Dietary benefits of fungal associates to an eruptive herbivore: potential implications of multiple associates on host population dynamics

We used the mountain pine beetle (Dendroctonus ponderosae Hopkins) and its two fungal associates, Grosmannia clavigera and Ophiostoma montium, to study potential nutritional benefits of fungi to bark beetles. We tested for potential effects of feeding on phloem colonized by fungi on beetle performance in field and laboratory studies. The fungi increased nitrogen levels in the phloem of attacked trees by 40%, indicating that it may be an important source of dietary nitrogen for mountain pine beetles. However, nitrogen levels of phloem inoculated with fungi in the laboratory were similar to uncolonized phloem, indicating that the fungi may redistribute nitrogen from the sapwood to the phloem rather than increase absolute levels of nitrogen. Beetles emerging from attacked trees carrying G. clavigera were larger than beetles carrying O. montium, which in turn were larger than beetles lacking fungi. Results of experimental laboratory studies varied, likely because of differences in the growth and sporulation of fungi under artificial conditions. Results indicate that the two fungi may offer complementary benefits to the mountain pine beetle because larvae preferentially fed on phloem colonized by both fungi together over phloem colonized by one fungus or uncolonized phloem. Teneral adults preemergence fed on spores in pupal chambers when they were produced and consumed little phloem before emerging. Teneral adults mined extensively in the phloem before emerging when spores were not produced in the pupal chamber. Our results provide evidence for a nutritional role of fungi in the diet of bark beetles and show that multiple associates may differentially affect beetle performance, which could have important implications for bark beetle population dynamics.     

In Vitro Interactions Between Yeasts and Bacteria and the Fungal Symbionts of the Mountain Pine Beetle (Dendroctonus ponderosae)

Multi-trophic interactions between prokaryotes, unicellular eukaryotes, and ecologically intertwined metazoans are presumably common in nature, yet rarely described. The mountain pine beetle, Dendroctonus ponderosae, is associated with two filamentous fungi, Grosmannia clavigera and Ophiostoma montium. Other microbes, including yeasts and bacteria, are also present in the phloem, but it is not known whether they interact with the symbiotic fungi or the host beetle. To test whether such interactions occur, we performed a suite of in vitro assays. Overall, relative yield of O. montium grown with microbes isolated from larval galleries was significantly greater than when the fungus was grown alone. Conversely, the yield of G. clavigera grown with these same microbes was less than or equal to when it was grown alone, suggesting that O. montium, and at least some microbes in larval galleries, have a mutualistic or commensal relationship, while G. clavigera and those same microbes have an antagonistic relationship. A bacterium isolated from phloem not colonized by beetles was found to inhibit growth of both G. clavigera and O. montium and appears to be an antagonist to both fungi. Our results suggest that bacteria and yeasts likely influence the distribution of mycangial fungi in the host tree, which, in turn, may affect the fitness of D. ponderosae.     

The impact of phloem nutrients on overwintering mountain pine beetles and their fungal symbionts

In the low nutrient environment of conifer bark, subcortical beetles often carry symbiotic fungi that concentrate nutrients in host tissues. Although bark beetles are known to benefit from these symbioses, whether this is because they survive better in nutrient-rich phloem is unknown. After manipulating phloem nutrition by fertilizing lodgepole pine trees (Pinus contorta Douglas var. latifolia), we found bolts from fertilized trees to contain more living individuals, and especially more pupae and teneral adults than bolts from unfertilized trees at our southern site. At our northern site, we found that a larger proportion of mountain pine beetle (Dendroctonus ponderosae Hopkins) larvae built pupal chambers in bolts from fertilized trees than in bolts from unfertilized trees. The symbiotic fungi of the mountain pine beetle also responded to fertilization. Two mutualistic fungi of bark beetles, Grosmannia clavigera (Rob.-Jeffr. & R. W. Davidson) Zipfel, Z. W. de Beer, & M. J. Wingf. and Leptographium longiclavatum Lee, S., J. J. Kim, & C. Breuil, doubled the nitrogen concentrations near the point of infection in the phloem of fertilized trees. These fungi were less capable of concentrating nitrogen in unfertilized trees. Thus, the fungal symbionts of mountain pine beetle enhance phloem nutrition and likely mediate the beneficial effects of fertilization on the survival and development of mountain pine beetle larvae.     

Target-specific PCR primers can detect and differentiate ophiostomatoid fungi from microbial communities associated with the mountain pine beetle Dendroctonus ponderosae

The aim of this study was to develop DNA probes that could identify the major fungal species associated with mountain pine beetles (MPB). The beetles are closely associated with fungal species that include ophiostomatoid fungi that can be difficult to differentiate morphologically. The most frequently isolated associates are the pine pathogens Grosmannia clavigera and Leptographium longiclavatum, the less pathogenic Ophiostoma montium, and an undescribed Ceratocystiopsis species (Cop. sp.). Because growing, isolating and extracting DNA from fungi vectored by MPB can be time and labour intensive, we designed three rDNA primer sets that specifically amplify short rDNA amplicons from O. montium, Cop. sp. and the pine Leptographium clade. We also designed two primer sets on a gene of unknown function that can differentiate G. clavigera and L. longiclavatum. We tested the primers on 76 fungal isolates that included MPB associates. The primers reliably identified their targets from DNA obtained from pure fungal cultures, pulverized beetles, beetle galleries, and tree phloem inoculated with G. clavigera. The primers will facilitate large-scale work on the ecology of the MPB-fungal-lodgepole pine ecosystem, as well as phytosanitary/quarantine sample screening.     

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.     

Distinguishing Ophiostoma ips and Ophiostoma montium,two bark beetle-associated sapstain fungi

Two synonymous sapstain species, Ophiostoma montium and Ophiostoma ips, which are vectored by Dendroctonus ponderosae and various bark beetles, respectively, were differentiated into separate species using growth and molecular characteristics. Analysis of 32 isolates of the two species from different countries showed that O. ips was able to grow at 35 degrees C while O. montium was not. This growth-based differentiation was strongly supported by sequence data for the internal transcribed spacer (ITS), 5.8S and partial 28S rDNA, and the beta-tubulin genes. The beta-tubulin gene sequence data indicate that the two species can easily be differentiated with a single polymerase chain reaction (PCR) assay.     

Patterns of functional enzyme activity in fungus farming ambrosia beetles

ABSTRACT: INTRODUCTION: In wood-dwelling fungus-farming weevils, the so-called ambrosia beetles, wood in the excavated tunnels is used as a medium for cultivating fungi by the combined action of digging larvae (which create more space for the fungi to grow) and of adults sowing and pruning the fungus. The beetles are obligately dependent on the fungus that provides essential vitamins, amino acids and sterols. However, to what extent microbial enzymes support fungus farming in ambrosia beetles is unknown. Here we measure (i) 13 plant cell-wall degrading enzymes in the fungus garden microbial consortium of the ambrosia beetle Xyleborinus saxesenii, including its primary fungal symbionts, in three compartments of laboratory maintained nests, at different time points after gallery foundation and (ii) four specific enzymes that may be either insect or microbially derived in X. saxesenii adult and larval individuals. RESULTS: We discovered that the activity of cellulases in ambrosia fungus gardens is relatively small compared to the activities of other cellulolytic enzymes. Enzyme activity in all compartments of the garden was mainly directed towards hemicellulose carbohydrates such as xylan, glucomannan and callose. Hemicellulolytic enzyme activity within the brood chamber increased with gallery age, whereas irrespective of the age of the gallery, the highest overall enzyme activity were detected in the gallery dump material expelled by the beetles. Interestingly endo-beta-1,3(4)-glucanase activity capable of callose degradation was identified in whole-body extracts of both larvae and adult X. saxesenii, whereas endo-beta-1,4-xylanase activity was exclusively detected in larvae. CONCLUSION: Similar to closely related fungi associated with bark beetles in phloem, the microbial symbionts of ambrosia beetles do not degrade cellulose. Instead, their enzyme activity is directed mainly towards comparatively more easily accessible hemicellulose components of the ray-parenchyma cells in the wood xylem. Furthermore, the detection of xylanolytic enzymes exclusively in larvae and not in adults indicates that larvae (pre-) digest plant cell wall structures exclusively in larvae (which feed on fungus colonized wood) and not in adults (which feed only on fungi). This implies that in X. saxesenii and likely also in many other ambrosia beetles, adults and larvae do not compete for the same food within their nests - in contrast, larvae increase colony fitness by facilitating enzymatic wood degradation and fungus cultivation.     

THE EVOLUTION OF AGRICULTURE IN BEETLES (CURCULIONIDAE: SCOLYTINAE AND PLATYPODINAE)

Abstract Beetles in the weevil subfamilies Scolytinae and Platypodinae are unusual in that they burrow as adults inside trees for feeding and oviposition. Some of these beetles are known as ambrosia beetles for their obligate mutualisms with asexual fungi—known as ambrosia fungi—that are derived from plant pathogens in the ascomycete group known as the ophiostomatoid fungi. Other beetles in these subfamilies are known as bark beetles and are associated with free‐living, pathogenic ophiostomatoid fungi that facilitate beetle attack of phloem of trees with resin defenses. Using DNA sequences from six genes, including both copies of the nuclear gene encoding enolase, we performed a molecular phylogenetic study of bark and ambrosia beetles across these two subfamilies to establish the rate and direction of changes in life histories and their consequences for diversification. The ambrosia beetle habits have evolved repeatedly and are unreversed. The subfamily Platypodinae is derived from within the Scolytinae, near the tribe Scolytini. Comparison of the molecular branch lengths of ambrosia beetles and ambrosia fungi reveals a strong correlation, which a fungal molecular clock suggests spans 60 to 21 million years. Bark beetles have shifted from ancestral association with conifers to angiosperms and back again several times. Each shift to angiosperms is associated with elevated diversity, whereas the reverse shifts to conifers are associated with lowered diversity. The unusual habit of adult burrowing likely facilitated the diversification of these beetle‐fungus associations, enabling them to use the biomass‐rich resource that trees represent and set the stage for at least one origin of eusociality.     

The enemy of my enemy is still my enemy: competitors add to predator load of a tree‐killing bark beetle

1 The mountain pine beetle Dendroctonus ponderosae is a major tree‐killing bark beetle in North America. We evaluated how the subsequent arrival of a competing bark beetle Ips pini influences the arrival of predators and their impact on both species. 2 The predators Temnochila chlorodia and Enoclerus sphegeus were strongly attracted to pheromones of D. ponderosae. By contrast, Enoclerus lecontei was mostly attracted to I. pini pheromones. The host compound myrcene synergized attraction of both D. ponderosae and E. sphegeus to the pheromone of D. ponderosae. However, it inhibited attraction of both I. pini and E. lecontei to I. pini’s pheromone. 3 Dendroctonus ponderosae were more attracted to trees than logs treated with its pheromones, whereas I. pini were more attracted to logs than trees treated with its pheromones. Some 78% of T. chlorodia were captured at hosts baited with D. ponderosae pheromones, whereas 83% of E. lecontei were captured at hosts baited with I. pini pheromones. We characterized the sequence of arrival to live trees baited with pheromones of D. ponderosae as: D. ponderosae, T. chlorodia, E. sphegeus, I. pini, E. lecontei. 4 Various combinations of I. pini and predators were added to logs colonized by D. ponderosae in the above sequence of arrival observed in live trees baited with D. ponderosae aggregation pheromones. Ips pini reduced D. ponderosae adult brood production. However, the combination of I. pini and E. lecontei did not raise D. ponderosae brood production above that observed with only I. pini present. Similarly, the combination of I. pini and T. chlorodia did not reduce D. ponderosae brood production below that observed with I. pini alone. By contrast, the combination of I. pini, T. chlorodia and E. lecontei caused more brood loss to D. ponderosae than I. pini alone. 5 Enoclerus lecontei did not reduce brood production by T. chlorodia, whereas T. chlorodia substantially reduced brood production by E. lecontei. 6 Secondary bark beetles that exploit the resource created by primary tree‐killing species exert negative effects through both competition and increased predator load. Implications to the population dynamics, ecology and evolution of tree‐killing bark beetles are discussed.     

Extreme ecological stoichiometry of a bark beetle–fungus mutualism

1. Ecological stoichiometry theory was applied to investigate how a consumer contends with an extreme elemental mismatch between its food and its body via symbiotic facilitation. 2. The beetle Dendroctonus brevicomis LeConte develops in bark, a substrate extremely low in nitrogen (N) and phosphorus (P). Its survival there depends on interactions with mutualist and antagonist fungi. 3. This study found that mutualists transfer N and P from sapwood and phloem into bark, where beetles feed, whereas the antagonist moves these elements only to phloem, resulting in starvation of the insect. However, even with mutualists, N and P concentrations remained low in bark, resulting in low N and extremely low P concentrations in the beetle. 4. The N:P ratios found in D. brevicomis larvae were the highest thus far reported for beetles and among the highest for insects and invertebrates. This suggests that the beetle has evolved additional, nutrient‐sparing adaptations.     

Survey and phylogenetic analysis of culturable microbes in the oral secretions of three bark beetle species

In a recent study, we reported a previously undescribed behavior in which a bark beetle exuded oral secretions containing bacteria that have antifungal properties, and hence defend their galleries against pervasive antagonistic Hyphomycete fungi. Actinobacteria, a group known for their antibiotic properties, were the most effective against fungi that invade the spruce beetle galleries. In the present study, we describe the isolation and identification of microorganisms from oral secretions of three bark beetles (Coleoptera: Curculionidae: Scolytinae): the spruce beetle, Dendroctonus rufipennis Kirby, the mountain pine beetle, Dendroctonus ponderosae Hopkins, and the pine engraver, Ips pini Say. Bacteria isolated from these three species span the major bacterial classes α-, β-, and γ-Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria, except for D. ponderosae , which yielded no α-proteobacteria or Bacteroidetes isolates. Spruce beetles and pine engraver beetles had similar numbers of α-proteobacteria isolates, but pine engravers yielded twice as many Bacteroidetes isolates as spruce beetles. In contrast, mountain pine beetles yielded more isolates in the β- and γ-proteobacteria than spruce beetles and pine engravers. The highest percentage of Actinobacteria was obtained from spruce beetles, followed by pine engravers and mountain pine beetles. All of the fungal isolates obtained from the three beetle species were Ascomycetes. The greatest fungal diversity was obtained in spruce beetles, which had nine species, followed by pine engravers with five, and mountain pine beetles with one.     

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.     

Spore dispersal of a resupinate ectomycorrhizal fungus, Tomentella sublilacina, via soil food webs

Patterns of fungal spore dispersal affect gene flow, population structure and fungal community structure. Many Basidiomycota produce resupinate (crust-like) basidiocarps buried in the soil. Although spores are actively discharged, they often do not appear to be well positioned for aerial dispersal. We investigated the potential spore dispersal mechanisms of one exemplar of this growth form, Tomentella sublilacina. It is a widespread ectomycorrhizal fungus that sporulates in the soil organic horizon, can establish from the spore bank shortly after disturbance, but also can be a dominant species in mature forest stands. We investigated whether its spores could be dispersed via spore-based food webs. We examined external surfaces, gut contents and feces from arthropod fungivores (mites, springtails, millipedes, beetles, fly larvae) and arthropod and vertebrate predators (centipedes, salamanders) from on and around T. sublilacina sporocarps. Spore densities were high in the guts of many individuals from all fungivore groups. Centipede gut contents, centipede feces and salamander feces contained undigested invertebrate exoskeletons and many apparently intact spores. DAPI staining of spores from feces of fungivores indicated that 7-73% of spores contained intact nuclei, whereas spores from predators had lower percentages of intact nuclei. The spiny spores often were lodged on invertebrate exoskeletons. To test the viability of spores that had passed through invertebrate guts we used fecal droppings of the millipede Harpaphe haydeniana to successfully inoculate seedlings of Pinus muricata (Bishop pine). These results indicate the potential for T. sublilacina spore dispersal via invertebrates and their predators in soil food webs and might help to explain the widespread distribution of this species. It is likely that this is a general mechanism of dispersal for fungi producing resupinate sporocarps, indicating a need to develop a fuller understanding of the linkages of soil food webs and spore dispersal.     

Kairomonal Responses of Natural Enemies and Associates of the Southern Ips (Coleoptera: Curculionidae: Scolytinae) to Ipsdienol, Ipsenol and Cis-Verbenol

Bark beetle infested pines are an ephemeral habitat utilized by a diverse assemblage of insects. Although many bark beetle insect associates have little or no measurable impact on bark beetle brood production, some reduce brood production by either competing with brood for the limited phloem tissue or by feeding on brood. Several studies have observed synchrony between the colonization of hosts by bark beetles and the arrival of insect associates. Some insect associates mediate synchrony with bark beetle mass attacks with kairomonal responses to bark beetle aggregation pheromones. The objectives of this study were to document the community of Coleoptera associated with the southern Ips (Ips avulsus, Ips calligraphus and Ips grandicollis) and to test the hypothesis that synchrony of insect associates with the southern Ips is mediated by kairomonal responses to aggregation pheromones. A large community of Coleoptera (109 species) was recorded from traps baited with southern Ips pheromones. A significant treatment effect was observed for the guilds of meristem feeders, natural enemies and woodborers. The southern Ips pheromone ipsenol was broadly attractive to meristem feeders, natural enemies and woodborers and in general blends were more attractive than individual compounds. These results demonstrate that a diverse community of Coleoptera is associated with the southern Ips and that several members of this community facilitate synchrony with kairomonal responses to southern Ips aggregation pheromones.     

Competition and Coexistence In a Multi-partner Mutualism: Interactions Between two Fungal Symbionts of the Mountain Pine Beetle In Beetle-attacked Trees

Despite overlap in niches, two fungal symbionts of the mountain pine beetle (Dendroctonus ponderosae), Grosmannia clavigera and Ophiostoma montium, appear to coexist with one another and their bark beetle host in the phloem of trees. We sampled the percent of phloem colonized by fungi four times over 1 year to investigate the nature of the interaction between these two fungi and to determine how changing conditions in the tree (e.g., moisture) affect the interaction. Both fungi colonized phloem at similar rates; however, G. clavigera colonized a disproportionately larger amount of phloem than O. montium considering their relative prevalence in the beetle population. High phloem moisture appeared to inhibit fungal growth shortly after beetle attack; however, by 1 year, low phloem moisture likely inhibited fungal growth and survival. There was no inverse relationship between the percent of phloem colonized by G. clavigera only and O. montium only, which would indicate competition between the species. However, the percent of phloem colonized by G. clavigera and O. montium together decreased after 1 year, while the percent of phloem from which no fungi were isolated increased. A reduction in living fungi in the phloem at this time may have significant impacts on both beetles and fungi. These results indicate that exploitation competition occurred after a year when the two fungi colonized the phloem together, but we found no evidence of strong interference competition. Each species also maintained an exclusive area, which may promote coexistence of species with similar resource use.     

Modeling cold tolerance in the mountain pine beetle, Dendroctonus ponderosae

Cold-induced mortality is a key factor driving mountain pine beetle, Dendroctonus ponderosae, population dynamics. In this species, the supercooling point (SCP) is representative of mortality induced by acute cold exposure. Mountain pine beetle SCP and associated cold-induced mortality fluctuate throughout a generation, with the highest SCPs prior to and following winter. Using observed SCPs of field-collected D. ponderosae larvae throughout the developmental season and associated phloem temperatures, we developed a mechanistic model that describes the SCP distribution of a population as a function of daily changes in the temperature-dependent processes leading to gain and loss of cold tolerance. It is based on the changing proportion of individuals in three states: (1) a non cold-hardened, feeding state, (2) an intermediate state in which insects have ceased feeding, voided their gut content and eliminated as many ice-nucleating agents as possible from the body, and (3) a fully cold-hardened state where insects have accumulated a maximum concentration of cryoprotectants (e.g. glycerol). Shifts in the proportion of individuals in each state occur in response to the driving variables influencing the opposite rates of gain and loss of cold hardening. The level of cold-induced mortality predicted by the model and its relation to extreme winter temperature is in good agreement with a range of field and laboratory observations. Our model predicts that cold tolerance of D. ponderosae varies within a season, among seasons, and among geographic locations depending on local climate. This variability is an emergent property of the model, and has important implications for understanding the insect's response to seasonal fluctuations in temperature, as well as population response to climate change. Because cold-induced mortality is but one of several major influences of climate on D. ponderosae population dynamics, we suggest that this model be integrated with others simulating the insect's biology.     

Gut microbiota of dung beetles correspond to dietary specializations of adults and larvae

Vertebrate dung is central to the dung beetle life cycle, constituting food for adults and a protective and nutritive refuge for their offspring. Adult dung beetles have soft mandibles and feed primarily on nutritionally rich dung particles, while larvae have sclerotized mandibles and consume coarser dung particles with a higher C/N ratio. Here, using the dung beetles Euoniticellus intermedius and E. triangulatus, we show that these morphological adaptations in mandibular structure are also correlated with differences in basic gut structure and gut bacterial communities between dung beetle life stages. Metagenome functional predictions based on 16S rDNA characterization further indicated that larval gut communities are enriched in genes involved in cellulose degradation and nitrogen fixation compared to adult guts. Larval gut communities are more similar to female gut communities than they are to those of males, and bacteria present in maternally provisioned brood balls and maternal ‘gifts’ (secretions deposited in the brood ball along with the egg) are also more similar to larval gut communities than to those of males. Maternal secretions and maternally provisioned brood balls, as well as dung, were important factors shaping the larval gut community. Differences between gut microbiota in the adults and larvae are likely to contribute to differences in nutrient assimilation from ingested dung at different life history stages.     

A major symbiont shift supports a major niche shift in a clade of tree-killing bark beetles

1. One small clade of bark beetles, out of thousands of species worldwide, has shifted from using phloem to using a combination of phloem and outer bark, or to completely using outer bark. 2. The shift to outer bark has been accompanied by a shift to non-typical bark beetle mutualist fungi in Entomocorticium (Basidiomycota) and Ceratocystiopsis (Ascomycota). 3. This study compared the growth and metabolic capabilities of fungi associated with a nearly phloem-independent species, Dendroctonus brevicomis, with those of mutualist fungi of Dendroctonus ponderosae, a completely phloem-colonising beetle in a sister clade associated with more typical Ascomycota in Grosmannia and Ophiostoma. 4. Only the basidiomycete associated with D. brevicomis could degrade cellulose and lignin, whereas both the ascomycete and basidiomycete could grow in outer bark. Ascomycetes associated with D. ponderosae could not degrade cellulose or lignin or grow in outer bark. 5. Beetles and fungi in these mutualisms may best be considered as co-niche constructors. For niche construction, one organism must modify a resource in a way that enhances its fitness while also influencing fitness of other organisms using the resource. Here, the beetles kill the tree, transport the fungi into the tree, and modify the woody substrate for use by the fungi. They have also evolved mycangia to ensure vertical dissemination of the fungi. In turn, the fungi modify tree tissues and provision nutrients to the host and have evolved traits that support their acquisition and transport by the beetle host in mycangia.     

The effect of larval aggregation behaviour on larval growth of the spruce bark beetle Dendroctonus micans

1. The great spruce bark beetle Dendroctonus micans is a primary pest of spruce in Europe. It is unusual among Eurasian scolytids in that apparently healthy trees are attacked by solitary adults, but larvae feed en masse , in response to a larval aggregation pheromone.
2. The effect of brood size on larval growth was determined in experiments on detached bark, logs and living trees. A positive relationship was found between brood size and larval growth in experiments started with either eggs or larvae up to fifth instar. The relationship appeared to be independent of the effects of both preformed and induced defences in bark.
3. No evidence was found to support the hypothesis that larvae feeding in groups spend a greater proportion of time feeding. Addition of resin to egg chambers in detached bark pieces resulted in high mortality of first-instar larvae. A possible role for larval aggregation in minimizing the effects of host defences is discussed.     

Evolutionary radiation of an inbreeding haplodiploid beetle lineage (Curculionidae, Scolytinae)

Haplodiploidy is a highly unusual genetic system that has arisen at least 17 times in animals of varying lifestyles, but most of these haplodiploid lineages remain relatively poorly known. In particular, the ecological and genetic circumstances under which haplodiploidy originates have been difficult to resolve. A recent molecular‐phylogenetic study has resolved the phylogenetic position of the haplodiploid clade of scolytine beetles as the sister group of the genus Dryocoetes. Haplodiploid bark beetles are remarkable in that the entire clade of over 1300 species are apparently extreme (sib‐mating) inbreeders, most of which cultivate fungi for food while some attack phloem, twigs or seeds. Here we present a much more detailed molecular‐phylogenetic study of this clade. Using partial sequences of elongation factor 1‐alpha and the mitochondrial small ribosomal subunit (12S), we reconstructed the phylogeny for 48 taxa within the haplodiploid clade, as well as two species of the diplodiploid sister genus Dryocoetes. Results indicate that the genus Ozopemon is the basal lineage of die haplodiploid clade. Since Ozopemon, Dryocoetes, and other outgroups are phloem‐feeding, this strongly suggest that haplodiploidy and inbreeding evolved in a phloem feeding ancestor. Following the divergence of Ozopemon there is a series of extremely short internodes near the base of the clade, suggesting a very rapid rate of diversification in early Miocene (based on fossil evidence and sequence divergence). Among the many substrates for breeding and food resources utilized within this species‐rich clade, the cultivation of yeast‐like ambrosia fungi in tunnels deep into the wood predominates (nearly 90% of the species). The number of transitions to feeding on such fungi was few, possibly only one, and is perhaps an irreversible transition. The habit of feeding on fungi cultured in xylem makes it possible for the beetles to use a great variety of plant taxa. This extreme resource generalism, in conjunction with the colonization advantage conferred by haplodiploidy and inbreeding, may have promoted the rapid diversification of this clade.