genomic regions of complex traits in forest trees species. Here, 1490 interior lodgepole pine (Pinus contorta Dougl. ex. Loud. var. latifolia Engelm) trees from four open-pollinated progeny trials were genotyped with 25,099 SNPs, and phenotyped for 15 growth, wood quality, pest resistance, drought tolerance, and defense chemical (monoterpenes) traits. The main objectives of this study were to: (1) identify genetic markers associated with these traits and determine their genetic architecture, and to compare the marker detected by single- (ST) and multiple-trait (MT) GWA models; (2) evaluate and compare the accuracy and control of bias of the genomic predictions for these traits underlying different ST and MT parametric and non-parametric GP methods. GWA, ST and MT analyses were compared using a linear transformation of genomic breeding values from the respective genomic best linear unbiased prediction (GBLUP) model. GP, ST and MT parametric and non-parametric (Reproducing Kernel Hilbert Spaces, RKHS) models were compared in terms of prediction accuracy (PA) and control of bias.

Results

MT-GWA analyses identified more significant associations than ST. Some SNPs showed potential pleiotropic effects. Averaging across traits, PA from the studied ST-GP models did not differ significantly from each other, with generally a slight superiority of the RKHS method. MT-GP models showed significantly higher PA (and lower bias) than the ST models, being generally the PA (bias) of the RKHS approach significantly higher (lower) than the GBLUP.

Conclusions

The power of GWA and the accuracy of GP were improved when MT models were used in this lodgepole pine population. Given the number of GP and GWA models fitted and the traits assessed across four progeny trials, this work has produced the most comprehensive empirical genomic study across any lodgepole pine population to date.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08747-7.

Performance of two Ips bark beetles and their associated pathogenic fungi on hosts reflects a species-specific association in the beetle-fungus complex

When Ips bark beetles invade and colonize the host plants, their associated pathogenic fungal partners are carried into the phloem of the host trees. Host trees are lethally attacked by the beetle-fungus complex and the collective damage severely limits forestry production worldwide. It is of great importance to verify whether bark beetles and their associated fungi show concordant performance in terms of biology, physiology, and biochemistry on host trees. In this study, the two Ips bark beetles Ips typographus and Ips subelongatus (Coleoptera: Curculionidae, Scolytinae), their respective associated pathogenic fungi Endoconidiophora polonica and Endoconidiophora fujiensis, and their respective host plants Picea jezoensis and Larix olgensis were selected as test material. Cross-inoculation experiments were conducted indoors and outdoors to investigate the differences in reproduction and development of two beetles and infectivity of two fungi on two plants, as well as the differences in physiological responses of two plants to two fungal infections. The results showed that I. typographus and E. polonica had excellent host performance on P. jezoensis; however, neither successfully colonized and infected L. olgensis. In contrast, I. subelongatus and E. fujiensis showed strong host suitability on L. olgensis and some degree of suitability on P. jezoensis, although the host suitability of P. jezoensis for E. polonica was significantly higher than that for E. fujiensis. In addition, we found that the absolute amount of ergosterol accumulated on the lesion was positively correlated with lesion area. The ergosterol amount and lesion area were both strongly correlated with the release of host monoterpenes, but had no obvious correlation with the concentration of fungi-induced phenols on the lesion area and the side-chain oxidation of lignin in the xylem of the infected sites. Based on these results, we confirmed that "I. typographus-E. polonica" and "I. subelongatus-E. fujiensis" complexes both showed the most suitable consistent performances on their own traditional hosts, establishing a stable species-specific association relationship in these two beetle-fungus complexes, with the "I. subelongatus-E. fujiensis" complex showing broader host suitability. From the perspective of physiological responses of plants to fungal infections, monoterpenes are an important indicator of host suitability.

Host Defense Metabolites Alter the Interactions between a Bark Beetle and its Symbiotic Fungi

Successful host plant colonization by tree-killing bark beetle-symbiotic fungal complexes depends on host suitability, which is largely determined by host defense metabolites such as monoterpenes. Studies have shown the ability of specific blends of host monoterpenes to influence bark beetles or their fungal symbionts, but how biologically relevant blends of host monoterpenes influence bark beetle-symbiotic fungal interaction is unknown. We tested how interactions between two host species (lodgepole pine or jack pine) and two fungal symbionts of mountain pine beetle (Grosmannia clavigera or Ophiostoma montium) affect the performance of adult female beetles in vitro. Beetles treated with the propagules of G. clavigera or O. montium or not treated (natural fungal load) were introduced into media amended with a blend of the entire monoterpene profile of either host species and beetle performance was compared. Overall, host blends altered beetle performance depending on the fungal species used in the beetle amendment. When beetles were amended with G. clavigera, their performance was superior over beetles amended with O. montium in either host blend. Furthermore, G. clavigera-amended beetles performed better in media amended with host blends than without a host blend; in contrast, O. montium-amended beetles performed better in media without a host blend than with a host blend. Overall, this study showed that host defense metabolites affect host suitability to bark beetles through influencing their fungal symbionts and that different species of fungal symbionts respond differentlly to host defense metabolites.

Pest defences under weak selection exert a limited influence on the evolution of height growth and drought avoidance in marginal pine populations

While droughts, intensified by climate change, have been affecting forests worldwide, pest epidemics are a major source of uncertainty for assessing drought impacts on forest trees. Thus far, little information has documented the adaptability and evolvability of traits related to drought and pests simultaneously. We conducted common-garden experiments to investigate how several phenotypic traits (i.e. height growth, drought avoidance based on water-use efficiency inferred from δ¹³C and pest resistance based on defence traits) interact in five mature lodgepole pine populations established in four progeny trials in western Canada. The relevance of interpopulation variation in climate sensitivity highlighted that seed-source warm populations had greater adaptive capability than cold populations. In test sites, warming generated taller trees with higher δ¹³C and increased the evolutionary potential of height growth and δ¹³C across populations. We found, however, no pronounced gradient in defences and their evolutionary potential along populations or test sites. Response to selection was weak in defences across test sites, but high for height growth particularly at warm test sites. Response to the selection of δ¹³C varied depending on its selective strength relative to height growth. We conclude that warming could promote the adaptability and evolvability of growth response and drought avoidance with a limited evolutionary influence from pest (biotic) pressures.

Combined drought and bark beetle attacks deplete non-structural carbohydrates and promote death of mature pine trees

How carbohydrate reserves in conifers respond to drought and bark beetle attacks are poorly understood. We investigated changes in carbohydrate reserves and carbon-dependent diterpene defences in ponderosa pine trees that were experimentally subjected to two levels of drought stress (via root trenching) and two types of biotic challenge treatments (pheromone-induced bark beetle attacks or inoculations with crushed beetles that include beetle-associated fungi) for two consecutive years. Our results showed that trenching did not influence carbohydrates, whereas both biotic challenges reduced amounts of starch and sugars of trees. However, only the combined trenched-bark beetle attacked trees depleted carbohydrates and died during the first year of attacks. While live trees contained higher carbohydrates than dying trees, amounts of constitutive and induced diterpenes produced did not vary between live and beetle-attacked dying trees, respectively. Based on these results we propose that reallocation of carbohydrates to diterpenes during the early stages of beetle attacks is limited in drought-stricken trees, and that the combination of biotic and abiotic stress leads to tree death. The process of tree death is subsequently aggravated by beetle girdling of phloem, occlusion of vascular tissue by bark beetle-vectored fungi, and potential exploitation of host carbohydrates by bark beetle symbionts as nutrients.

Production of complementary defense metabolites reflects a co-evolutionary arms race between a host plant and a mutualistic bark beetle-fungal complex

Intra-specific variation in conifers has been extensively studied with respect to defense against herbivores and pathogens. While studies have shown the ability of individual or specific mixtures of compounds to influence insects and microbes, research testing biologically relevant mixtures of defense compounds reflecting intra-specific variation amongst tree populations to enemy complexes is needed. We characterized the variations in lodgepole pine monoterpenes from a progeny trial in western Canada and grouped trees in four clusters using their monoterpene profiles. We then selected 11 representative families across four clusters and amended their entire monoterpene profiles (with the exception of β-phellandrene) in media to determine how representative families affect the performance of the mountain pine beetle or its fungal symbiont. We placed adult beetles or inoculated fungus on the amended media and measured beetle performance and fungal growth as a proxy to host suitability. We found that different clusters or families differentially influenced beetle or fungal responses. However, monoterpene profiles of trees suitable to the beetle or the fungus were dissimilar. These outcomes reflect a co-evolutionary arms-race between the host and the bark beetle-fungus complex, which has resulted in the production of complementary defense metabolites among different pine populations to enhance tree survival.

Primary and Secondary Metabolite Profiles of Lodgepole Pine Trees Change with Elevation, but Not with Latitude

Climate change has a large influence on plant functional and phenotypic traits including plant primary and secondary metabolites. One well-established approach to investigating the variation in plant metabolites involves studying plant populations along elevation and latitude gradients. We considered how two space-for-time climate change gradients (elevation and latitude) influence carbohydrate reserves (soluble sugars, starches) and secondary metabolites (monoterpenes, diterpene resin acids) of lodgepole pine trees in western Canada. We were particularly interested in the relationship of terpenes and carbohydrates with a wide range of tree, site, and climatic factors. We found that only elevation had a strong influence on the expression of both terpenes and carbohydrates of trees. Specifically, as elevation increased, concentrations of monoterpenes and diterpenes generally increased and soluble sugars (glucose, sucrose, total sugars) decreased. In contrast, latitude had no impact on either of terpenes or carbohydrates. Furthermore, we found a positive relationship between concentrations of starch and total terpenes and diterpenes in the elevation study; whereas neither starches nor sugars were correlated to terpenes in the latitude study. Similarly, both terpenes and carbohydrates had a much greater number of significant correlations to site characteristics such as slope, basal area index, and sand basal area, in the elevational than in the latitude study. Overall, these results support the conclusion that both biotic and abiotic factors likely drive the patterns of primary and secondary metabolite profiles of lodgepole pine along geographical gradients. Also, presence of a positive relationship between terpenes and starches suggests an interaction between primary ad secondary metabolites of lodgepole pine trees.

Selection of entomopathogenic fungus Beauveria bassiana (Deuteromycotina: Hyphomycetes) for the biocontrol of Dendroctonus ponderosae (Coleoptera: Curculionidae, Scolytinae) in Western Canada

The mountain pine beetle, Dendroctonus ponderosae, has infested over ~16 Mha of pine forests in British Columbia killing >50% of mature lodgepole pine, Pinus contorta, trees in affected stands. At present, it is functionally an invasive species in Alberta, killing and reproducing in evolutionarily naïve populations of lodgepole pine (P. contorta), novel jack pine (P. banksiana), and their hybrids. The entomopathogenic fungus Beauveria bassiana has shown some potential as a biocontrol agent of several bark beetle species. In this study, nine isolates of B. bassiana were examined for insect virulence characteristics, including conidiation rate, pigmentation, and infection rate in laboratory-reared D. ponderosae, to assess for their potential as biocontrol agents. The strains were categorized into three phenotypic groups based on pigmentation, conidial density, and myceliation rate. Virulence screening utilizing insect-based agar medium (D. ponderosae and European honeybee Apis mellifera carcasses) revealed no difference in selection of fungal growth. However, infection studies on D. ponderosae and A. mellifera showed contrasting results. In vivo A. mellifera infection model revealed ~5% mortality, representing the natural death rate of the hive population, whereas laboratory-reared D. ponderosae showed 100% mortality and mycosis. The LT₅₀ (median lethal time 50) ranges from 2 to 5 ± 0.33 days, and LT₁₀₀ ranges from 4 to 6 ± 0.5 days. We discuss the selective advantages of the three phenotypic groups in terms of virulence, pigmentation, conidial abundance, and tolerance to abiotic factors like UV and host tree monoterpenes. These results can further provide insights into the development of several phenotypically diverse B. bassiana strains in controlling the spread of the invasive D. ponderosae in Western Canada. KEY POINTS: • Three B. bassiana morphotype groups have been demonstrated to kill D. ponderosae. • A range of effective lethal times (LT₅₀ and LT₁₀₀) was established against D. ponderosae. • Variable tolerance to UV light and pine monoterpenes were observed in B. bassiana.

Review: Plant eco-evolutionary responses to climate change: Emerging directions

Contemporary climate change is exposing plant populations to novel combinations of temperatures, drought stress, [CO₂] and other abiotic and biotic conditions. These changes are rapidly disrupting the evolutionary dynamics of plants. Despite the multifactorial nature of climate change, most studies typically manipulate only one climatic factor. In this opinion piece, we explore how climate change factors interact with each other and with biotic pressures to alter evolutionary processes. We evaluate the ramifications of climate change across life history stages,and examine how mating system variation influences population persistence under rapid environmental change. Furthermore, we discuss how spatial and temporal mismatches between plants and their mutualists and antagonists could affect adaptive responses to climate change. For example, plant-virus interactions vary from highly pathogenic to mildly facilitative, and are partly mediated by temperature, moisture availability and [CO₂]. Will host plants exposed to novel, stressful abiotic conditions be more susceptible to viral pathogens? Finally, we propose novel experimental approaches that could illuminate how plants will cope with unprecedented global change, such as resurrection studies combined with experimental evolution, genomics or epigenetics.

Selection of entomopathogenic fungus Beauveria bassiana (Deuteromycotina: Hyphomycetes) for the biocontrol of Dendroctonus ponderosae (Coleoptera: Curculionidae, Scolytinae) in Western Canada

The mountain pine beetle, Dendroctonus ponderosae, has infested over ~16 Mha of pine forests in British Columbia killing >50% of mature lodgepole pine, Pinus contorta, trees in affected stands. At present, it is functionally an invasive species in Alberta, killing and reproducing in evolutionarily naïve populations of lodgepole pine (P. contorta), novel jack pine (P. banksiana), and their hybrids. The entomopathogenic fungus Beauveria bassiana has shown some potential as a biocontrol agent of several bark beetle species. In this study, nine isolates of B. bassiana were examined for insect virulence characteristics, including conidiation rate, pigmentation, and infection rate in laboratory-reared D. ponderosae, to assess for their potential as biocontrol agents. The strains were categorized into three phenotypic groups based on pigmentation, conidial density, and myceliation rate. Virulence screening utilizing insect-based agar medium (D. ponderosae and European honeybee Apis mellifera carcasses) revealed no difference in selection of fungal growth. However, infection studies on D. ponderosae and A. mellifera showed contrasting results. In vivo A. mellifera infection model revealed ~5% mortality, representing the natural death rate of the hive population, whereas laboratory-reared D. ponderosae showed 100% mortality and mycosis. The LT₅₀ (median lethal time 50) ranges from 2 to 5 ± 0.33 days, and LT₁₀₀ ranges from 4 to 6 ± 0.5 days. We discuss the selective advantages of the three phenotypic groups in terms of virulence, pigmentation, conidial abundance, and tolerance to abiotic factors like UV and host tree monoterpenes. These results can further provide insights into the development of several phenotypically diverse B. bassiana strains in controlling the spread of the invasive D. ponderosae in Western Canada. KEY POINTS: • Three B. bassiana morphotype groups have been demonstrated to kill D. ponderosae. • A range of effective lethal times (LT₅₀ and LT₁₀₀) was established against D. ponderosae. • Variable tolerance to UV light and pine monoterpenes were observed in B. bassiana.

Changes in soil fungal community composition depend on functional group and forest disturbance type

Disturbances have altered community dynamics in boreal forests with unknown consequences for belowground ecological processes. Soil fungi are particularly sensitive to such disturbances; however, the individual response of fungal guilds to different disturbance types is poorly understood. Here, we profiled soil fungal communities in lodgepole pine forests following a bark beetle outbreak, wildfire, clear-cut logging, and salvage-logging. Using Illumina MiSeq to sequence ITS1 and SSU rDNA, we characterized communities of ectomycorrhizal, arbuscular mycorrhizal, saprotrophic, and pathogenic fungi in sites representing each disturbance type paired with intact forests. We also quantified soil fungal biomass by measuring ergosterol. Abiotic disturbances changed the community composition of ectomycorrhizal fungi and shifted the dominance from ectomycorrhizal to saprotrophic fungi compared to intact forests. The disruption of the soil organic layer with disturbances correlated with the decline of ectomycorrhizal and the increase of arbuscular mycorrhizal fungi. Wildfire changed the community composition of pathogenic fungi but did not affect their proportion and diversity. Fungal biomass declined with disturbances that disrupted the forest floor. Our results suggest that the disruption of the forest floor with disturbances, and the changes in C and nutrient dynamics it may promote, structure the fungal community with implications for fungal biomass-C.

Insect defoliation is linked to a decrease in soil ectomycorrhizal biomass and shifts in needle endophytic communities

Insect outbreaks of increasing frequency and severity in forests are predicted due to climate change. Insect herbivory is known to promote physiological changes in forest trees. However, little is known about whether these plant phenotypic adjustments have cascading effects on tree microbial symbionts such as fungi in roots and foliage. We studied the impact of defoliation by the pine processionary moth in two infested Pinus nigra forests through a multilevel sampling of defoliated and non-defoliated trees. We measured tree growth, nutritional status and carbon allocation to chemical defenses. Simultaneously, we analysed the putative impact of defoliation on the needle endophytes and on the soil fungal communities. Higher concentrations of chemical defenses were found in defoliated trees, likely as a response to defoliation; however, no differences in non-structural carbohydrate reserves were found. In parallel to the reductions in tree growth and changes in chemical defenses, we observed shifts in the composition of needle endophytic and soil fungal communities in defoliated trees. Defoliated trees consistently corresponded with a lower biomass of ectomycorrhizal fungi in both sites, and a higher alpha diversity and greater relative abundance of belowground saprotrophs and pathogens. However, ectomycorrhizal alpha diversity was similar between non-defoliated and defoliated trees. Specific needle endophytes in old needles were strongly associated with non-defoliated trees. The potential role of these endophytic fungi in pine resistance should be further investigated. Our study suggests that lower biomass of ectomycorrhizal fungi in defoliated trees might slow down tree recovery since fungal shifts might affect tree-mycorrhizal feedbacks and can potentially influence carbon and nitrogen cycling in forest soils.

Light Limitation Impacts Growth but Not Constitutive or Jasmonate Induced Defenses Relevant to Emerald Ash Borer (Agrilus planipennis) in White Fringetree (Chionanthus virginicus) or Black Ash (Fraxinus nigra)

White fringetree is a host for the invasive emerald ash borer (EAB) but is of lower quality than the related and highly susceptible black ash. Field observations suggest that host trees grown in full sun are more resistant to EAB than those in shade, however the impact of light limitation on chemical defenses has not been assessed. We quantified constitutive and jasmonate-induced phloem defenses and growth patterns of white fringetree and black ash under differential light conditions and related them to EAB larval performance. White fringetree had significantly lower constitutive and induced activities of peroxidase, polyphenol oxidase, β-glucosidase, chitinase and lignin content, but significantly higher gallic acid equivalent soluble phenolic, soluble sugar, and oleuropein concentrations compared to black ash. Multivariate analyses based on tissue chemical attributes displayed clear separation of species and induced defense responses. Further, EAB performed significantly worse on white fringetree than black ash, consistent with previous studies. Light limitation did not impact measured defenses or EAB larval performance, but it did decrease current year growth and increase photosynthetic efficiency. Overall our results suggest that phenolic profiles, metabolite abundance, and growth traits are important in mediating white fringetree resistance to EAB, and that short-term light limitation does not influence phloem chemistry or larval success.

Resin ducts as resistance traits in conifers: linking dendrochronology and resin-based defences

Conifers have evolved different chemical and anatomical defences against a wide range of antagonists. Resin ducts produce, store and translocate oleoresin, a complex terpenoid mixture that acts as both a physical and a chemical defence. Although resin duct characteristics (e.g., number, density, area) have been positively related to biotic resistance in several conifer species, the literature reporting this association remains inconclusive. Axial resin ducts recorded in annual growth rings are an archive of annual defensive investment in trees. This whole-life record of defence investment can be analysed using standard dendrochronological procedures, which allows us to assess interannual variability and the effect of understudied drivers of phenotypic variation on resin-based defences. Understanding the sources of phenotypic variation in defences, such as genetic differentiation and environmental plasticity, is essential for assessing the adaptive potential of forest tree populations to resist pests under climate change. Here, we reviewed the evidence supporting the importance of resin ducts in conifer resistance, and summarized current knowledge about the sources of variation in resin duct production. We propose a standardized methodology to measure resin duct production by means of dendrochronological procedures. This approach will illuminate the roles of resin ducts in tree defence across species, while helping to fill pivotal knowledge gaps in plant defence theory, and leading to a robust understanding of the patterns of variation in resin-based defences throughout the tree's lifespan.

Exposure to Fungal Volatiles Can Influence Volatile Emissions From Other Ophiostomatoid Fungi

Fungal volatile organic compounds (FVOCs) can act as intra- and inter-kingdom communication signals that influence the growth and behaviors of organisms involved in antagonistic or mutualistic relationships with fungi. There is growing evidence suggesting that FVOCs can mediate interactions between organisms within and across different ecological niches. Bark beetles have established mutualistic relationships with ophiostomatoid fungi which can serve as a food source and condition host plant tissues for developing beetle larvae. While the profiles (both composition and concentrations) of volatile emission from ophiostomatoid fungi can be influenced by abiotic factors, whether emissions from a given fungal species can be influenced by those from another is still unknown. Here, we analyzed FVOCs emitted from the two ophiostomatoid fungi, Grosmannia clavigera and Ophiostoma ips, associated with mountain pine beetle and pine engraver beetle, respectively, when each fungus was growing alone or in a shared headspace. We used two isolates of each fungus species. Overall, we detected a total of eight volatiles in both G. clavigera alone or in combination with O. ips including acetoin, ethyl acetate, cis-grandisol, isoamyl alcohol, isobutanol, 2-methyl-1-butanol, phenethyl acetate, and phenethyl alcohol. The profiles of volatiles emitted differed between the two fungal species but not between the two isolates of the same fungus. Six compounds were common between the species, whereas two compounds were detected only when G. clavigera was present. Moreover, the majority of volatiles were detected less frequently and at lower concentrations when the two fungi were grown together in a shared headspace. These results are likely due to reduced volatile emissions from O. ips in the presence of G. clavigera. However, changes in the profiles of fungal volatiles did not correspond with the observed changes in the growth of either species. Overall, these results suggest that the similarities in fungal volatiles among different species of fungi may reflect a common ecological niche and that the differences may correspond to species-specific adaptation to their respective host beetles or genetic factors.

Nitrogen and Ergosterol Concentrations Varied in Live Jack Pine Phloem Following Inoculations With Fungal Associates of Mountain Pine Beetle

Bark beetles form symbiotic associations with multiple species of fungi that supplement their metabolic needs. However, the relative contributions of each symbiont to the nutrition of bark beetles have been largely unexplored. Thus, we evaluated the ability of three fungal symbionts of mountain pine beetle to concentrate nitrogen and produce ergosterol while infecting phloem of a novel host jack pine. Ergosterol was used as proxy to determine the fungal biomass (hyphal density) in the current study. We inoculated 80 trees in two forest stands with one of the three fungal species or a non-fungal (control) agar. Six weeks later, we collected phloem from the necrotic lesions induced by the fungi, uninfected tissues adjacent to lesions, and non-inoculated control trees. We found that nutritional contributions varied with fungal species. Nitrogen in lesions was higher in trees inoculated with Ophiostoma montium or control trees, relative to Grosmannia clavigera or Leptographium longiclavatum. Furthermore, concentrations of ergosterol were higher in O. montium lesions compared to other tissues or treatments. These results suggest that O. montium differs from G. clavigera and L. longiclavatum in terms of acquiring nitrogen from host tissues and producing ergosterol.

Induced Defenses of a Novel Host Tree Affect the Growth and Interactions of Bark Beetle-Vectored Fungi

Mountain pine beetle (MPB) has recently expanded its host range to the novel jack pine forests in Alberta. Invasion success of MPB may depend on the outcome of interactions between its symbiotic fungus Grosmannia clavigera and Ophiostoma ips, a fungal associate of a potential competitor Ips pini. However, how the quality of jack pine phloem could influence interactions between the fungi is unknown. We investigated whether introduced concentrations of host nitrogen and monoterpenes affect the growth of and interaction between the fungi. Nitrogen concentrations did not affect the growth rate of either fungus. In the absence of monoterpenes, the presence of O. ips promoted G. clavigera growth. Monoterpenes either promoted or inhibited the growth of both fungi, and altered the outcome of species interactions from facilitation to no-effect. Overall, these results suggest that jack pine phloem quality and the presence of a niche-sharing fungus could influence MPB development.

Caterpillars on a phytochemical landscape: The case of alfalfa and the Melissa blue butterfly

Modern metabolomic approaches that generate more comprehensive phytochemical profiles than were previously available are providing new opportunities for understanding plant-animal interactions. Specifically, we can characterize the phytochemical landscape by asking how a larger number of individual compounds affect herbivores and how compounds covary among plants. Here we use the recent colonization of alfalfa (Medicago sativa) by the Melissa blue butterfly (Lycaeides melissa) to investigate the effects of indivdiual compounds and suites of covarying phytochemicals on caterpillar performance. We find that survival, development time, and adult weight are all associated with variation in nutrition and toxicity, including biomolecules associated with plant cell function as well as putative anti-herbivore action. The plant-insect interface is complex, with clusters of covarying compounds in many cases encompassing divergent effects on different aspects of caterpillar performance. Individual compounds with the strongest associations are largely specialized metabolites, including alkaloids, phenolic glycosides, and saponins. The saponins are represented in our data by more than 25 individual compounds with beneficial and detrimental effects on L. melissa caterpillars, which highlights the value of metabolomic data as opposed to approaches that rely on total concentrations within broad defensive classes.

Tree defence and bark beetles in a drying world: carbon partitioning, functioning and modelling

Drought has promoted large-scale, insect-induced tree mortality in recent years, with severe consequences for ecosystem function, atmospheric processes, sustainable resources and global biogeochemical cycles. However, the physiological linkages among drought, tree defences, and insect outbreaks are still uncertain, hindering our ability to accurately predict tree mortality under on-going climate change. Here we propose an interdisciplinary research agenda for addressing these crucial knowledge gaps. Our framework includes field manipulations, laboratory experiments, and modelling of insect and vegetation dynamics, and focuses on how drought affects interactions between conifer trees and bark beetles. We build upon existing theory and examine several key assumptions: (1) there is a trade-off in tree carbon investment between primary and secondary metabolites (e.g. growth vs defence); (2) secondary metabolites are one of the main component of tree defence against bark beetles and associated microbes; and (3) implementing conifer-bark beetle interactions in current models improves predictions of forest disturbance in a changing climate. Our framework provides guidance for addressing a major shortcoming in current implementations of large-scale vegetation models, the under-representation of insect-induced tree mortality.

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

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

Spatial characteristics of volatile communication in lodgepole pine trees: Evidence of kin recognition and intra-species support

Plant interactions using volatile organic compounds, particularly in the context of kin recognition have received considerable attention in recent years, but several discrepancies and conflicting results have restricted our understanding. We propose that some of these discrepancies in literature are in part due to integral spatial characteristics of sites, and plant attributes. Chemotypic plasticity is commonly used to characterize kin, particularly in conifers. We studied constitutive and induced monoterpene chemotypes of non-attacked lodgepole pine trees within 30 m radii of pine trees attacked by mountain pine beetle. We tested the effects of volatile compounds emitted from the attacked trees on the non-attacked trees by challenge inoculations with a mountain pine beetle associated fungus. We found no relationship between constitutive monoterpene concentrations of the non-attacked trees and distance or direction from the attacked trees or site aspects. In contrast, the effects of volatile compounds were evident after inoculations, depending on distance from the attacked trees and site aspects. However, these interactions only emerged among chemotypically related trees. These results suggest that plants discriminate between chemical cues from kin and strangers, and the emitters likely aid only chemotypically related plants by emitting specific blends of volatiles that can only be deciphered by the receiving kin. These results further demonstrate the importance of incorporating spatial characteristics of sites and plant attributes in studies aimed at investigating intra-species interactions using volatile organic compounds.

Interacting Effects of Global Change on Forest Pest and Pathogen Dynamics

Pathogens and insect pests are important drivers of tree mortality and forest dynamics, but global change has rapidly altered or intensified their impacts. Predictive understanding of changing disease and outbreak occurrence has been limited by two factors: ( a) tree mortality and morbidity are emergent phenomena determined by interactions between plant hosts, biotic agents (insects or pathogens), and the environment; and ( b) disparate global change drivers co-occur, obscuring net impacts on each of these components. To expand our understanding of changing forest diseases, declines, and outbreaks, we adopt a framework that identifies and organizes observed impacts of diverse global change drivers on the primary mechanisms underlying agent virulence and host susceptibility. We then discuss insights from ecological theory that may advance prediction of forest epidemics and outbreaks. This approach highlights key drivers of changing pest and pathogen dynamics, which may inform forest management aimed at mitigating accelerating rates of tree mortality globally.

Soil Available Water Holding Capacity Can Alter the Reproductive Performance of Mountain Pine Beetle (Coleoptera: Curculionidae) in Jack Pine (Pinales: Pinaceae) Through Phloem Nitrogen Concentration

Mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae, Scolytidae) has recently invaded novel jack pine (Pinus banksiana Lamb., Pinales: Pinaceae) forests in western Canada. Jack pine seems to be a suitable host for mountain pine beetle, but how growing conditions influence jack pine's quality as a host, and hence, its susceptibility for mountain pine beetle, is unknown. Specifically, how soil nutrient concentrations and available water holding capacity (AWHC) affect jack pine quality should be investigated. Host plant quality is an important determinant of mountain pine beetle host colonization and reproduction and is usually assessed by primary (nutrients) and secondary (defense chemistry) constituents of host phloem. We evaluated mountain pine beetle host acceptance and brood production by recording the percentage of female mountain pine beetle that entered the phloem and oviposited in 30 jack pine bolts from two sites that differed in soil nutrient concentrations and AWHC. We also compared the concentrations of phloem nutrients and defense monoterpenes among the selected trees and found that trees at the Low AWHC site had higher amounts of nitrogen, phosphorus, and potassium. Monoterpene concentrations did not differ among trees at the two sites. Host acceptance by and brood production of mountain pine beetle were greater in bolts from the Low AWHC site. We conclude that AWHC of the soil may influence mountain pine beetle host acceptance and brood production through altering host plant quality, particularly nitrogen in the phloem, and will potentially influence any further range expansion of the beetle in eastern North America.

Pathophysiological responses of pine defensive metabolites largely lack differences between pine species but vary with eliciting ophiostomatoid fungal species

Phytopathogenic ophiostomatoid fungi are common associates of bark beetles and contribute to beetle-associated mortality of trees. Mountain pine beetle outbreaks in Canada are facilitating novel associations between its vectored fungi (Grosmannia clavigera, Leptographium longiclavatum and Ophiostoma montium) and jack pine. How the induced defense-related metabolite responses of jack and lodgepole pines vary in response to the fungi is unknown. Understanding this variation is important to clarifying pine susceptibility to and the physiological impacts of infection. We used a comparative metabolite profiling approach to investigate the defense-related signaling, carbon utilization/mobilization, and synthesis responses of both pines to the fungi. Both pine species largely exhibited similar metabolite responses to the fungi. The magnitude of pine metabolite responses positively reflected pathogen virulence. Our findings indicate that pines can recognize and metabolomically respond to novel pathogens, likely due to signals common between the novel fungi and fungi coevolved with the pine. Thus, jack pine is likely as susceptible as lodgepole pine to infections by each of the MPB-vectored fungi. Furthermore, the magnitude of the metabolite responses of both pines varied by the eliciting fungal species, with the most virulent pathogen causing the greatest reduction in carbohydrates and the highest accumulation of defensive terpenes.