Volatile Organic Compounds Emitted by Fungal Associates of Conifer Bark Beetles and their Potential in Bark Beetle Control

Extreme specificity in obligate mutualism-A role for competition?

Obligate mutualisms, reciprocally obligate beneficial interactions, are some of the most important mutualisms on the planet, providing the basis for the evolution of the eukaryotic cell, the formation and persistence of terrestrial ecosystems and the establishment and expansion of coral reefs. In addition, these mutualisms can also lead to the diversification of interacting partner species. Accompanying this diversification is a general pattern of a high degree of specificity among interacting partner species. A survey of obligate mutualisms demonstrates that greater than half of these systems have only one or two mutualist species on each side of the interaction. This is in stark contrast to facultative mutualisms that can have dozens of interacting mutualist species. We posit that the high degree of specificity in obligate mutualisms is driven by competition within obligate mutualist guilds that limits species richness. Competition may be particularly potent in these mutualisms because mutualistic partners are totally dependent on each other's fitness gains, which may fuel interspecific competition. Theory and the limited number of empirical studies testing for the role of competition in determining specificity suggest that competition may be an important force that fuels the high degree of specificity. Further empirical research is needed to dissect the relative roles of trait complementarity, mutualism regulation, and competition among mutualist guild members in determining mutualism specificity at local scales.

How Effective Are Push-Pull Semiochemicals as Deterrents for Bark Beetles? A Global Meta-Analysis of Thirty Years of Research

Bark beetles (Coleoptera: Curculionidae: Scolytinae) are among the most damaging tree pests globally. Rising temperatures, drought, fire, storms, cyclones, and poor forest management cause stress and loss of vigour in trees, and these conditions favour bark beetle outbreaks. While research has been conducted on push-pull strategies to deter bark beetles, using attractive and deterrent semiochemicals, the potential of this strategy to reduce bark beetle populations, particularly in the genera Dendroctonus and Ips, remains uncertain. Here, we conducted a global meta-analysis of 52 research articles to quantify the effects of semiochemical treatments on managing different species of Dendroctonus and Ips for forest protection. Based on this analysis, we found that push-pull semiochemicals can significantly reduce Dendroctonus and Ips populations measured by a reduction in the attraction to lure/trap catches, tree mortality, and attacks on trees. The overall efficacy of the push-pull semiochemical treatment shows a 66% reduction for Ips compared to control and a 54% reduction compared to control for Dendroctonus, while, at the species level, there was a 69% reduction for Dendroctonus ponderosae (Hopkins) and a 94% reduction in Ips perturbatus (Eichhoff), and a 93% reduction in Ips latidens (LeConte). Interestingly, among different treatment sources, the efficacy of conspecific semiochemicals in combination with heterospecific semiochemicals and non-host volatiles showed a 92% reduction in Dendroctonus spp., and conspecific semiochemicals in combination with non-host volatiles showed a 77% significant reduction in Ips spp., while the efficacy of heterospecific semiochemicals in reducing Ips population was about 69%, and 20% in Dendroctonus. Among different ecological regions, the use of a push-pull strategy showed a 70% reduction in Dendroctonus in central-west North America, and Ips showed a 75% reduction in southwest North America. Our results demonstrate that semiochemical-based push-pull techniques have the potential to reduce Dendroctonus and Ips bark beetle populations. Furthermore, based on our analysis, the efficacy of such eco-friendly interventions could be further improved and provide a good tool for forest managers to control these pests, at least under some circumstances.

Odors Attracting the Long-Legged Predator Medetera signaticornis Loew to Ips typographus L. Infested Norway Spruce Trees

Predatory long-legged flies of the genus Medetera are important, but currently understudied, natural enemies of Scolytinae bark beetles such as Ips typographus. Medetera flies lay eggs on beetle-infested trees, where the developing larvae find their prey, but the chemical cues used by Medetera to locate infested trees are currently unknown. To identify odors attracting Medetera signaticornis, a species in Europe, headspace samples were collected at several time-points through different stages of I. typographus attacks on logs of Norway spruce (Picea abies). The headspace samples were analyzed using combined gas chromatography and mass spectrometry (GC-MS), and gas chromatography coupled with electroantennographic detection (GC-EAD) to determine compounds that stimulate M. signaticornis antennae. Antennae of M. signaticornis males and females were found to detect (-)-cis-verbenol, ( +)-trans-verbenol and myrtenol, which are known to be produced by bark beetles. Antennal responses were also observed for verbenene, isoterpinolene, α-pinene oxide, camphor, pinocamphone, terpinene-4-ol, myrtenal, borneol, α-terpineol, geranyl acetone, and verbenone, which are primarily produced by microorganisms, and α-pinene, α-fenchene, β-pinene, camphene, 3-carene, limonene, γ-terpinene, and terpinolene, known spruce tree compounds. In field experiments testing two synthetic blends containing 18 antennal active and two additional compounds 2-methyl-3-buten-2-ol and ipsdienol we observed significant attraction of M. signaticornis within 24 h. These attractive blends can form the basis for development of Medetera monitoring lures for use in future forest and pest management.

Attackers gain the upper hand over plants in the face of rapid global change

Interactions among organisms in natural ecosystems are the foundational underpinnings of nearly all ecological studies. It has never been more important to increase our awareness of how these interactions are altered by human activity, threatening biodiversity and disrupting ecosystem functioning. Much of the historic focus of species conservation has been the preservation of endangered and endemic species at risk from hunting, over-exploitation, and habitat destruction. However, there is increasing evidence that differences between plants and their attacking organisms in the speed and direction of physiological, demographic, and genetic (adaptation) responses to global change are having devastating consequences, resulting in large-scale losses of dominant or abundant plant species, particularly in forest ecosystems. From the elimination in the wild of the American chestnut to the extensive regional damage caused by insect outbreaks in temperate forest ecosystems, these losses of dominant species change the ecological landscape and functioning, and represent important threats to biodiversity at all scales. Introductions due to human activity, range shifts due to climate change, and their combination are the principal drivers behind these profound ecosystem changes. In this Review, we argue that there is an urgent need to increase our recognition and hone our predictive power for how these imbalances may occur. Moreover, we should seek to minimize the consequences of these imbalances in order to ensure the preservation of the structure, function and biodiversity of entire ecosystems, not just rare or highly endangered species.

Why Do These Yeasts Smell So Good? Volatile Organic Compounds (VOCs) Produced by Malassezia Species in the Exponential and Stationary Growth Phases

Malassezia synthesizes and releases volatile organic compounds (VOCs), small molecules that allow them to carry out interaction processes. These lipid-dependent yeasts belong to the human skin mycobiota and are related to dermatological diseases. However, knowledge about VOC production and its function is lacking. This study aimed to determine the volatile profiles of Malassezia globosa, Malassezia restricta, and Malassezia sympodialis in the exponential and stationary growth phases. The compounds were separated and characterized in each growth phase through headspace solid-phase microextraction (HS-SPME) and gas chromatography–mass spectrometry (GC–MS). We found a total of 54 compounds, 40 annotated. Most of the compounds identified belong to alcohols and polyols, fatty alcohols, alkanes, and unsaturated aliphatic hydrocarbons. Unsupervised and supervised statistical multivariate analyses demonstrated that the volatile profiles of Malassezia differed between species and growth phases, with M. globosa being the species with the highest quantity of VOCs. Some Malassezia volatiles, such as butan-1-ol, 2-methylbutan-1-ol, 3-methylbutan-1-ol, and 2-methylpropan-1-ol, associated with biological interactions were also detected. All three species show at least one unique compound, suggesting a unique metabolism. The ecological functions of the compounds detected in each species and growth phase remain to be studied. They could interact with other microorganisms or be an important clue in understanding the pathogenic role of these yeasts.

Exchange of Microbiomes in Plant-Insect Herbivore Interactions

Prokaryotic and eukaryotic microbial symbiotic communities span through kingdoms. The vast microbial gene pool extends the host genome and supports adaptations to changing environmental conditions. Plants are versatile hosts for the symbionts, carrying microbes on the surface, inside tissues, and even within the cells. Insects are equally abundantly colonized by microbial symbionts on the exoskeleton, in the gut, in the hemocoel, and inside the cells. The insect gut is a prolific environment, but it is selective on the microbial species that enter with food. Plants and insects are often highly dependent on each other and frequently interact. Regardless of the accumulating evidence on the microbiomes of both organisms, it remains unclear how much they exchange and modify each other's microbiomes. In this review, we approach this question from the point of view of herbivores that feed on plants, with a special focus on the forest ecosystems. After a brief introduction to the subject, we concentrate on the plant microbiome, the overlap between plant and insect microbial communities, and how the exchange and modification of microbiomes affects the fitness of each host.

Conifer-killing bark beetles locate fungal symbionts by detecting volatile fungal metabolites of host tree resin monoterpenes

Outbreaks of the Eurasian spruce bark beetle (Ips typographus) have decimated millions of hectares of conifer forests in Europe in recent years. The ability of these 4.0 to 5.5 mm long insects to kill mature trees over a short period has been sometimes ascribed to two main factors: (1) mass attacks on the host tree to overcome tree defenses and (2) the presence of fungal symbionts that support successful beetle development in the tree. While the role of pheromones in coordinating mass attacks has been well studied, the role of chemical communication in maintaining the fungal symbiosis is poorly understood. Previous evidence indicates that I. typographus can distinguish fungal symbionts of the genera Grosmannia, Endoconidiophora, and Ophiostoma by their de novo synthesized volatile compounds. Here, we hypothesize that the fungal symbionts of this bark beetle species metabolize spruce resin monoterpenes of the beetle's host tree, Norway spruce (Picea abies), and that the volatile products are used as cues by beetles for locating breeding sites with beneficial symbionts. We show that Grosmannia penicillata and other fungal symbionts alter the profile of spruce bark volatiles by converting the major monoterpenes into an attractive blend of oxygenated derivatives. Bornyl acetate was metabolized to camphor, and α- and β-pinene to trans-4-thujanol and other oxygenated products. Electrophysiological measurements showed that I. typographus possesses dedicated olfactory sensory neurons for oxygenated metabolites. Both camphor and trans-4-thujanol attracted beetles at specific doses in walking olfactometer experiments, and the presence of symbiotic fungi enhanced attraction of females to pheromones. Another co-occurring nonbeneficial fungus (Trichoderma sp.) also produced oxygenated monoterpenes, but these were not attractive to I. typographus. Finally, we show that colonization of fungal symbionts on spruce bark diet stimulated beetles to make tunnels into the diet. Collectively, our study suggests that the blends of oxygenated metabolites of conifer monoterpenes produced by fungal symbionts are used by walking bark beetles as attractive or repellent cues to locate breeding or feeding sites containing beneficial microbial symbionts. The oxygenated metabolites may aid beetles in assessing the presence of the fungus, the defense status of the host tree and the density of conspecifics at potential feeding and breeding sites.

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.

Identification of sex-specific compounds in the invasive four-eyed fir bark beetle Polygraphus proximus

Polygraphus proximus, a four-eyed fir bark beetle, is an invasive bark beetle species which has caused extensive damage to forests of Abies sibirica in southern and western Siberia and to Abies species in the European part of Russia. There is a high risk that the pest insect will spread to areas where it is currently not considered present, such as the European Union. In these areas, it threatens to attack conifer forests of various species which may result in major environmental and economic impact. The aim of this study was to identify pheromone components of P. proximus that can be used as pheromone baits. Males and females of P. proximus were allowed to bore into the bark of stem sections of Abies sibirica at the laboratory, and volatiles were collected with solid-phase microextraction (SPME). Analyses of these extracts with gas chromatography-mass spectrometry (GC–MS) revealed several sex-specific compounds. In total, twelve male-specific compounds and one female-specific compound were identified. The major male-specific compound determined by GC peak area was (Z)‐2‐(3,3‐dimethylcyclohexylidene)‐ethanol [(Z)-DMCHE] and the minor male-specific compounds were 3-methyl-3-buten-1-ol, 3-methyl-2-buten-1-ol, 3-methyl-2-butenal, benzyl alcohol, fragranol, 7-methyl-3-methylene-6-octen-1-ol, (Z)- and (E)-2-(3,3-dimethylcyclohexylidene)-acetaldehyde, geraniol, geranial and papayanol. The only female-specific compound was identified as 1-hexanol. Two of the male-specific compounds, (Z)‐DMCHE and 3-methyl-2-buten-1-ol were shown to attract males and females of P. proximus in field studies. Thus, we now for the first time can present the structures of two male-specific components that are biologically active parts of P. proximus aggregation pheromone. However, some chemical communication overlap between P. proximus and P. subopacus needs to be further investigated as (Z)‐DMCHE also attracted males and females of P. subopacus.

Ophiostomatoid fungi synergize attraction of the Eurasian spruce bark beetle, Ips typographus to its aggregation pheromone in field traps

Eurasian spruce bark beetle, Ips typographus is a destructive pest of the Norway spruce (Picea abies). Recent outbreaks in Europe have been attributed to global warming and other anthropogenic impacts. Bark beetles are guided by multiple complex olfactory cues throughout their life cycle. Male-produced aggregation pheromones, comprising 2-methyl-3-buten-2-ol and cis-verbenol, have been identified as the most powerful attractants for dispersing conspecifics. In addition to host trees, bark beetles interact with multiple organisms, including symbiotic ophiostomatoid fungi, which may promote beetle colonization success and offspring development. Previously, in a short-distance laboratory assay, we demonstrated that I. typographus adults are attracted to the volatile organic compounds (VOCs) produced by three symbiotic fungi: Grosmannia penicillata, Endoconidiophora polonica, and Leptographium europhioides. Furthermore, the abundant fusel alcohols and their acetates were found to be the most attractive odorants in the fungal VOC profile. In this study, using a long-distance field-trapping experiment, we analyzed the role of fungal VOCs as attractants for dispersing I. typographus. Two types of fungal lures were tested in combination with pheromones in traps: (1) live cultures of fungi grown on potato dextrose agar (PDA) and (2) dispensers containing synthetic fusel alcohols and their acetates in equal proportions. Subsequently, the composition of VOCs emitted from live fungal lures were analyzed. We found that the symbiotic fungi synergistically increased the attraction of beetles to pheromones in field traps and the attractiveness of live fungal lures depended on the fungal load. While one Petri dish with E. polonica, when combined with pheromones synergistically increased trapping efficiency, three Petri dishes with L. europhioides were required to achieve the same. The synthetic mix of fungal fusel alcohols and acetates improved the catch efficiency of pheromones only at a low tested dose. VOC analysis of fungal cultures revealed that all the three fungi produced fusel alcohols and acetates but in variable composition and amounts. Collectively, the results of this study show that, in addition to pheromones, bark beetles might also use volatile cues from their symbiotic fungi to improve tree colonization and reproductive success in their breeding and feeding sites.

Biogenesis of spiroketals by submerged cultured basidiomycete Trametes hirsuta

Volatile spiroketals are well-documented semiochemicals secreted by beetles and wasps for the intra- and interspecies communication. Its use in insect traps and as natural herbicide makes them of commercial interest. Besides insects, fungi are well-known producers, but the fungal biogenesis of spiroketals has remained speculative. Product formation along fatty acid degradation based on non-labeled feeding experiments was assumed. Thus, the observed occurrence of conophthorin and (E)- and (Z)-chalcograns in submerged cultures of the basidiomycete Trametes hirsuta prompted a precursor study aiming at a more detailed insight into their formation. Supplementation of (9Z,12 Z)-octadecadienoic (linoleic) acid resulted in elevated product yields and the identification of a fourth spiroketal, 2,8-dimethyl-1,7-dioxaspiro[5.5]-undecane. However, no intermediates of fatty acid degradation suitable as spiroketal precursors were identified. In addition, the hyphae lacked lipoxygenase activity, which was formerly supposed to be mandatory for spiroketal formation. Supplementation of 1-/2-13C acetate showed incorporation of the label into chalcogran. Therefore, a formation along the polyketide pathway analogous to insects was concluded.

Graphical abstract

European Spruce Bark Beetle, Ips typographus (L.) Males Are Attracted to Bark Cores of Drought-Stressed Norway Spruce Trees with Impaired Defenses in Petri Dish Choice Experiments

The European spruce bark beetle, Ips typographus (L.), is a major pest of Norway spruce. During outbreaks, the beetles can colonize moderately stressed trees via mass attacks mediated by aggregation pheromones, while at endemic population levels, beetles infest trees with impaired defenses. I. typographus introduces ophiostomatoid fungi into the phloem, which can support host colonization. Low-density fungal infections are locally contained by hypersensitive wound reactions; larger necrotic lesions indicate lower tree resistance. Here, we made links between drought stress, susceptibility to fungal infections, and the attractiveness of spruce for host-searching I. typographus males. We sampled bark cores from roofed, non-roofed and untreated control trees of a rainfall exclusion field site. Drought stress was assessed using pre-dawn twig water potentials, and tree defenses were assessed using inoculations with Grosmannia penicillata. Subsequently, we performed Petri dish arena choice tests in the lab, where male beetles could choose between the bark samples of differentially stressed trees. We found that the attractiveness of bark cores increased with drought stress and the extent of hypersensitive wound reactions to fungal infection. Furthermore, beetles stayed longer in those Petri dish sections with the sample of their final choice. The bioassays provide evidence for the primary attraction of male I. typographus to tissues of Norway spruce and preference of beetles for stressed trees.

Human Impacts on Insect Chemical Communication in the Anthropocene

The planet is presently undergoing dramatic changes caused by human activities. We are living in the era of the Anthropocene, where our activities directly affect all living organisms on Earth. Insects constitute a major part of the world’s biodiversity and currently, we see dwindling insect biomass but also outbreaks of certain populations. Most insects rely on chemical communication to locate food, mates, and suitable oviposition sites, but also to avoid enemies and detrimental microbes. Emissions of, e.g., CO2, NOx, and ozone can all affect the chemical communication channel, as can a rising temperature. Here, we present a review of the present state of the art in the context of anthropogenic impact on insect chemical communication. We concentrate on present knowledge regarding fruit flies, mosquitoes, moths, and bark beetles, as well as presenting our views on future developments and needs in this emerging field of research. We include insights from chemical, physiological, ethological, and ecological directions and we briefly present a new international research project, the Max Planck Centre for Next Generation Insect Chemical Ecology (nGICE), launched to further increase our understanding of the impact of human activities on insect olfaction and chemical communication.

Fungal communities in feces of the frugivorous bat Ectophylla alba and its highly specialized Ficus colubrinae diet

BACKGROUND

Bats are important long-distance dispersers of many tropical plants, yet, by consuming fruits, they may disperse not only the plant's seeds, but also the mycobiota within those fruits. We characterized the culture-dependent and independent fungal communities in fruits of Ficus colubrinae and feces of Ectophylla alba to determine if passage through the digestive tract of bats affected the total mycobiota.

RESULTS

Using presence/absence and normalized abundance data from fruits and feces, we demonstrate that the fungal communities were significantly different, even though there was an overlap of ca. 38% of Amplicon Sequence Variants (ASVs). We show that some of the fungi from fruits were also present and grew from fecal samples. Fecal fungal communities were dominated by Agaricomycetes, followed by Dothideomycetes, Sordariomycetes, Eurotiomycetes, and Malasseziomycetes, while fruit samples were dominated by Dothideomycetes, followed by Sordariomycetes, Agaricomycetes, Eurotiomycetes, and Laboulbeniomycetes. Linear discriminant analyses (LDA) show that, for bat feces, the indicator taxa include Basidiomycota (i.e., Agaricomycetes: Polyporales and Agaricales), and the ascomycetous class Eurotiomycetes (i.e., Eurotiales, Aspergillaceae). For fruits, indicator taxa are in the Ascomycota (i.e., Dothideomycetes: Botryosphaeriales; Laboulbeniomycetes: Pyxidiophorales; and Sordariomycetes: Glomerellales). In our study, the differences in fungal species composition between the two communities (fruits vs. feces) reflected on the changes in the functional diversity. For example, the core community in bat feces is constituted by saprobes and animal commensals, while that of fruits is composed mostly of phytopathogens and arthropod-associated fungi.

CONCLUSIONS

Our study provides the groundwork to continue disentangling the direct and indirect symbiotic relationships in an ecological network that has not received enough attention: fungi-plants-bats. Findings also suggest that the role of frugivores in plant-animal mutualistic networks may extend beyond seed dispersal: they may also promote the dispersal of potentially beneficial microbial symbionts while, for example, hindering those that can cause plant disease.

Perspectives for Synergic Blends of Attractive Sources in South American Palm Weevil Mass Trapping: Waiting for the Red Palm Weevil Brazil Invasion

Coupling several natural and synthetic lures with aggregation pheromones from the palm weevils Rhynchophorus palmarum and R. ferrugineus reveals a synergy that results in an increase in pest captures. The combined attraction of pure pheromones, ethyl acetate, and decaying sweet and starchy plant tissue increases the net total of mass-trapped weevils. The 2018 entrance of the red palm weevil (RPW) into South America has threatened palm-product income in Brazil and other neighboring countries. The presence of the new A1 quarantine pest necessitates the review of all available options for a sustainable mass-trapping, monitoring, and control strategy to ultimately target both weevils with the same device. The effective lure-blend set for the mass-trapping system will attract weevils in baiting and contaminating stations for entomopathogenic fungi that the same weevils will spread.

RNA Interference-Based Forest Protection Products (FPPs) Against Wood-Boring Coleopterans: Hope or Hype?

Forest insects are emerging in large extension in response to ongoing climatic changes, penetrating geographic barriers, utilizing novel hosts, and influencing many hectares of conifer forests worldwide. Current management strategies have been unable to keep pace with forest insect population outbreaks, and therefore novel and aggressive management strategies are urgently required to manage forest insects. RNA interference (RNAi), a Noble Prize-winning discovery, is an emerging approach that can be used for forest protection. The RNAi pathway is triggered by dsRNA molecules, which, in turn, silences genes and disrupts protein function, ultimately causing the death of the targeted insect. RNAi is very effective against pest insects; however, its proficiency varies significantly among insect species, tissues, and genes. The coleopteran forest insects are susceptible to RNAi and can be the initial target, but we lack practical means of delivery, particularly in systems with long-lived, endophagous insects such as the Emerald ash borer, Asian longhorn beetles, and bark beetles. The widespread use of RNAi in forest pest management has major challenges, including its efficiency, target gene selection, dsRNA design, lack of reliable dsRNA delivery methods, non-target and off-target effects, and potential resistance development in wood-boring pest populations. This review focuses on recent innovations in RNAi delivery that can be deployed against forest pests, such as cationic liposome-assisted (lipids), nanoparticle-enabled (polymers or peptides), symbiont-mediated (fungi, bacteria, and viruses), and plant-mediated deliveries (trunk injection, root absorption). Our findings guide future risk analysis of dsRNA-based forest protection products (FPPs) and risk assessment frameworks incorporating sequence complementarity-based analysis for off-target predictions. This review also points out barriers to further developing RNAi for forest pest management and suggests future directions of research that will build the future use of RNAi against wood-boring coleopterans.

Entomopathogenic fungi to control bark beetles: a review of ecological recommendations

There is considerable interest in applying entomopathogenic fungi as a biological control to limit insect populations due to their low environmental and human applicator impacts. However, despite many promising laboratory tests, there are few examples where these fungi were successfully applied to manage bark beetles. Here, we explore how environmental conditions unique to bark beetle habitats may have limited previous entomopathogenic fungus applications, including variable temperatures, ultraviolet light, bark beetle symbiotic microorganisms, tree phytochemicals, and cryptic bark beetle behaviors. Based on the existing literature, we provide a framework for interpreting the pathogenicity of entomopathogenic fungi to bark beetles, with emphasis on both standardizing and improving laboratory approaches to enhance field applications. Our synthesis indicates that most previous laboratory evaluations are conducted under conditions that are not representative of actual bark beetle systems; this may render fungal isolates functionally non-pathogenic in field settings. We recommend that future studies place particular effort into understanding entomopathogen response to the presence of bark beetle symbiotic microorganisms, plant phytochemicals, and potential as a tree endophyte. Additionally, field application methods should aid entomopathogens in overcoming stressful conditions and allow the fungus to infect multiple bark beetle life stages. © 2021 Society of Chemical Industry.

Fungal Interactions and Host Tree Preferences in the Spruce Bark Beetle Ips typographus

The spruce bark beetle Ips typographus is the most damaging pest in European spruce forests and has caused great ecological and economic disturbances in recent years. Although native to Eurasia, I. typographus has been intercepted more than 200 times in North America and could establish there as an exotic pest if it can find suitable host trees. Using in vitro bioassays, we compared the preference of I. typographus for its coevolved historical host Norway spruce (Picea abies) and two non-coevolved (naïve) North American hosts: black spruce (Picea mariana) and white spruce (Picea glauca). Additionally, we tested how I. typographus responded to its own fungal associates (conspecific fungi) and to fungi vectored by the North American spruce beetle Dendroctonus rufipennis (allospecific fungi). All tested fungi were grown on both historical and naïve host bark media. In a four-choice Petri dish bioassay, I. typographus readily tunneled into bark medium from each of the three spruce species and showed no preference for the historical host over the naïve hosts. Additionally, the beetles showed a clear preference for bark media colonized by fungi and made longer tunnels in fungus-colonized media compared to fungus-free media. The preference for fungus-colonized media did not depend on whether the medium was colonized by conspecific or allospecific fungi. Furthermore, olfactometer bioassays demonstrated that beetles were strongly attracted toward volatiles emitted by both con- and allospecific fungi. Collectively, these results suggest that I. typographus could thrive in evolutionary naïve spruce hosts if it becomes established in North America. Also, I. typographus could probably form and maintain new associations with local allospecific fungi that might increase beetle fitness in naïve host trees.

Research Progress on Phytopathogenic Fungi and Their Role as Biocontrol Agents

Phytopathogenic fungi decrease crop yield and quality and cause huge losses in agricultural production. To prevent the occurrence of crop diseases and insect pests, farmers have to use many synthetic chemical pesticides. The extensive use of these pesticides has resulted in a series of environmental and ecological problems, such as the increase in resistant weed populations, soil compaction, and water pollution, which seriously affect the sustainable development of agriculture. This review discusses the main advances in research on plant-pathogenic fungi in terms of their pathogenic factors such as cell wall-degrading enzymes, toxins, growth regulators, effector proteins, and fungal viruses, as well as their application as biocontrol agents for plant pests, diseases, and weeds. Finally, further studies on plant-pathogenic fungal resources with better biocontrol effects can help find new beneficial microbial resources that can control diseases.

Mutualistic Ophiostomatoid Fungi Equally Benefit from Both a Bark Beetle Pheromone and Host Tree Volatiles as Nutrient Sources

Interactions between mutualistic bark beetles and ophiostomatoid fungi have received considerable attention in recent years. Studies have shown how volatile organic compounds emitted from mutualist fungi affect the behaviors of several bark beetle species. However, we currently lack sufficient knowledge regarding whether bark beetle pheromones can influence mutualist fungi. Here, we measured growth and biomass of two mutualistic fungi of the mountain pine beetle in response to headspace of a beetle pheromone (trans-verbenol), a blend of host tree volatiles, the combination of both, or control (no volatile source) in vitro experiments consisting of a nitrogen-based medium. The surface area and ergosterol content of the mycelia were used as surrogates for fungal growth and biomass respectively. We found that both growth and biomass of Grosmannia clavigera and Ophiostoma montium were greater in medium exposed to any type of volatile sources than the control. While growth and ergosterol content of G. clavigera were highest in the combination treatment, there were no differences in growth or biomass among the types of volatiles introduced for O. montium. These results suggest that both mutualistic fungi can utilize both bark beetle pheromone and host tree volatiles as nutrient sources. Overall, these results support the on-going studies on the role of volatile organic compounds mediating mutualistic bark beetle-fungi interactions.

Combining phytochemicals and multitrophic interactions to control forest insect pests

Forest pests can cause massive ecological and economic damage worldwide. Ecologically sound solutions to diminish forest insect pest impacts include the use of their natural enemies, such as predators and parasitoids, as well as entomopathogenic fungi, bacteria or viruses. Phytochemical compounds mediate most interactions between these organisms, but knowledge of such chemically mediated multitrophic relationships is still at its infancy for forest systems, particularly when compared to agricultural systems. Here, we highlight the main gaps in how phytochemicals of forest trees facilitate or interfere with trophic interactions between trees, insect herbivores, and interacting organisms including predators, parasitoids and microbes. We propose future avenues of research on phytochemical-based biocontrol of forest pests taking into account the characteristics of trees and forests.

Interactions among Norway spruce, the bark beetle Ips typographus and its fungal symbionts in times of drought

Resilience and functionality of European Norway spruce forests are increasingly threatened by mass outbreaks of the bark beetle Ips typographus promoted by heat, wind throw and drought. Here, we review current knowledge on Norway spruce and I. typographus interactions from the perspective of drought-stressed trees, host selection, colonisation behaviour of beetles, with multi-level effects of symbiotic ophiostomatoid fungi. By including chemo-ecological, molecular and behavioural perspectives, we provide a comprehensive picture on this complex, multitrophic system in the light of climate change. Trees invest carbon into specialised metabolism to produce defence compounds against biotic invaders; processes that are strongly affected by physiological stress such as drought. Spruce bark contains numerous terpenoid and phenolic substances, which are important for bark beetle aggregation and attack success. Abiotic stressors such as increased temperatures and drought affect composition, amounts and emission rates of volatile compounds. Thus, drought events may influence olfactory responses of I. typographus, and further the pheromone communication enabling mass attack. In addition, I. typographus is associated with numerous ophiostomatoid fungal symbionts with multiple effects on beetle life history. Symbiotic fungi degrade spruce toxins, help to exhaust tree defences, produce beetle semiochemicals, and possibly provide nutrition. As the various fungal associates have different temperature optima, they can influence the performance of I. typographus differently under changing environmental conditions. Finally, we discuss why effects of drought on tree-killing by bark beetles are still poorly understood and provide an outlook on future research on this eruptive species using both, field and laboratory experiments.

GC-MS analysis of mango stem bark extracts (Mangifera indica L.), Haden variety. Possible contribution of volatile compounds to its health effects

Mango stem bark extracts (MSBE) have been used as bioactive ingredients for nutraceutical, cosmeceutical, and pharmaceutical formulations due to their antioxidant, anti-inflammatory, and analgesic effects. We performed the MSBE preparative column liquid chromatography, which led to the resolution and identification by GC-MS of 64 volatile compounds: 7 hydrocarbons, 3 alcohols, 1 ether, 3 aldehydes/ketones, 7 phenols, 20 terpenoids (hydrocarbons and oxygenated derivatives), 9 steroids, 4 nitrogen compounds, and 1 sulphur compound. Major components were β-elemene, α-guaiene, aromadendrene, hinesol, 1-octadecene, β-eudesmol, methyl linoleate, juniper camphor, hinesol, 9-methyl (3β,5α)-androstan-3-ol, γ-sitosterol, β-chamigrene, 2,5-dihydroxymethyl-phenetylalcohol, N-phenyl-2-naphtaleneamine, and several phenolic compounds. The analysis of MSBE, Haden variety, by GC-MS is reported for the first time, which gives an approach to understand the possible synergistic effect of volatile compounds on its antioxidant, analgesic, and anti-inflammatory effects. The identification of relevant bioactive volatile components from MSBE extracts, mainly terpenes from the eudesmane family, will contribute to correlate its chemical composition to previous determined pharmacological effects.

Ethanol-Enriched Substrate Facilitates Ambrosia Beetle Fungi, but Inhibits Their Pathogens and Fungal Symbionts of Bark Beetles

Bark beetles (sensu lato) colonize woody tissues like phloem or xylem and are associated with a broad range of micro-organisms. Specific fungi in the ascomycete orders Hypocreales, Microascales and Ophistomatales as well as the basidiomycete Russulales have been found to be of high importance for successful tree colonization and reproduction in many species. While fungal mutualisms are facultative for most phloem-colonizing bark beetles (sensu stricto), xylem-colonizing ambrosia beetles are long known to obligatorily depend on mutualistic fungi for nutrition of adults and larvae. Recently, a defensive role of fungal mutualists for their ambrosia beetle hosts was revealed: Few tested mutualists outcompeted other beetle-antagonistic fungi by their ability to produce, detoxify and metabolize ethanol, which is naturally occurring in stressed and/or dying trees that many ambrosia beetle species preferentially colonize. Here, we aim to test (i) how widespread beneficial effects of ethanol are among the independently evolved lineages of ambrosia beetle fungal mutualists and (ii) whether it is also present in common fungal symbionts of two bark beetle species (Ips typographus, Dendroctonus ponderosae) and some general fungal antagonists of bark and ambrosia beetle species. The majority of mutualistic ambrosia beetle fungi tested benefited (or at least were not harmed) by the presence of ethanol in terms of growth parameters (e.g., biomass), whereas fungal antagonists were inhibited. This confirms the competitive advantage of nutritional mutualists in the beetle’s preferred, ethanol-containing host material. Even though most bark beetle fungi are found in the same phylogenetic lineages and ancestral to the ambrosia beetle (sensu stricto) fungi, most of them were highly negatively affected by ethanol and only a nutritional mutualist of Dendroctonus ponderosae benefited, however. This suggests that ethanol tolerance is a derived trait in nutritional fungal mutualists, particularly in ambrosia beetles that show cooperative farming of their fungi.

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.

Evaluation of Terpene-Volatile Compounds Repellent to the Coffee Berry Borer, Hypothenemus hampei (Ferrari) (Coleoptera: Curculionidae)

The coffee berry borer (CBB) is one of the main coffee pests in the world including Colombia. This pest is difficult to manage because of its cryptic habits and the continuous availability of coffee fruits. Among the new management strategies being tested is the use of volatile compounds as insect repellents. In this work, the behavioral response of female adult CBBs to terpenes previously identified in the CBB-repellent plant species Lantana camara was evaluated. α-Terpinene, (R)-limonene, farnesene and β-caryophyllene terpenes were tested via a Y-tube olfactometer in which ripe coffee fruits were accompanied by terpenes at concentrations between 25 and 200 ppm. Only β-caryophyllene induced a significant and consistent CBB repellent effect at all tested doses. The protective effect of microencapsulated β-caryophyllene was then determined under laboratory conditions by incorporating the terpene in a colloidosome-gel system at 2.8 × 10⁵ ng/h in the middle of coffee fruits with adult CBBs. The coffee fruits in turn presented a decrease in fruit infestation. Furthermore, the protection of coffee fruits when β-caryophyllene gels were hung in coffee trees was evaluated in the field; infestations were artificially induced by the use of raisins (CBB-infested old coffee fruits) placed on the ground. Compared with unprotected trees, the trees treated with caryophyllene gels exhibited a 33 to 45% lower degree of infestation. Taken together, the results show that β-caryophyllene is a promising compound for an integrated pest management (IPM) program in commercial coffee plantations.

Ecology and Evolution of Insect-Fungus Mutualisms

The evolution of a mutualism requires reciprocal interactions whereby one species provides a service that the other species cannot perform or performs less efficiently. Services exchanged in insect-fungus mutualisms include nutrition, protection, and dispersal. In ectosymbioses, which are the focus of this review, fungi can be consumed by insects or can degrade plant polymers or defensive compounds, thereby making a substrate available to insects. They can also protect against environmental factors and produce compounds antagonistic to microbial competitors. Insects disperse fungi and can also provide fungal growth substrates and protection. Insect-fungus mutualisms can transition from facultative to obligate, whereby each partner is no longer viable on its own. Obligate dependency has (a) resulted in the evolution of morphological adaptations in insects and fungi, (b) driven the evolution of social behaviors in some groups of insects, and (c) led to the loss of sexuality in some fungal mutualists.

Volatile organic compounds influence the interaction of the Eurasian spruce bark beetle (Ips typographus) with its fungal symbionts

Insects have mutualistic symbioses with a variety of microorganisms. However, the chemical signals that maintain these insect-microbe relationships are poorly known compared to those from insect-plant symbioses. The spruce bark beetle, Ips typographus, the most destructive forest pest in Europe, has a symbiotic relationship with several fungi that are believed to contribute to its successful invasion of Norway spruce. Here we tested the hypothesis that volatile organic compounds (VOCs) emitted from fungal symbionts could be cues for bark beetles to recognize and distinguish among members of its microbial community. Behavioral experiments with fungi showed that immature adults of I. typographus are attracted to food sources colonized by their fungal symbionts but not to saprophytic fungi and that this attraction is mediated by volatile cues. GC-MS measurements revealed that the symbionts emitted VOCs. Testing the activity of these compounds on beetle antennae using single sensillum recordings showed that beetles detect many fungal volatiles and possess olfactory sensory neurons specialized for these compounds. Finally, synthetic blends of fungal volatiles attracted beetles in olfactometer experiments. These findings indicate that volatile compounds produced by fungi may act as recognition signals for bark beetles to maintain specific microbial communities that might have impact on their fitness.

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.

Volatiles from the hypoxylaceous fungi Hypoxylon griseobrunneum and Hypoxylon macrocarpum

The volatiles emitted by the ascomycetes Hypoxylon griseobrunneum and Hypoxylon macrocarpum (Hypoxylaceae, Xylariales) were collected by use of a closed-loop stripping apparatus (CLSA) and analysed by GC–MS. The main compound class of both species were polysubstituted benzene derivatives. Their structures could only be unambiguously determined by comparison to all isomers with different substitution patterns. The substitution pattern of the main compound from H. griseobrunneum, the new natural product 2,4,5-trimethylanisole, was explainable by a polyketide biosynthesis mechanism that was supported by a feeding experiment with (methyl-²H₃)methionine.

Symbiont selection via alcohol benefits fungus farming by ambrosia beetles

Ambrosia beetles are among the true fungus-farming insects and cultivate fungal gardens on which the larvae and adults feed. After invading new habitats, some species destructively attack living or weakened trees growing in managed and unmanaged settings. Ambrosia beetles adapted to weakened trees tunnel into stem tissues containing ethanol to farm their symbiotic fungi, even though ethanol is a potent antimicrobial agent that inhibits the growth of various fungi, yeasts, and bacteria. Here we demonstrate that ambrosia beetles rely on ethanol for host tree colonization because it promotes the growth of their fungal gardens while inhibiting the growth of “weedy” fungal competitors. We propose that ambrosia beetles use ethanol to optimize their food production.

Animal–microbe mutualisms are typically maintained by vertical symbiont transmission or partner choice. A third mechanism, screening of high-quality symbionts, has been predicted in theory, but empirical examples are rare. Here we demonstrate that ambrosia beetles rely on ethanol within host trees for promoting gardens of their fungal symbiont and producing offspring. Ethanol has long been known as the main attractant for many of these fungus-farming beetles as they select host trees in which they excavate tunnels and cultivate fungal gardens. More than 300 attacks by Xylosandrus germanus and other species were triggered by baiting trees with ethanol lures, but none of the foundresses established fungal gardens or produced broods unless tree tissues contained in vivo ethanol resulting from irrigation with ethanol solutions. More X. germanus brood were also produced in a rearing substrate containing ethanol. These benefits are a result of increased food supply via the positive effects of ethanol on food-fungus biomass. Selected Ambrosiella and Raffaelea fungal isolates from ethanol-responsive ambrosia beetles profited directly and indirectly by (i) a higher biomass on medium containing ethanol, (ii) strong alcohol dehydrogenase enzymatic activity, and (iii) a competitive advantage over weedy fungal garden competitors (Aspergillus, Penicillium) that are inhibited by ethanol. As ambrosia fungi both detoxify and produce ethanol, they may maintain the selectivity of their alcohol-rich habitat for their own purpose and that of other ethanol-resistant/producing microbes. This resembles biological screening of beneficial symbionts and a potentially widespread, unstudied benefit of alcohol-producing symbionts (e.g., yeasts) in other microbial symbioses.

Behavioral Responses of Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae) to Volatiles of Black Walnut and Geosmithia morbida (Ascomycota: Hypocreales: Bionectriaceae), the Causal Agent of Thousand Cankers Disease

Thousand cankers disease (TCD) is a pest complex formed by the association of the walnut twig beetle (WTB), Pityophthorus juglandis Blackman (Coleoptera: Curculionidae: Scolytinae), with the fungal pathogen Geosmithia morbida Kolařík, Freeland, Utley and Tisserat (Ascomycota: Hypocreales: Bionectriaceae). Current monitoring and detection efforts for WTB rely on a pheromone lure that is effective over a limited distance while plant- and fungal-derived volatiles that may facilitate host location remain poorly understood. In this study, we test the hypothesis that adult beetles are attracted to volatiles of black walnut, Juglans nigra L. (Juglandaceae), and the pathogen, G. morbida. We measured the response of beetles to head-space volatiles collected from leaves and stems of 12 genotypes of black walnut and found genotypic variation in the attractiveness of host trees to adult WTB. Volatile profiles of the most attractive genotypes contained more α-pinene and β-pinene, and adult beetles were attracted to both of these compounds in olfactometer bioassays. In bioassays, we also demonstrated that adult WTB are attracted to volatiles of G. morbida. These findings suggest that, in addition to the aggregation pheromone, dispersing WTB potentially use host plant and fungal volatiles to locate suitable larval hosts. Finally, we conducted a field experiment to determine the extent to which ethanol, a common attractant for bark beetles, and limonene, a known bark beetle repellent, influence the behavior of adult WTB to pheromone-baited traps. Although ethanol did not increase trap capture, WTB were repelled by limonene, suggesting that this compound could be used to manipulate and manage WTB populations.

Influence of microbial symbionts on insect pheromones

Symbiotic microorganisms can influence the fitness of their insect hosts by modulating pheromone production and perception.

Evolutionary feedbacks between insect sociality and microbial management

Fitness-determining interactions with microbes-in particular fungi-have often been considered a by-product of social evolution in insects. Here, we take the view that both beneficial and harmful microbial consortia are major drivers of social behaviours in many insect systems-ranging from aggregation to eusociality. We propose evolutionary feedbacks between the insect sociality and microbial communities that strengthen mutualistic interactions with beneficial (dietary or defensive) microbes and simultaneously increase the capacity to defend against pathogens (i.e. social immunity). We identified variation in habitat stability-as determined by breeding site predictability and ephemerality-as a main ecological factor that constrains these feedbacks. To test this hypothesis we suggest following the evolution of insect social traits upon experimental manipulation of habitat stability and microbial consortia.

Fungal volatiles - a survey from edible mushrooms to moulds

Covering: up to January 2017This review gives a comprehensive overview of the production of fungal volatiles, including the history of the discovery of the first compounds and their distribution in the various investigated strains, species and genera, as unravelled by modern analytical methods. Biosynthetic aspects and the accumulated knowledge about the bioactivity and biological functions of fungal volatiles are also covered. A total number of 325 compounds is presented in this review, with 247 cited references.