Complex interactions among host pines and fungi vectored by an invasive bark beetle

Different genotypes and species of symbiotic fungi mediate the behavioral response of invasive Sirex noctilio fabricius (Hymenoptera: Siricidae)

In northeast China, the invasive woodwasp., Sirex noctilio, attacks Pinus sylvestris var. mongolica Litv and often shares habitat with native Sirex nitobei. Previous research showed that S. noctilio can utilize the volatiles from its symbiotic fungus (A. areolatum IGS-BD) to locate host trees. Consequently, symbiotic fungi (A. areolatum IGS-D and A. chailletii) carried by S. nitobei may influence the behavioral selection of S. noctilio. This study aimed to investigate the impact of fungal odor sources on S. noctilio's behavior in laboratory and field experiments. Our observations revealed that female woodwasps exhibited greater attraction toward the fungal volatiles of 14-day-old Amylostereum IGS-D in a "Y"-tube olfactometer and wind tunnel. When woodwasps were released into bolts inoculated separately with three strains in the field, females of S. noctilio exhibited a preference for those bolts pre-inoculated with A. areolatum IGS-BD. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that the volatiles emitted by the two genotypes of A. areolatum were similar yet significantly distinct from those of Ampelopsis chailletii. Hence, we postulate that the existence of native A. areolatum IGS-D could potentially facilitate the colonization of S. noctilio in scenarios with minimal or no A. areolatum IGS-BD present in the host.

Modeling the pest-pathogen threats in a warming world for the red turpentine beetle (Dendroctonus valens) and its symbiotic fungus (Leptographium procerum)

BACKGROUND

Dendroctonus valens along with its symbiotic fungi have caused unprecedented damage to pines in China. Leptographium procerum, its primary symbiotic fungus, facilitates the invasion and colonization of the pest, thereby aggravating ecological threats. Assessing shifts in the niches and ranges of D. valens and its symbiotic fungus could provide a valuable basis for pest control. Here, we conducted niche comparisons between native and invasive populations of D. valens. Then, we employed standard ecological niche models and ensembles of small models to predict the potential distributions of D. valens and L. procerum under climate change conditions and to estimate areas of overlap.

RESULTS

The niche of invasive population of D. valens in Chinese mainland only occupied a limited portion of the niche of native population in North America, leaving a substantial native niche unfilled and without any niche expansion. The suitable regions for D. valens are predicted in central and southern North America and central and northeastern Chinese mainland. The overlap with the suitable regions of L. procerum included eastern North America and the central and northeastern Chinese mainland under historical climatic scenarios. The regions susceptible to their symbiotic damage will shift northward in response to future climate change.

CONCLUSIONS

Projected distributions of D. valens and its symbiotic fungus, along with areas vulnerable to their symbiotic damage, provide essential insights for devising strategies against this association. Additionally, our study contributes to comprehending how biogeographic approaches aid in estimating potential risks of pest-pathogen interactions in forests within a warming world. © 2024 Society of Chemical Industry.

G-Protein Coupled Receptor Gpr-1 Is Important for the Growth and Nutritional Metabolism of an Invasive Bark Beetle Symbiont Fungi Leptographium procerum

Leptographium procerum has been demonstrated to play important roles in the invasive success of red turpentine beetle (RTB), one of the most destructive invasive pests in China. Our previous studies found that bacterial volatile ammonia plays an important role in the maintenance of the RTB-L. procerum invasive complex. In this study, we found a GPCR gene Gpr-1 that was a response to ammonia but not involved in the ammonia-induced carbohydrate metabolism. Deletion of Gpr-1 significantly inhibited the growth and pathogenicity but thickened the cell wall of L. procerum, resulting in more resistance to cell wall-perturbing agents. Further analyses suggested that Gpr-1 deletion caused growth defects that might be due to the dysregulation of the amino acid and lipid metabolisms. The thicker cell wall in the ΔGpr-1 mutant was induced through the cell wall remodeling process. Our results indicated that Gpr-1 is essential for the growth of L. procerum by regulating the nutritional metabolism, which can be further explored for potential applications in the management of RTB.

Composition and Diversity of the Endobacteria and Ectobacteria of the Invasive Bark Beetle Hylurgus ligniperda (Fabricius) (Curculionidae: Scolytinae) in Newly Colonized Areas

Hylurgus ligniperda (Fabricius) (Curculionidae: Scolytinae) is a new invasive pest beetle in China, which colonized the Shandong province, causing devastating damage. Originating in Europe, it has spread to Oceania, Asia, North and South America. Bacterial associates have been frequently reported to play a vital role in strengthening the ecological adaptations of bark and ambrosia beetles. The environmental adaptability of H. ligniperda may be supported by their associated bacteria. Bacterial communities colonizing different body parts of insects may have different functions. However, little is known about the bacteria associated with H. ligniperda and their potential involvement in facilitating the adaptation and invasion of the beetles into new environments. In this study, we employed high-throughput sequencing technology to analyze the bacterial communities associated with male and female adults of H. ligniperda by comparing those colonizing the elytra, prothorax, and gut. Results showed that the bacterial communities of male and female adults were similar, and the elytra samples had the highest bacterial diversity and richness, followed by the gut, while the prothorax had the lowest. The dominant phyla were Proteobacteria, Firmicutes, and Actinobacteriota, while the dominant genera were Serratia, Lactococcus, Rhodococcus, unclassified Enterobacteriaceae, and Gordonia. Among these, Rhodococcus and Gordonia were the specific genera of endobacteria and ectobacteria, respectively. Differences in the distribution of associated bacteria may suggest that they have different ecological functions for H. ligniperda. The results of functional prediction showed that bacteria were enriched in terpenoid backbone biosynthesis, degradation of aromatic compounds, limonene and pinene degradation, neomycin, kanamycin and gentamicin biosynthesis, indicating that they may assist their beetles in synthesizing pheromones, degrading toxic secondary metabolites of host trees, and antagonizing pathogenic fungi. These results help us understand the interaction between H. ligniperda and bacteria and highlight possible contributions to the invasion process.

Symbiotic microbes aid host adaptation by metabolizing a deterrent host pine carbohydrate d-pinitol in a beetle-fungus invasive complex

The red turpentine beetle (RTB) is one of the most destructive invasive pests in China and solely consumes pine phloem containing high amounts of d-pinitol. Previous studies reported that d-pinitol exhibits deterrent effects on insects. However, it remains unknown how insects overcome d-pinitol during their host plant adaptation. We found that d-pinitol had an antagonistic effect on RTB, which mainly relied on gallery microbes to degrade d-pinitol to enhance host adaptation with mutualistic Leptographium procerum and two symbiotic bacteria, Erwinia and Serratia, responsible for this degradation. Genomic, transcriptomic, and functional investigations revealed that all three microbes can metabolize d-pinitol via different branches of the inositol pathway. Our results collectively highlight the contributions of symbiotic microbes in RTB's adaptation to living on pine, thereby facilitating outbreaks of RTB in China. These findings further enrich our knowledge of symbiotic invasions and contribute to the further understanding of plant-insect interactions.

Chromosome-level genome assembly and population genomic analyses provide insights into adaptive evolution of the red turpentine beetle, Dendroctonus valens

BACKGROUND

Biological invasions are responsible for substantial environmental and economic losses. The red turpentine beetle (RTB), Dendroctonus valens LeConte, is an important invasive bark beetle from North America that has caused substantial tree mortality in China. The lack of a high-quality reference genome seriously limits deciphering the extent to which genetic adaptions resulted in a secondary pest becoming so destructive in its invaded area.

RESULTS

Here, we present a 322.41 Mb chromosome-scale reference genome of RTB, of which 98% of assembled sequences are anchored onto fourteen linkage groups including the X chromosome with a N50 size of 24.36 Mb, which is significantly greater than other Coleoptera species. Repetitive sequences make up 45.22% of the genome, which is higher than four other Coleoptera species, i.e., Mountain pine beetle Dendroctonus ponderosae, red flour beetle Tribolium castaneum, blister beetle Hycleus cichorii, and Colorado potato beetle Leptinotarsa decemlineata. We identify rapidly expanded gene families and positively selected genes in RTB, which may be responsible for its rapid environmental adaptation. Population genetic structure of RTB was revealed by genome resequencing of geographic populations in native and invaded regions, suggesting substantial divergence of the North American population and illustrates the possible invasion and spread route in China. Selective sweep analysis highlighted the enhanced ability of Chinese populations in environmental adaptation.

CONCLUSIONS

Overall, our high-quality reference genome represents an important resource for genomics study of invasive bark beetles, which will facilitate the functional study and decipher mechanism underlying invasion success of RTB by integrating the Pinus tabuliformis genome.

Abundance and Diversity of Ophiostomatoid Fungi Associated With the Great Spruce Bark Beetle (Dendroctonus micans) in the Northeastern Qinghai-Tibet Plateau

The role of several virulent tree pathogens in host death has been overlooked because of the aggressiveness of their associated bark beetles. The great spruce bark beetle (Dendroctonus micans) is a widely distributed beetle that infests coniferous plants in Eurasia; however, its associated fungi have been poorly studied. Therefore, in this study, we elucidated the diversity of ophiostomatoid fungi associated with D. micans in the northeastern Qinghai-Tibet Plateau through field investigation, laboratory isolation, and culture analyses. A total of 220 strains of ophiostomatoid fungi were isolated from adults and tunnel galleries of D. micans infesting Picea crassifolia. We identified that the isolated strains belonged to eight ophiostomatoid species, including five new species (Ophiostoma huangnanense sp. nov., Ophiostoma maixiuense sp. nov., Ophiostoma sanum sp. nov., Leptographium sanjiangyuanense sp. nov., and Leptographium zekuense sp. nov.), one undefined species (Ophiostoma sp. 1), and two known species (Ophiostoma bicolor and Endoconidiophora laricicola), using phylogenetic analysis of multigene DNA sequences and morphological characteristics. This is the first time that E. laricicola, a pioneer invader and virulent pathogen, has been reported in China. We found that E. laricicola was the dominant species, accounting for 40.91% of the total number of ophiostomatoid communities. This study enriched the knowledge of the fungal associates of D. micans and elucidated that it carried the virulent pathogen E. laricicola at a surprisingly high frequency. Our findings show increased species association between D. micans and ophiostomatoid fungi and provide a basis for understanding the occurrence of forest diseases and pests.

Mongolian pine forest decline by the combinatory effect of European woodwasp and plant pathogenic fungi

Interactions between the decline of Mongolian pine woodlands and fungal communities and invasive pests in northeastern China are poorly understood. In this study, we investigated the fungal communities occurring in three tree samples: the woodwasp Sirex noctilio infested, healthy uninfested and unhealthy uninfested Mongolian pine trees. We analyzed the relationships of the Mongolian pine decline with fungal infection and woodwasp infestation. Twenty-six fungal species were identified from the sampled trees. Each tree sample harbored a fungal endophyte community with a unique structure. Pathogenic fungi richness was four times higher in infested and unhealthy un-infested trees compared to that in healthy uninfested trees. Sphaeropsis sapinea was the most dominant pathogenic fungus in the sampled Mongolian pine trees. The number of S. noctilio was higher than native bark beetles in the declining Mongolian pine trees. The invasion of the woodwasp appeared to be promoted by the fungal infection in the Mongolian pine trees. The incidence of S. noctilio infestation was higher in the fungi infected trees (83.22%) than those without infection (38.72%). S. sapinea population exhibited positive associations with within-tree colonization of S. noctilio and bark beetle. Collectively, these data indicate that the fungal disease may have caused as the initial reason the decline of the Mongolian pine trees, and also provided convenient conditions for the successful colonization of the woodwasp. The woodwasps attack the Mongolian pine trees infected by fungi and accelerated its decline.

Fungal associates of an invasive pine-infesting bark beetle, Dendroctonus valens, including seven new Ophiostomatalean fungi

The red turpentine beetle (RTB; Dendroctonus valens) is a bark beetle that is native to Central and North America. This insect is well-known to live in association with a large number of Ophiostomatalean fungi. The beetle is considered a minor pest in its native range, but has killed millions of indigenous pine trees in China after its appearance in that country in the late 1990s. In order to increase the base of knowledge regarding the RTB and its symbionts, surveys of the beetle's fungal associates were initially undertaken in China, and in a subsequent study in its native range in North America. A total of 30 Ophiostomatalean species that included several undescribed taxa, were identified in these surveys. In the present study, seven of the undescribed taxa collected during the surveys were further characterised based on their morphological characteristics and multi-gene phylogenies. We proceeded to describe five of these as novel Leptographium spp. and two as new species of Ophiostoma. Four of the Leptographium spp. resided in the G. galeiformis-species complex, while one formed part of the L. olivaceum-species complex. One Ophiostoma sp. was a member of the O. ips-species complex, while the only new species from China was closely related to O. floccosum. Two of the previously undescribed taxa from North America were shown to be congeneric with L. terebrantis, implying that this species was most often isolated in association with the RTB in North America. The undescribed taxon from North America was identified as O. ips, and like L. terebrantis, this species was also not recognized during the initial North American survey. Resolving the identities of these taxa provides essential baseline information to better understand the movement of fungal pathogens with this beetle. This then enhances our ability to accurately assess and predict the risks of invasions by these and related fungi.

Fungal microbiomes are determined by host phylogeny and exhibit widespread associations with the bacterial microbiome

Interactions between hosts and their resident microbial communities are a fundamental component of fitness for both agents. Though recent research has highlighted the importance of interactions between animals and their bacterial communities, comparative evidence for fungi is lacking, especially in natural populations. Using data from 49 species, we present novel evidence of strong covariation between fungal and bacterial communities across the host phylogeny, indicative of recruitment by hosts for specific suites of microbes. Using co-occurrence networks, we demonstrate marked variation across host taxonomy in patterns of covariation between bacterial and fungal abundances. Host phylogeny drives differences in the overall richness of bacterial and fungal communities, but the effect of diet on richness was only evident in the mammalian gut microbiome. Sample type, tissue storage and DNA extraction method also affected bacterial and fungal community composition, and future studies would benefit from standardized approaches to sample processing. Collectively these data indicate fungal microbiomes may play a key role in host fitness and suggest an urgent need to study multiple agents of the animal microbiome to accurately determine the strength and ecological significance of host-microbe interactions.

Impacts of radiation exposure on the bacterial and fungal microbiome of small mammals in the Chernobyl Exclusion Zone

Environmental impacts of the 1986 Chernobyl Nuclear Power Plant accident are much debated, but the effects of radiation on host microbiomes have received little attention to date. We present the first analysis of small mammal gut microbiomes from the Chernobyl Exclusion Zone in relation to total absorbed dose rate, including both caecum and faeces samples. We provide novel evidence that host species determines fungal community composition, and that associations between microbiome (both bacterial and fungal) communities and radiation exposure vary between host species. Using ambient versus total weighted absorbed dose rates in analyses produced different results, with the latter more robust for interpreting microbiome changes at the individual level. We found considerable variation between results for faecal and gut samples of bank voles, suggesting faecal samples are not an accurate indicator of gut composition. Associations between radiation exposure and microbiome composition of gut samples were not robust against geographical variation, although we identified families of bacteria (Lachnospiraceae and Muribaculaceae) and fungi (Steccherinaceae and Strophariaceae) in the guts of bank voles that may serve as biomarkers of radiation exposure. Further studies considering a range of small mammal species are needed to establish the robustness of these potential biomarkers.

Chemical signal interactions of the bark beetle with fungal symbionts, and host/non-host trees

The symbiosis between the bark beetle (Ips subelongatus) and its fungal symbiont (Endoconidiophora fujiensis) poses a serious threat to larch forests. However, the signaling pathways between these symbiotic partners and their host/non-host trees are not fully understood. Inoculation of the host larch (Larix principis-rupprechtii) with two strains of E. fujiensis induced a rapid and long-term release of monoterpenes. Although the fungi had a level of tolerance to these compounds, many monoterpenes inhibited fungal growth in culture. Moreover, monoterpenes with stronger inhibitory effects on fungal growth exhibited weaker synergistic effects on the attraction of I. subelongatus to aggregation pheromone. Surprisingly, individual isomers of aggregation pheromone components promoted fungal symbiont growth in a culture medium. Non-host volatiles (NHVs) were tested and shown to completely inhibit the growth of fungal symbionts in culture but had no effects on beetle responses to aggregation pheromone, with the exception of (Z)-3-hexen-1-ol. These results reveal convergence and mutualism patterns in the evolution of I. subelongatus and E. fujiensis with respect to host tree volatiles but not in response to NHVs. Ultimately, we put forward a hypothesis that host plants are ecological and evolutionary determinants of bark beetle-fungus symbioses in terms of their complex signaling interactions.

An invasive beetle-fungus complex is maintained by fungal nutritional-compensation mediated by bacterial volatiles

Mutualisms between symbiotic microbes and animals have been well documented, and nutritional relationships provide the foundation for maintaining beneficial associations. The well-studied mutualism between bark beetles and their fungi has become a classic model system in the study of symbioses. Despite the nutritional competition between bark beetles and beneficial fungi in the same niche due to poor nutritional feeding substrates, bark beetles still maintain mutualistic associations with beneficial fungi over time. The mechanism behind this phenomenon, however, remains largely unknown. Here, we demonstrated the bark beetle Dendroctonus valens LeConte relies on the symbiotic bacterial volatile ammonia, as a nitrogen source, to regulate carbohydrate metabolism of its mutualistic fungus Leptographium procerum to alleviate nutritional competition, thereby maintaining the stability of the bark beetle–fungus mutualism. Ammonia significantly reduces competition of L. procerum for carbon resources for D. valens larval growth and increases fungal growth. Using stable isotope analysis, we show the fungus breakdown of phloem starch into d-glucose by switching on amylase genes only in the presence of ammonia. Deletion of amylase genes interferes with the conversion of starch to glucose. The acceleration of carbohydrate consumption and the conversion of starch into glucose benefit this invasive beetle–fungus complex. The nutrient consumption–compensation strategy mediated by tripartite beetle–fungus–bacterium aids the maintenance of this invasive mutualism under limited nutritional conditions, exacerbating its invasiveness with this competitive nutritional edge.

Red turpentine beetle primary attraction to (-)-β-pinene+ethanol in US Pacific Northwest ponderosa pine forests

Red turpentine beetle, Dendroctonus valens (Coleoptera: Curculionidae: Scolytinae) is a non-aggressive pine bark beetle native to North America, and more aggressive invader in China. Dispersing pioneer beetles are attracted to potential host trees by oleoresin monoterpene kairomones, but respond more strongly to those combined with ethanol, a mixture often released from stressed, dying, or recently dead trees. (+)-3-Carene, usually the dominant or co-dominant monoterpene in ponderosa pine, Pinus ponderosa, is a stronger attractant than α-pinene or β-pinene where tested over a large portion of the D. valens range, while (+)-3-carene+ethanol was shown previously to attract twice the beetles of (+)-3-carene. A field test comparing D. valens attraction among the three monoterpenes when all are released with ethanol has never been reported, and was our objective. In three US Pacific Northwestern pine forests, (–)-β-pinene+ethanol lures attracted 1.4 to 1.9 times more beetles than (+)-3-carene+ethanol. (+)- or (±)-α-pinene+ethanol lures were least attractive. A 1:1:1 monoterpene mixture+ethanol lure attracted more beetles than the 1:1:1 lure, but it was not statistically higher. Monoterpenes were dispensed from low density polyethylene bottles and their release rates monitored in laboratory and field tests. Under laboratory conditions (+)-3-carene was released much more rapidly than (+)-α-pinene or (–)-β-pinene when dispensed separately, or in a 1:1:1 mixture. (+)-3-Carene in the 1:1:1 mixture increased the release of both pinenes over their rates when dispensed separately. (–)-β-Pinene+ethanol is currently the strongest kairomone lure for D. valens attraction in US northwest pine forests, and has value for beetle detection, monitoring, research, and management.

New Approaches in Urban Forestry to Minimize Invasive Species Impacts: The Case of Xiongan New Area in China

China is implementing an extensive urban forestry plan in Xiongan New Area (XNA), a new city in Hebei province. The city has been designated to serve Beijing’s noncapital functions and promote the integration of the broader Beijing–Tianjin–Hebei city-region. As part of a green initiative to minimize environmental impacts and its carbon footprint, a massive urban forestry system has been planned on an unprecedented scale, expected to cover over 600 km² by 2030. Using science to inform policy, one major goal is to simultaneously minimize impacts of invasive species, while making urban forests more resilient to potential invasive species threats. In this review, we introduce these urban forestry plans such as basic concepts and principles for afforestation, tree species to be planted, delineation of existing pests already established, and expected forest invasive species of concern threatening the new area. Finally, we introduce a framework for invasive pest management strategies in XNA based on a “big data” approach and decision system to minimize impacts of invasive species. This new approach to urban forestry has the potential to become an exemplary global model for urban forestry planning, one that integrates research activities focused on forest health surveys and monitoring with sustainable forestry management. Finally, we provide an overview of the forest health policy required for the design of an unprecedentedly large new urban forest from initial planning to full implementation of an integrated forest management program.

Acquisition of fungi from the environment modifies ambrosia beetle mycobiome during invasion

Microbial symbionts can play critical roles when their host attempts to colonize a new habitat. The lack of symbiont adaptation can in fact hinder the invasion process of their host. This scenario could change if the exotic species are able to acquire microorganisms from the invaded environment. Understanding the ecological factors that influence the take-up of new microorganisms is thus essential to clarify the mechanisms behind biological invasions. In this study, we tested whether different forest habitats influence the structure of the fungal communities associated with ambrosia beetles. We collected individuals of the most widespread exotic (Xylosandrus germanus) and native (Xyleborinus saxesenii) ambrosia beetle species in Europe in several old-growth and restored forests. We characterized the fungal communities associated with both species via metabarcoding. We showed that forest habitat shaped the community of fungi associated with both species, but the effect was stronger for the exotic X. germanus. Our results support the hypothesis that the direct contact with the mycobiome of the invaded environment might lead an exotic species to acquire native fungi. This process is likely favored by the occurrence of a bottleneck effect at the mycobiome level and/or the disruption of the mechanisms sustaining co-evolved insect-fungi symbiosis. Our study contributes to the understanding of the factors affecting insect-microbes interactions, helping to clarify the mechanisms behind biological invasions.

Complex Insect-Pathogen Interactions in Tree Pandemics

Tree pandemics are a major cause of economic and ecological loss in forest and urban ecosystems. They often depend on the introduction of a non-native pathogen, which is occupying the niche of a native, non-aggressive organism. Complex interactions with native insects carrying fungi and nematodes can be established based on the proximity of the aggressive pathogenic agents. Here we review three major pandemics of forest and urban trees in temperate ecosystems at world scale, i.e., the Dutch elm disease, the cypress canker, and the pine wilt disease. For each system, the relationships between aggressive and non-aggressive fungi and nematodes with the native insect vectors are presented. Hidden players such as insects, microorganisms or plants, which may have the role of facilitating or contrasting the performance of the agents, are also considered. Results suggest that pandemics rely on the introduction of a non-native pathogen that exploits well-developed interactions between native non-aggressive organisms and insects associated with trees. The success of the invaders depends on the morpho-physiological proximity of the players and on the mutual benefits resulting from the associations. Deciphering such interactions in native systems may help to predict the outcome of the introduction of new pathogens and the development of new tree pandemics.

Effects of endophytic fungi diversity in different coniferous species on the colonization of Sirex noctilio (Hymenoptera: Siricidae)

Diversity of endophyte communities of the host tree affects the oviposition behavior of Sirex noctilio and the growth of its symbiotic fungus Amylostereum areolatum. In this study, we evaluated the structure and distribution of endophyte communities in the host tree (Pinus sylvestris var. mongolica) of S. noctilio and eight potential host tree species in China. Overall, 1626 fungal strains were identified by using internal transcribed spacer sequencing and morphological features. Each tree species harbored a fungal endophyte community with a unique structure, with the genus Trichoderma common to different communities. The isolation and colonization rate of endophytes from Pinus tabulaeformis, followed by P. sylvestris var. mongolica, were lower than those of other species. The proportion of endophytic fungi that strongly inhibited S. noctilio and symbiotic fungus growth was significantly lower in P. tabulaeformis, P. sylvestris var. mongolica and P. yunnanensis. Further, the diversity of the endophyte communities appeared to be predominantly influenced by tree species and the region, and, to a lesser extent, by the trunk height. Collectively, the data indicated that P. tabulaeformis might be at a higher risk of invasion and colonization by S. noctilio than other trees.

Brown planthopper honeydew-associated symbiotic microbes elicit momilactones in rice

Plants use many natural products to counter pests and diseases in nature. In rice, direct defense mechanisms include broad range of secondary metabolites, such as phenolamides (PA), diterpene phytoalexins, and flavonoid sakuranetin. Recently, accumulation of PAs in rice was shown to be under control of microbial symbionts in honeydew (HD), digestive waste from the rice brown planthopper (Nilaparvata lugens; BPH), but whether HD microbiota can also promote diterpene phytoalexins, momilactone A (MoA) and MoB, has not been reported. Here, we demonstrate that crude HD, but not a filtered one, induces MoA and MoB in rice, suggesting the involvement of BPH-HD endosymbionts. Consequently, microbial strains previously isolated from HD could promote MoA and MoB levels in wounded rice leaves, suggesting that rice indeed responds to BPH by cumulative chemical defense that involves both PA and diterpene phytoalexin pathways.

Bacterial microbiota protect an invasive bark beetle from a pine defensive compound

BACKGROUND

There is growing evidence that some devastating biotic invasions are facilitated by microbial symbionts. The red turpentine beetle (RTB), an innocuous secondary insect attacking weakened trees in North America, has formed an invasive complex with the fungus Leptographium procerum in China, and this invasive beetle-fungus symbiotic complex is capable of attacking and killing healthy pines. A previous study demonstrated that three Chinese-resident fungi, newly acquired by RTB in China, induce high levels of a phenolic defensive chemical, naringenin, in pines and this invasive beetle-fungus complex is suppressed by elevated levels of naringenin while the beetle uses its gallery as an external detoxification system in which particular yeast-like fungi and bacterial species biodegrade naringenin. However, the functional roles of key microbial players in the symbiosis, contained within the microbiome of the bark beetle gallery, have not been well elucidated.

RESULTS

In this report, the symbiotic naringenin-degrading microbiota were found to increase RTB survivorship in the presence of induced host defenses, and potential genes associated with degradation pathways were discovered. While fungi in the gallery microbiota had little involvement in naringenin degradation, bacterial community structure within the beetle gallery was highly correlated to naringenin degrading activity. Phylotypes of the Gram-negative bacterial genus Novosphingobium, which possessed genes involved in degradation pathways, were highly correlated to naringenin degradation activities and RTB associated with an isolated species of this genus acquired protection against naringenin and gained fitness.

CONCLUSIONS

Our results demonstrated that symbiotic bacterial community of RTB galleries enhances the survivorship and overall fitness of invasive beetles by degrading the host phenolic naringenin, ultimately overcoming the tree defenses and facilitating the success of the invasive beetle-fungi complex. This dynamic interplay between the invasive insect pest and multipartite microbes suggests a putative mechanism in invasion ecology for mitigating biotic resistance to symbiotic invasion.

A novel application of RNase H2-dependent quantitative PCR for detection and quantification of Grosmannia clavigera, a mountain pine beetle fungal symbiont, in environmental samples

Mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) is an economically and ecologically important pest of pine species in western North America. Mountain pine beetles form complex multipartite relationships with microbial partners, including the ophiostomoid fungi Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield, Ophiostoma montium (Rumbold) von Arx, Grosmannia aurea (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield, Leptographium longiclavatum (Lee, Kim, and Breuil) and Leptographium terebrantis (Barras and Perry). These fungi are vectored by MPB to new pine hosts, where the fungi overcome host defenses to grow into the sapwood. A tree's relative susceptibility to these fungi is conventionally assessed by measuring lesions that develop in response to fungal inoculation. However, these lesions represent a symptom of infection, representing both fungal growth and tree defense capacity. In order to more objectively assess fungal virulence and host tree susceptibility in studies of host-pathogen interactions, a reliable, consistent, sensitive method is required to accurately identify and quantify MPB-associated fungal symbionts in planta. We have adapted RNase H2-dependent PCR, a technique originally designed for rare allele discrimination, to develop a novel RNase H2-dependent quantitative PCR (rh-qPCR) assay that shows greater specificity and sensitivity than previously published PCR-based methods to quantify MPB fungal symbionts in pine xylem and MPB whole beetles. Two sets of assay probes were designed: one that amplifies a broad range of ophiostomoid species, and a second that amplifies G. clavigera but not other MPB-associated ophiostomoid species. Using these primers to quantify G. clavigera in pine stems, we provide evidence that lesion length does not accurately reflect the extent of fungal colonization along the stem nor the quantity of fungal growth within this colonized portion of stem. The sensitivity, specificity, reproducibility, cost effectiveness and high-throughput potential of the rh-qPCR assay makes the technology suitable for identification and quantification of a wide array of pathogenic and beneficial microbes that form associations with plants and other organisms, even when the microbial partner is present in low abundance.

Does cryptic microbiota mitigate pine resistance to an invasive beetle-fungus complex? Implications for invasion potential

Microbial symbionts are known to assist exotic pests in their colonization of new host plants. However, there has been little evidence linking symbiotic invasion success to mechanisms for mitigation of native plant resistance. The red turpentine beetle (RTB) was introduced with a fungus, Leptographium procerum, to China from the United States and became a destructively invasive symbiotic complex in natural Pinus tabuliformis forests. Here, we report that three Chinese-resident fungi, newly acquired by RTB in China, induce high levels of a phenolic defensive chemical, naringenin, in pines. This invasive beetle-fungus complex is suppressed by elevated levels of naringenin. However, cryptic microbiotas in RTB galleries strongly degrade naringenin, and pinitol, the main soluble carbohydrate of P. tabuliformis, is retained in L. procerum-infected phloem and facilitate naringenin biodegradation by the microbiotas. These results demonstrate that cryptic microbiota mitigates native host plant phenolic resistance to an invasive symbiotic complex, suggesting a putative mechanism for reduced biotic resistance to symbiotic invasion.

Inducible pine rosin defense mediates interactions between an invasive insect-fungal complex and newly acquired sympatric fungal associates

Mutualism between insects and fungi drives insect evolutionary diversification and niche expansion; for invasive insects, however, mechanisms by which they maintain mutualistic relationships with beneficial fungi have not been clearly explored. Here, we report that an invasive herbivorous insect, the red turpentine beetle (RTB), with its co-invasive mutualistic fungus, Leptographium procerum, has newly acquired a set of sympatric fungi during invasion, which could potentially outcompete the RTB mutualistic fungus. Host pine Pinus tabuliformis exhibited more rosin-based responses to the sympatric fungi than to RTB mutualistic fungus and, in return, the rapidly induced rosin suppressed the sympatric fungi more significantly than L. procerum. In addition, from direct fungal pairing competitions, we found that the antagonistic effects of sympatric fungi on L. procerum were drastically reduced under induced rosin defense. Our results together with previous findings imply that pine oleoresin defense (turpentine and rosin) might have been exploited by the invasive mutualistic fungus L. procerum, which helps to explain its invasion success and, by extension, its mutualistic partner RTB in China.

Altitudinal Barrier to the Spread of an Invasive Species: Could the Pyrenean Chain Slow the Natural Spread of the Pinewood Nematode?

Mountain ranges may delimit the distribution of native species as well as constitute potential barriers to the spread of invasive species. The invasive pinewood nematode, Bursaphelenchus xylophilus, is a severe forest pest inducing pine wilt disease. It is vectored in Europe by a native long-horned beetle, Monochamus galloprovincialis. This study explored the potential of the Pyrenean chain to slow or prevent the natural spread of nematode-infested beetles from the Iberian Peninsula, where the nematode is established and is expanding its range, towards France and the rest of Europe. An analysis of the genetic structure and migration patterns of the beetle populations throughout the Pyrenean mountain range was combined with a spread model simulating the potential movements of nematode-infested beetles across it. The central part of the Pyrenees, which corresponds to the highest elevation zone, was shown to prevent gene flow between the French and Spanish populations of M. galloprovincialis on each side of the mountains. Conversely, strong admixture was detected between populations located on both sides of low elevation hills, and especially at the east and west extremities of the mountain range. Simulations of the spread of nematode-infested beetles under various thresholds of beetle survival and pine wilt disease expression gave results consistent with the variation in genetic make-up, suggesting that western and eastern hillsides may represent corridors favoring natural spread of the nematode from the Iberian Peninsula to France. Simulations also showed that temperature rise due to climate change may significantly reduce the extent of the barrier formed by highest elevations. Our results support the hypothesis that the Pyrenean chain represents a partial barrier to the natural spread of nematode-infested beetles. These results, which have to be considered together with potential human-assisted long-distance spread of the nematode, highlight priority zones for future pest monitoring and management programs. More generally, such an integrated approach could be used to assess the role of mountain chains in the potential spread of other invasive pests.

IMA Genome-F 3: Draft genomes of Amanita jacksonii, Ceratocystis albifundus, Fusarium circinatum, Huntiella omanensis, Leptographium procerum, Rutstroemia sydowiana, and Sclerotinia echinophila

The genomes of fungi provide an important resource to resolve issues pertaining to their taxonomy, biology, and evolution. The genomes of Amanita jacksonii, Ceratocystis albifundus, a Fusarium circinatum variant, Huntiella omanensis, Leptographium procerum, Sclerotinia echinophila, and Rutstroemia sydowiana are presented in this genome announcement. These seven genomes are from a number of fungal pathogens and economically important species. The genome sizes range from 27 Mb in the case of Ceratocystis albifundus to 51.9 Mb for Rutstroemia sydowiana. The latter also encodes for a predicted 17 350 genes, more than double that of Ceratocystis albifundus. These genomes will add to the growing body of knowledge of these fungi and provide a value resource to researchers studying these fungi.

Taxonomy and phylogeny of the Leptographium procerum complex, including Leptographium sinense sp. nov. and Leptographium longiconidiophorum sp. nov

Leptographium procerum (Ophiostomatales, Ascomycota) is a well-known fungal associate of pine root-infesting bark beetles and weevils, occurring in several countries of the world. The fungus is not a primary pathogen but has been associated with white pine root decline in the USA and with serious damage caused by the introduced red turpentine beetle (RTB) Dendroctonus valens in China. Several species closely related to L. procerum have been described during the past decade. The aim of this study was to reevaluate species boundaries in the L. procerum complex using multigene phylogenetic analyses and morphological comparisons. Phylogenetic analyses of seven gene regions (ITS2-LSU, actin, β-tubulin, calmodulin, translation elongation factor 1-α, and the mating type genes MAT1-1-3 and MAT1-2-1) distinguished between nine species in the complex. These included L. procerum, L. bhutanense, L. gracile, L. profanum, L. pini-densiflorae, L. sibiricum, L. sinoprocerum, as well as two new species described here as Leptographium sinense sp. nov. from Hylobitelus xiaoi on Pinus elliottii in China, and Leptographium longiconidiophorum sp. nov. from Pinus densiflora in Japan. Leptographium latens is reduced to synonymy with L. gracile, and an epitype is designated for L. procerum, because a living culture associated with the holotype of L. procerum did not exist. Amplification patterns of the mating type genes suggest that all known species in the L. procerum complex are heterothallic, although sexual states have not been observed for any of the species. The results also suggest that Eastern Asia is most probably the centre of species diversity for the L. procerum complex.

Influence of water deficit on the molecular responses of Pinus contorta × Pinus banksiana mature trees to infection by the mountain pine beetle fungal associate, Grosmannia clavigera

Conifers exhibit a number of constitutive and induced mechanisms to defend against attack by pests and pathogens such as mountain pine beetle (Dendroctonus ponderosae Hopkins) and their fungal associates. Ecological studies have demonstrated that stressed trees are more susceptible to attack by mountain pine beetle than their healthy counterparts. In this study, we tested the hypothesis that water deficit affects constitutive and induced responses of mature lodgepole pine × jack pine hybrids (Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex S. Wats. × Pinus banksiana Lamb.) to inoculation with the mountain pine beetle fungal associate Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield. The degree of stress induced by the imposed water-deficit treatment was sufficient to reduce photosynthesis. Grosmannia clavigera-induced lesions exhibited significantly reduced dimensions in water-deficit trees relative to well-watered trees at 5 weeks after inoculation. Treatment-associated cellular-level changes in secondary phloem were also observed. Quantitative RT-PCR was used to analyze transcript abundance profiles of 18 genes belonging to four families classically associated with biotic and abiotic stress responses: aquaporins (AQPs), dehydration-responsive element binding (DREB), terpene synthases (TPSs) and chitinases (CHIs). Transcript abundance profiles of a TIP2 AQP and a TINY-like DREB decreased significantly in fungus-inoculated trees, but not in response to water deficit. One TPS, Pcb(+)-3-carene synthase, and the Class II CHIs PcbCHI2.1 and PcbCHI2.2 showed increased expression under water-deficit conditions in the absence of fungal inoculation, while another TPS, Pcb(E)-β-farnesene synthase-like, and two CHIs, PcbCHI1.1 and PcbCHI4.1, showed attenuated expression under water-deficit conditions in the presence of fungal inoculation. The effects were observed both locally and systemically. These results demonstrate that both constitutive and induced carbon- and nitrogen-based defenses are affected by water deficit, suggesting potential consequences for mountain pine beetle dynamics, particularly in novel environments.

Yeast diversity associated with invasive Dendroctonus valens killing Pinus tabuliformis in China using culturing and molecular methods

Bark beetle-associated yeasts are much less studied than filamentous fungi, yet they are also considered to play important roles in beetle nutrition, detoxification, and chemical communication. The red turpentine beetle, Dendroctonus valens, an invasive bark beetle introduced from North America, became one of the most destructive pests in China, having killed more than 10 million Pinus tabuliformis as well as other pine species. No investigation of yeasts associated with this bark beetle in its invaded ranges has been conducted so far. The aim of this study was to assess the diversity of yeast communities in different microhabitats and during different developmental stages of Den. valens in China using culturing and denaturing gradient gel electrophoresis (DGGE) approaches and to compare the yeast flora between China and the USA. The yeast identity was confirmed by sequencing the D1/D2 domain of LSU ribosomal DNA (rDNA). In total, 21 species (13 ascomycetes and eight basidiomycetes) were detected by culturing method, and 12 species (11 ascomycetes and one basidiomycetes) were detected by molecular methods from China. The most frequent five species in China were Candida piceae (Ogataea clade), Cyberlindnera americana, Candida oregonensis (Metschnikowia clade), Candida nitratophila (Ogataea clade) and an undescribed Saccharomycopsis sp., detected by both methods. Seven species were exclusively detected by DGGE. Ca. oregonensis (Metschnikowia clade) was the most frequently detected species by DGGE method. Eight species (all were ascomycetes) from the USA were isolated; seven of those were also found in China. We found significant differences in yeast total abundance as well as community composition between different developmental stages and significant differences between the surface and the gut. The frass yeast community was more similar to that of Den. valens surface or larvae than to the community of the gut or adults. Possible functions of the yeast associates are discussed.

A native fungal symbiont facilitates the prevalence and development of an invasive pathogen-native vector symbiosis

Invasive pathogen-insect symbioses have been extensively studied in many different ecological niches. Whether the damage of symbioses in different introduced regions might be influenced by other microorganisms has, however, received little attention. Eight years of field data showed that the varied levels of the nematode and beetle populations and infested trees of the invasive Bursaphelenchus xylophilus--Monochamus alternatus symbiosis were correlated with patterns in the isolation frequencies of ophiostomatoid fungi at six sites, while the laboratory experiments showed that the nematode produced greater numbers of offspring with a female-biased sex ratio and developed faster in the presence of one native symbiotic ophiostomatoid fungus, Sporothrix sp. 1. Diacetone alcohol (DAA) from xylem inoculated with Sporothrix sp. 1 induced B. xylophilus to produce greater numbers of offspring. Its presence also significantly increased the growth and survival rate of M. alternatus, and possibly explains the prevalence of the nematode-vector symbiosis when Sporothrix sp. 1 was dominant in the fungal communities. Studying the means by which multispecies interactions contributed to biogeographical dynamics allowed us to better understand the varied levels of damage caused by biological invasion across the invaded range.

Large shift in symbiont assemblage in the invasive red turpentine beetle

Changes in symbiont assemblages can affect the success and impact of invasive species, and may provide knowledge regarding the invasion histories of their vectors. Bark beetle symbioses are ideal systems to study changes in symbiont assemblages resulting from invasions. The red turpentine beetle (Dendroctonus valens) is a bark beetle species that recently invaded China from its native range in North America. It is associated with ophiostomatalean fungi in both locations, although the fungi have previously been well-surveyed only in China. We surveyed the ophiostomatalean fungi associated with D. valens in eastern and western North America, and identified the fungal species using multi-gene phylogenies. From the 307 collected isolates (147 in eastern North America and 160 in western North America), we identified 20 species: 11 in eastern North America and 13 in western North America. Four species were shared between eastern North America and western North America, one species (Ophiostoma floccosum) was shared between western North America and China, and three species (Grosmannia koreana, Leptographium procerum, and Ophiostoma abietinum) were shared between eastern North America and China. Ophiostoma floccosum and O. abietinum have worldwide distributions, and were rarely isolated from D. valens. However, G. koreana and L. procerum are primarily limited to Asia and North America respectively. Leptographium procerum, which is thought to be native to North America, represented >45% of the symbionts of D. valens in eastern North America and China, suggesting D. valens may have been introduced to China from eastern North America. These results are surprising, as previous population genetics studies on D. valens based on the cytochrome oxidase I gene have suggested that the insect was introduced into China from western North America.

Transcriptome resources and functional characterization of monoterpene synthases for two host species of the mountain pine beetle, lodgepole pine (Pinus contorta) and jack pine (Pinus banksiana)

BACKGROUND

The mountain pine beetle (MPB, Dendroctonus ponderosae) epidemic has affected lodgepole pine (Pinus contorta) across an area of more than 18 million hectares of pine forests in western Canada, and is a threat to the boreal jack pine (Pinus banksiana) forest. Defence of pines against MPB and associated fungal pathogens, as well as other pests, involves oleoresin monoterpenes, which are biosynthesized by families of terpene synthases (TPSs). Volatile monoterpenes also serve as host recognition cues for MPB and as precursors for MPB pheromones. The genes responsible for terpene biosynthesis in jack pine and lodgepole pine were previously unknown.

RESULTS

We report the generation and quality assessment of assembled transcriptome resources for lodgepole pine and jack pine using Sanger, Roche 454, and Illumina sequencing technologies. Assemblies revealed transcripts for approximately 20,000 - 30,000 genes from each species and assembly analyses led to the identification of candidate full-length prenyl transferase, TPS, and P450 genes of oleoresin biosynthesis. We cloned and functionally characterized, via expression of recombinant proteins in E. coli, nine different jack pine and eight different lodgepole pine mono-TPSs. The newly identified lodgepole pine and jack pine mono-TPSs include (+)-α-pinene synthases, (-)-α-pinene synthases, (-)-β-pinene synthases, (+)-3-carene synthases, and (-)-β-phellandrene synthases from each of the two species.

CONCLUSION

In the absence of genome sequences, transcriptome assemblies are important for defence gene discovery in lodgepole pine and jack pine, as demonstrated here for the terpenoid pathway genes. The product profiles of the functionally annotated mono-TPSs described here can account for the major monoterpene metabolites identified in lodgepole pine and jack pine.

Saccharide-mediated antagonistic effects of bark beetle fungal associates on larvae

Bark beetles are among the most destructive of pine forest pests and they form close symbiotic relationships with ophiostomatoid fungi. Although some fungi are considered to be mutualistic symbionts of bark beetles with respect to the supply of nutrients, detrimental effects of fungal symbionts on larval growth have also been frequently reported. The mechanisms of such antagonistic effects are hypothesized to be a decrease in nutritional resources caused by competition for saccharides by the fungi. Here, we provide experimental evidence that three beetle-associated fungi modify the nutritional content of an artificial phloem diet, leading to a detrimental effect on the growth of Dendroctonus valens larvae. When larvae were fed a diet of pine phloem in agar medium colonized with any of these fungi, feeding activity was not affected but weight significantly decreased. Additional analysis showed that fungi depleted the fructose and glucose concentrations in the phloem media. Furthermore, these detrimental effects were neutralized by supplementing the media with fructose or glucose, suggesting that fungi may affect larval growth by modifying diet saccharide contents. These data indicate that fungus-induced nutritional changes in bark beetle diet can affect larval growth, and that the mechanism involves fungus-induced saccharide depletion from the larval diet.

An experimental test of insect-mediated colonisation of damaged Pinus radiata trees by sapstain fungi

Vector-pathogen dynamics play a central role in understanding tree health and forest dynamics. There is substantial evidence that bark beetles act as spore vectors for many species of fungi that cause 'sapstain' discolouration of damaged trees and timber. However, the direct quantitative link between vector-mediated spore dispersal and subsequent sapstain colonisation of wood is not fully understood. Here, we used caged versus uncaged experimental logs to test whether the exclusion of bark beetles quantitatively alters the distribution and intensity of sapstain fungal spread within damaged trees. Using generalised linear mixed models, we tested the effect of bark beetle exclusion on sapstain intensity within and among cut logs at two plantation forest sites. Overall, sapstain was found on all logs regardless of caging treatment, indicating that sapstain colonisation can occur (to some degree) without arthropod vectors, probably via wind, rain-splash and, potentially, latent endophytic development. This was supported by the dominance of Diplodia pinea in fungal isolations taken from trees felled at the site, as this fungal species is known to disperse independently of bark beetles. However, the intensity of sapstain within and among experimental logs was significantly greater in uncaged than in caged logs, where beetle colonisation was significantly greater. This appeared to be driven by a significant within-log association between the intensity of staining and the intensity of beetle, and other arthropod, tunnelling and feeding activities. Taken together, these results strongly suggest that the dominant mechanism underlying the role of bark beetles in sapstain development in this study system is not vector-mediated spore dispersal, per se, but rather the facilitation of spore entry and hyphal development through tunnelling and feeding activities. We discuss the implications of these findings for forest management and the effective salvage-harvest of trees damaged by stochastic climate events such as storm and fire damage.

Red turpentine beetle: innocuous native becomes invasive tree killer in China

The red turpentine beetle (RTB), Dendroctonus valens LeConte (Coleoptera: Curculionidae: Scolytinae), is a secondary pest of pines in its native range in North and Central America. Outbreaks and tree mortality attributed to RTB alone are rare in its native range. RTB was introduced into China in the early 1980s and spread rapidly from Shanxi Province to four adjacent provinces; it has infested over 500,000 ha of pine forest and has caused extensive tree mortality since 1999. We provide a historical background on RTB outbreaks, explanations for its invasive success, management options, and economic impacts of RTB in China. Genetic variation in RTB fungal associates, interactions between RTB and its associated fungi, behavioral differences in Chinese RTB, and other factors favoring RTB outbreaks are considered in an effort to explain the invasiveness of RTB in China. The promise of semiochemicals as a management tool is also discussed.

Does mutualism drive the invasion of two alien species? The case of Solenopsis invicta and Phenacoccus solenopsis

Although mutualism between ants and honeydew-producing hemipterans has been extensively recognized in ecosystem biology, however few attempts to test the hypothesis that mutualism between two alien species leads to the facilitation of the invasion process. To address this problem, we focus on the conditional mutualism between S. invicta and P. solenopsis by field investigations and indoor experiments. In the laboratory, ant colony growth increased significantly when ants had access to P. solenopsis and animal-based food. Honeydew produced by P. solenopsis also improved the survival of ant workers. In the field, colony density of P. solenopsis was significantly greater on plots with ants than on plots without ants. The number of mealybug mummies on plants without fire ants was almost three times that of plants with fire ants, indicating a strong effect of fire ants on mealybug survival. In addition, the presence of S. invicta successfully contributed to the spread of P. solenopsis. The quantity of honeydew consumption by S. invicta was significantly greater than that of a presumptive native ant, Tapinoma melanocephalum. When compared with the case without ant tending, mealybugs tended by ants matured earlier and their lifespan and reproduction increased. T. melanocephalum workers arrived at honeydew more quickly than S. invicta workers, while the number of foraging S. invicta workers on plants steadily increased, eventually exceeding that number of T. melanocephalum foragers. Overall, these results suggest that the conditional mutualism between S. invicta and P. solenopsis facilitates population growth and fitness of both species. S. invicta tends to acquire much more honeydew and drive away native ants, promoting their predominance. These results suggest that the higher foraging tempo of S. invicta may provide more effective protection of P. solenopsis than native ants. Thus mutualism between these two alien species may facilitate the invasion success of both species.

Model analysis for plant disease dynamics co-mediated by herbivory and herbivore-borne phytopathogens

Plants are subject to diseases caused by pathogens, many of which are transmitted by herbivorous arthropod vectors. To understand plant disease dynamics, we studied a minimum hybrid model combining consumer-resource (herbivore-plant) and susceptible-infected models, in which the disease is transmitted bi-directionally between the consumer and the resource from the infected to susceptible classes. Model analysis showed that: (i) the disease is more likely to persist when the herbivore feeds on the susceptible plants rather than the infected plants, and (ii) alternative stable states can exist in which the system converges to either a disease-free or an endemic state, depending on the initial conditions. The second finding is particularly important because it suggests that the disease may persist once established, even though the initial prevalence is low (i.e. the R(0) rule does not always hold). This situation is likely to occur when the infection improves the plant nutritive quality, and the herbivore preferentially feeds on the infected resource (i.e. indirect vector-pathogen mutualism). Our results highlight the importance of the eco-epidemiological perspective that integration of tripartite interactions among host plant, plant pathogen and herbivore vector is crucial for the successful control of plant diseases.

The ratio and concentration of two monoterpenes mediate fecundity of the pinewood nematode and growth of its associated fungi

The pinewood nematode (PWN) Bursaphelenchus xylophilus, vectored primarily by the sawyer beetle, Monochamus alternatus, is an important invasive pest and causal agent of pine wilt disease of Chinese Masson pine, Pinus massoniana. Previous work demonstrated that the ratios and concentrations of α-pinene∶β-pinene differed between healthy trees and those trees containing blue-stain fungus (and M. alternatus pupae). However, the potential influence of the altered monoterpene ratios and concentrations on PWN and associated fungi remained unknown. Our current results show that low concentrations of the monoterpenes within petri dishes reduced PWN propagation, whereas the highest concentration of the monoterpenes increased PWN propagation. The propagation rate of PWN treated with the monoterpene ratio representative of blue-stain infected pine (α-pinene∶β-pinene = 1∶0.8, 137.6 mg/ml) was significantly higher than that (α-pinene∶β-pinene = 1∶0.1, 137.6 mg/ml) representative of healthy pines or those damaged by M. alternatus feeding, but without blue stain. Furthermore, inhibition of mycelial growth of associated fungi increased with the concentration of the monoterpenes α-pinene and β-pinene. Additionally, higher levels of β-pinene (α-pinene∶β-pinene = 1∶0.8) resulted in greater inhibition of the growth of the associated fungi Sporothrix sp.2 and Ophiostoma ips strains, but had no significant effects on the growth of Sporothrix sp.1, which is the best food resource for PWN. These results suggest that host monoterpenes generally reduce the reproduction of PWN. However, PWN utilizes high monoterpene concentrations and native blue-stain fungus Sporothrix sp.1 to improve its own propagation and overcome host resistance, which may provide clues to understanding the ecological mechanisms of PWN's successful invasion.

Associations of Conifer-Infesting Bark Beetles and Fungi in Fennoscandia

Bark beetles (Coleoptera, Scolytinae) have a widespread association with fungi, especially with ophiostomatoid fungi (Ascomycota) that cause blue staining of wood, and in some cases, serious tree diseases. In Fennoscandia, most studies of these fungi have focused on economically important bark beetle species and this is likely to have led to a biased view of the fungal biodiversity in the region. Recently, the associations between fungi and bark beetles in Fennoscandia have been shown to be more diverse than previously thought. Furthermore, they form complex and dynamic associations that are only now beginning to emerge. This review examines the current knowledge of the rather poorly known interactions between bark beetles, fungi and their conifer host trees in Fennoscandia. The diversity of ophiostomatoid species is discussed and the possible factors that influence the assemblages of fungal associates are considered for all species that are known to occur in the region. For many ophiostomatoid species found in Fennoscandia, little or nothing is known regarding their pathogenicity, particularly if they were to be transferred to new environments. We, therefore, draw attention to the possible threats of timber trade and climate change-induced invasions of new habitats by bark beetles and the fungi that can be moved along with them.

Mutual interactions between an invasive bark beetle and its associated fungi

Interactions between invasive insects and their fungal associates have important effects on the behavior, reproductive success, population dynamics and evolution of the organisms involved. The red turpentine beetle (RTB), Dendroctonus valens LeConte (Coleoptera: Scolytinae), an invasive forest pest in China, is closely associated with fungi. By carrying fungi on specialized structures in the exoskeleton, RTB inoculates fungi in the phloem of pines (when females dig galleries for egg laying and when males join them for mating). After eggs hatch, larvae gregariously feed on the phloem colonized by the fungi. We examined the effects of five isolates of RTB associated fungi (two from North America, Leptographium terebrantis and L. procerum, and three from China, Ophiostoma minus, L. sinoprocerum and L. procerum) on larval feeding activity, development and mortality. We also studied the effects of volatile chemicals produced in the beetle hindgut on fungal growth. Ophiostoma minus impaired feeding activity and reduced weight in RTB larvae. Leptographium sinoprocerum, L. terebrantis and L. procerum did not dramatically influence larval feeding and development compared to fungi-free controls. Larval mortality was not influenced by any of the tested fungi. Hindgut volatiles of RTB larvae, verbenol, myrtenol and myrtenal, inhibited growth rate of all the fungi. Our results not only show that D. valens associated fungus, O. minus, can be detrimental to its larvae; but, most importantly, they also show that these notorious beetles have an outstanding adaptive response evidenced by the ability to produce volatiles that inhibit growth of harmful fungus.