Bark Beetles and their Ophiostomatoid Fungal Symbionts. The family Scolytidae (subfamily Scolytinae) includes ips, ambrosia, and bark beetles, some of

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1 Meg Dudley BSPM April 2013 Bark Beetles and their Ophiostomatoid Fungal Symbionts Abstract The family Scolytidae (subfamily Scolytinae) includes ips, ambrosia, and bark beetles, some of which are considered major pests to high-value trees. Many of the more than 400 species within the Scolytinae subfamily include fungal parasites or symbionts. Often these fungi are of the Order Ophistomatales or Ophiostomatoid fungi. Many Ophiostomatoid (ascomycete) fungi are phytopathogenic, and are the cause a variety of vascular wilt diseases in their hosts. The relationships between fungi and bark beetles range from commensalistic to mutualistic, and can be found throughout the world. In North America, the two fungal species responsible for Dutch elm disease, Ophiostoma ulmi (Buisman) Nannfeldt and Ophiostoma novo-ulmi Brasier (both of which were introduced from Europe), are transmitted by the native elm bark beetle (Hylurgopinus rufipes (Eichoff)) and by two invasive beetles, the European elm bark beetle (Scolytus multistriatus (Marsham)) and the banded elm bark beetle (Scolytus schevyrewi Semenov). In this mutualistic relationship, the fungi are vectored to new hosts, and the beetles gain easier access to new hosts weakened by the fungal pathogen. The western balsam bark beetle, Dryocoetes confusus Swaine, along with its canker-causing fungal associate, Ophiostoma dryocoetidis (Kendrick and Molnar) de Hoog & R.J. Scheff, are found on subalpine fir (Abies lasiocarpa (Hook) Nutt.). Dryocoetes confusus possesses mandibular mycangia, in which it transports spores or mycelia of O. dryocoetidis. Both the beetle and the fungus are independently lethal to the host; trees which resist beetle attack are often killed by canker formation. The 1 P a g e

2 relationship between Ips pini (Say) and its fungal counterpart, Ophiostoma ips (Rumbold) Nannfeldt, is generally mutualistic, though with a caveat; studies indicate that trees inoculated with O. ips tend to be less heavily colonized by I. pini, but fungal presence has a positive effect on brood size. Further, O. pini appears to grow faster in trees that have not yet been colonized by I. pini. Another well-known beetle-fungus interaction is the mutualistic relationship between the Mountain Pine Beetle (Dendroctonus ponderosae Hopkins) and three fungi, Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel de Beer and Wingf., Leptographium longiclavatum Lee, S., J.J. Kim & C. Breuil, and Ophiostoma montium (Rumbold) von Arx. Of the fungi associated with D. ponderosae, G. clavigera is the most phytopathogenic, and infection appears to increase MPB brood fitness. The interactions described here illustrate the widespread and diverse relationships between members of Scolytinae and Ophistomatales. Introduction The bark beetles (i.e. the beetles of the subfamily Scolytinae (Curculionidae)) represent more than 6,000 described species worldwide, and, along with ambrosia beetles (Platypodinae) are the most destructive insect pests of the world s forests (Wood, 1982). In his seminal 1982 monograph of the Scolytidae of the Americas, S.L. Wood notes that these phloeophagus (i.e. phloem-feeding) insects fulfill specific ecological niches. Very often, a bark beetle species feeds on a few or just a single host species, and frequently on specific parts of the host tree (Wood, 1982). Lifecycle details naturally vary by species, but in general, the newly-emerged Scolytinae adults search for a new host tree through the detection of host volatile compounds (e.g. oleoresins, alcohols) and directed flight (Wood, 1982). Depending on the species, the adults may then feed on green phloem for a time, or seek a new brood tree in which to mate and lay eggs (Wood, 1982). The so-called pioneers, or those beetles which initially colonize a suitable host 2 P a g e

3 tree, and begin to emit aggregation pheromones to attract other members of its species (Wood, 1982). This sets the stage for a new generation of beetles. Common fungal associates of bark and ambrosia beetles are various members of the order Ophistomatales, or the Ophiostomatoid fungi (Kendrick). Many phytopathogenic species within Ophistomatales cause vascular wilt diseases in woody plants, and in some cases, canker formation. Some common genera include Ophiostoma, Ceratocystis, Leptographium, and Chalara, which are the causal agents of Dutch Elm Disease (O. novo-ulmi), oak wilt (C. fagacearum), black stain root disease (L. wageneri), and European ash decline (C. fraxinea) (Kendrick; Kowalski and Holdenrieder, 2009). Species within the order Ophistomatales are frequently found in bark beetle tunnels and galleries, and produce erect spore-bearing structures (i.e. ascomata) which support a droplet of spores and liquid (Kendrick). Any passing beetle will pick up the spore mass and transport it to a new host (Kendrick). Bark beetles can transport their fungal associates either in specialized structures within the exoskeleton called mycangia, or in pits or other grooves on the elytra or thorax (Six and Wingfield, 2011). In general, the association between bark beetles and their ophiostomatoid fungal counterparts is commensalistic, if not mutualistic; the fungus often provides nutritional support, or favorably alters host tissues to enhance beetle survival (Bleiker et al. 2009). In return, the beetle transports the fungus to new hosts, which fungus could not efficiently do on its own (Whitney, 1971; Bleiker, et al. 2009). I. Scolytus multistriatus, Scolytus schevyrewi, and Ophiostoma ulmi, O. novo-ulmi Populations of American and European elm species (Ulmus spp.) have been largely decimated over the past century, due to the spread of a vascular wilt fungus and its bark beetle vectors (Agrios, 530). The original pathogen responsible for Dutch Elm Disease was the ascomycetous fungus Ophiostoma ulmi (Agrios 530). The disease was named Dutch elm disease because it was 3 P a g e

4 first described in the Netherlands in 1921, although records indicate that it was observed in France in 1917 (Agrios, 530). This species (and its close relative, O. novo-ulmi) is native to Asia and the Middle East (D Arcy, 2000; Negron et al. 2005). DED was first introduced into North America in the 1920s, when woodworkers along the east coast of the US imported infested European elm logs (D Arcy, 2000). Since the mid-twentieth century, O. ulmi has gradually been replaced by a much more aggressive species, O. novo-ulmi (D Arcy, 2000). Both species produce whitish mycelium and conidia (asexual spores) and ascospores. These pathogens gain entry to new hosts by bark beetle feeding wounds, or by root grafts between diseased and healthy trees (D Arcy, 2000). The disease cycle begins with the growth of the fungus into the xylem vessels, where the asexual, conidial spores are produced on branching hyphae, thus producing secondary infection within the host (D Arcy, 2000). Once the tree has died or is in decline, ascospores may be produced, usually within beetle galleries (D Arcy, 2000). The sticky spores are produced on long stalks, called synnema, which are readily dispersed when adult bark beetles pass through the gallery tunnels (D Arcy, 2000). There are three beetle vectors of these pathogens; the native elm bark beetle (Hylurgopinus rufipes), the European elm bark beetle (Scolytus multistriatus), and the banded elm bark beetle (Scolytus schevyrewi) (Agrios, 531; Negron et al. 2005). Of these beetles, the two invasive species, Scolytus multistriatus and S. schevyrewi, are the more efficient vectors of the DED pathogens (Negron et al. 2005). Scolytus multistriatus was first introduced into the United States in 1909, and until the introduction of S. schevyrewi, was the main vector of Ophiostoma ulmi and O. novo-ulmi (Negron et al. 2005). Recent trapping surveys indicate that S. schevyrewi has displaced S. multistriatus in Colorado, Wyoming, and Utah (Lee et al. 2009). Though feeding by S. schevyrewi alone may be sufficient to kill its host, a study showed that the two Scolytus 4 P a g e

5 species are equally efficient vectors of O. novo-ulmi (Negron et al. 2005; Jacobi et al. 2013). The lifecycles of these beetle species is similar: young adults feed on phloem tissue of small diameter twig crotches, high in the tree canopy (Agrios, 532). It is during this initial feeding that the thousands of O. novo-ulmi spores the beetles carry adhere to and spread through the damaged host tissue (Agrios, 532). Later, the beetles locate dead or dying elm and excavate brood tunnels and reproduce in the trunk of the tree (Agrios, 532). This interaction, between a fungal pathogen (Ophiostoma novo-ulmi) and a bark beetle associate (Scolytus multistriatus and S. schevyrewi), can be arguably described as a mutualistic relationship; the fungus is carried to new hosts that may not have been accessible through a root graft, and the bark beetles utilize sickened elms as brood trees. Although there are certainly other disease and damage agents that weaken elm trees (e.g., Verticillium fungus), given its virulence, Ophiostoma novo-ulmi should be considered foremost among them. II. Dryocoetes confusus and Ophiostoma dryocoetidis Of the seven species of Dryocoetes bark beetles that occur in North America, D. confusus arguably the most destructive (Negron and Popp, 2009). This bark beetle, which possesses mandibular mycangia, is primarily found on subalpine fir (Abies lasiocarpa) from western Canada to the American southwest (Farris, 1969; Wood, 1982). The western balsam bark beetle, as D. confusus is commonly known, can also be found on red fir (Abies magnifica A. Murray bis), white fir (Abies concolor (Gord. & Glend.) Lindl ex Hildebr.) and Engelmann spruce (Picea engelmannii Parry ex Engelm.) (Wood, 1982). In general, this bark beetle targets host trees weakened by disease or other factors, and first attacks one side of the bole, with successive generations girdling the tree over time (Wood, 1982). In Colorado, adults emerge in early June, with peak flights occurring in early to mid July (Negron and Popp, 2009). Dryocoetes confusus 5 P a g e

6 has a two-year life cycle, and spends its first winter as a larva, and second as an adult (Negron and Popp, 2009). Data also suggest that this species utilizes more than one brood tree, and may produce a second cohort of eggs in a single season (Negron and Popp, 2009). Molnar and Kendrick (1965) were the first to describe the fungus now known as Ophiostoma dryocoetidis, the fungal counterpart to Dryocoetes confusus. A subsequent study by Molnar (1965) showed that O. dryocoetidis formed necrotic lesions in host tissue surrounding beetle entrance holes. Among trees that had successfully fended off beetle attack but were infested with the fungus, either lesions coalesced and girdled the tree, killing it, or the fungal growth was restricted (by hypersensitive response) (Molnar, 1965). More recently, Bleiker and Uzunovic (2004) showed that fast-growing subalpine fir trees responded more quickly to beetle-fungus attacks through the hypersensitive response, and that slow-growing trees responded more slowly to the same treatment. The literature on this beetle-fungus pair is somewhat scarce relative to other insect-microbe systems, however this interaction seems to be a commensalistic one; the spores or mycelia O. dryocoetidis are transported in the mandibular mycangia of D. confusus to new host trees with no apparent harm to the beetle (Farris, 1969). III. Ips pini and Ophiostoma ips The pine engraver beetle (Ips pini) has the widest distribution of all bark beetles in North America, ranging from Alaska south to the state of Chihuahua in Mexico (Wood, 1982). Pine hosts of this beetle include jack pine (Pinus banksiana Lamb.), lodgepole pine (Pinus contorta Douglas ex Loudon), ponderosa pine (Pinus ponderosae Lawson & C. Lawson), and eastern white pine (Pinus strobus L.) (Wood, 1982). This insect is known to kill immature lodgepole and ponderosa pines through mass attack (Furniss et al. 1995). Two generations are produced per 6 P a g e

7 year; the first generation emerges in early spring and attacks downed trees or logging slash, and the second generation emerges in summer and attacks standing trees (Gast et al. 1993). The fungus Ophiostoma ips was first described by Leach et al. (1934) after the observation that a bluestain fungus was present on all pine boles exposed to attack by Ips pini and absent on all boles lacking attack by I. pini. Like many other fungi of the Ophiostoma genus, growth of O. ips mycelium through the host vascular system causes occlusion and tyloses, known as vascular wilt (Agrios, 528). Unlike other beetle-fungus relationships, the pairing of I. pini and O. ips is not strictly a beneficial one for the beetle (Kopper et al. 2004). According to Kopper et al. (2004), in the presence of O. ips, fewer female I. pini colonized pine logs, which as the study authors note, indicates that the beetles may be able to detect whether the fungus is already present in a potential host tree. There is no known anti-aggregation pheromone produced by I. pini, and thus the beetles may be detecting fungal metabolites emitted from the potential host (Kopper et al. 2004). However, when beetles were bred in logs already infected with O. ips, brood emergence was greater, and the length of time that the logs were usable to the beetles for breeding was extended (Kopper et al. 2004). Kopper et al. (2004) theorize that the fungus may maintain higher moisture and nutrient levels than would be present in the logs without fungal colonization. Thus, it is difficult to categorize the relationship between these two organisms, although one could argue that since the negative influence of decreased host colonization is neutralized by the positive influences on brood emergence and environmental suitability, thus making this association commensalistic in nature. 7 P a g e

8 IV. Dendroctonus ponderosae, Grosmannia clavigera and Ophiostoma montium The Mountain Pine Beetle (Dendroctonus ponderosae) is the organism responsible for the current massive mortality of many pines species across the western U.S. and Canada (MT DNRC). This insect feeds on a variety of pine species, including native (e.g. Pinus contorta, P. ponderosae, P. flexilis) and non-native trees (Pinus sylvestris) (MT DNRC). Six and Wingfield (2011) categorize host infestation by mountain pine beetle (MPB) into three distinct phases: dispersal, attack, and colonization. During the dispersal phase, adult beetles with their fungal symbionts (located in mycangia or on the exoskeleton) fly in search of a new host tree (Six and Wingfield, 2011). The attack phase begins when the pioneer beetle (i.e. the initial individual to arrive at the new host) selects a tree host and begins to burrow through the bark (Six and Wingfield, 2011). The pioneer then releases aggregation pheromones, which attract male and female D. ponderosae (Six and Wingfield, 2011). More beetles arrive and produce more aggregation pheromone, resulting in mass attack of the tree, which generally occurs over a period 2-5 days (Six and Wingfield, 2011). Once the host defenses have been overwhelmed, the beetles produce an anti-aggregation pheromone, to discourage overexploitation of the food source (Six and Wingfield, 2011). This marks the beginning of the third phase, colonization, as the adults excavate brood galleries and lay eggs, effectively inoculating the host with the phytopathogenic fungi (Six and Wingfield, 2011). Upon emergence from the egg mass, larvae burrow tunnels throughout the tree, and eventually form pupae in pupal chambers (Six and Wingfield, 2011). As the new adults (i.e. teneral adults) emerge, they feed upon spores produced by their fungal symbionts (Six and Wingfield, 2011). D. ponderosae is associated with several species of Ophiostomatoid fungi (Khadempour et al. 2012; Goodsman et al. 2012; Kim et al. 2005). Lee et al. (2012) reported the isolation (from both 8 P a g e

9 beetles and gallery host tissue) of over one thousand individual fungal colonies belonging to nine separate species. The two most common fungal species associated with MPB are Grosmannia clavigera and Ophiostoma montium (Lee et al. 2005). Grosmannia clavigera is the most aggressive fungal symbiont found on Dendroctonus ponderosae beetles, larvae, and gallery tissue (Lee et al. 2006). Upon gaining entry to a new host tree, this fungus grows through phloem and xylem tissues, causing occlusion of the vessels as it colonizes the host (Yamaoka et al. 1995). The virulence of this phytopathogen is thought to be in part due to its ability to utilize monoterpenes, a major chemical defense in trees, as its sole carbon source (DiGuistini et al. 2011). The second common fungal associate of MPB, O. montium, is generally slower to colonize it host, but, like G. clavigera, causes vascular wilt as its hyphae spread into host tissues (Yamaoka et al. 1995). Both species are so-called blue stain fungi due to the blue-grey streaking of the host sapwood, which is a result of melanin production by the fungus (Yamaoka et al. 1995). The relative abundance of these two Ophiostomatoid species varies with temperature and time (Six and Bentz, 2007). G. clavigera is more abundant during cooler periods and in association with third- and fourth- instar larvae, and O. montium is more abundant during warmer periods and in association with eggs, young larvae, and newly-emerged adults (Six and Bentz, 2007). As daily maximum temperatures approach 25 C, the abundance of G. clavigera decreases greatly, with O. montium becoming the dominant symbiont at temperatures ranging from C (Six and Bentz, 2007). Six and Bentz (2007) theorize that G. clavigera is most abundant early in MPB infestation due to both its virulence and its tolerance of low oxygen conditions, which exist within living host tissues. Conversely, O. montium is weakly virulent and requires elevated oxygen, as is the condition of dying host tissues (Six and Bentz, 2007). 9 P a g e

10 This relationship, between MPB and G. clavigera and O. montium, is clearly a mutualistic one; as is the case with other bark beetle-fungus symbioses, the blue stain fungi are transported to host trees via the mycangia or on the exoskeleton of their MPB associates, and the beetles are rewarded with a nutritious food source in the form of fungal hyphae and spores (Six and Wingfield, 2011). Adams and Six (2007) observed that mid- and late-instar MPB larvae frequently stop tunneling into fresh phloem, and return to older portions of the galleries where the fungal symbiont is present. This is presumably to feed on hyphae and spores, and to reinoculate fresh phloem tissues upon their return to the tips of the feeding galleries (Adams and Six, 2007). Goodsman et al. (2012) found that G. clavigera and O. montium act as nitrogen sources for MPB in various stages of development, and that nitrogen levels were highest in spores and hyphae within or near MPB galleries. Conclusion This review offers a few examples of the myriad of relationship types that exist between bark beetles and their fungal symbionts. Wood (1982) writes that it is very probable that all of the more than six thousand described species of Scolytinae (i.e. bark beetles) have some sort of relationship with one or more fungal species. Such relationships, Wood (1982) notes, range from the most casual, or perhaps accidental, contact to intimate mutualistic bonds in which neither the fungus nor the beetle could survive without the other. (Pg. 23). 10 P a g e

11 References Cited Agrios, George N. "Plant Diseases Caused By Fungi." Plant Pathology. 5th ed. London: Elsevier Academic, Print. Bleiker, K.P., Potter, S.E., Lauzon, C.R., and D.L. Six Transport of fungal symbionts by Mountain Pine Beetles. Can Entomol 141: D Arcy,C.J Dutch elm disease. The Plant Health Instructor. DOI: /PHI-I Updated DiGuistini, S., Wang, Y., Liao, N.Y. et al Genome and transcriptome analyses of the mountain pine beetle-symbiont Grosmannia clavigera, a lodgepole pine pathogen. P Natl Acad Sci USA 108: Farris, S.H Occurrence of mycangia in the bark beetle Dryocoetes confusus (Coleoptera: Scolytidae). Can Entomol 101: Furniss, M.M., Harvey, A.E., and H. Solheim Transmission of Ophiostoma ips (Ophiostomatales: Ophiostomataceae) by Ips pini (Coleoptera: Scolytidae) to ponderosa pine in Idaho. An Entomol Soc Am 88: Gast, S.J., Stock, M.W., and M.M. Furniss Physiological factors affecting attractions of Ips pini (Coleoptera: Scolytidae) to host odor or male pheromone in Idaho. An Entomol Soc Am 86: Goodsman, D.W., Erblingen, N., and V.J. Lieffers The impact of phloem nutrients on overwintering mountain pine beetles and their fungal symbionts. Env Entomol 41: Jacobi, W.R., Koski, R.D., and J. F. Negron Dutch Elm Disease pathogen transmission by the Banded Elm Bark Beetle, Scolytus schevyrewi. For Path (in press). Kendrick, B. Order Ophiostomatales Class Sordariomycetes. The Fifth Kingdom. Sydneyby-the-Sea, B.C., Canada. Accessed Kendrick, W.B., and A.C. Molnar Ceratocystis dryocoetidis n. sp. associated with the bark beetle Dryocoetes confusus Sw. Can J Botany 43: Khadempour, L., LeMay, V., Jack, D., Bohlmann, J., and C. Breuil The relative abundance of mountain pine beetle fungal associates through the beetle life cycle in pine trees. Fung Mycol 64: P a g e

12 Kim, J., Allen, E.A., Humble, L.M., and C. Breuil Ophiostomatoid and basidiomycetous fungi associated with green, red, and grey lodgepole pines after mountain pine beetle (Dendroctonus poderosae) infestation. Can J Forest Res 35: Kopper, B.J., Klepzig, K.D., and K.F. Raffa Components of antagonism and mutualism in Ips pini-fungal interactions: relationship to a life history of colonizing highly stressed and dead trees. Env Entomol 33: Kowalski, T. & Holdenrieder, O., Pathogenicity of Chalara fraxinea. For Path 39(1), pp Lee, J.C., Aguayo, I., Aslin, R., Durham, G., Hamud, S.M., Moltzan, B.D., Munson, A.S., Negron, J.F., Peterson, T., Ragenovich, I.R., Witcosky, J.J., and S.J. Seybold Cooccurrence of the invasive banded and European bark beetles (Coleoptera: Scolytidae) in North America. Ann Entomol Soc Amer 102: Lee, S., Kim, J., and C. Breuil Diversity of fungi associated with the mountain pine beetle, Dendroctonus ponderosae and infested lodgepole pines in British Columbia. Fungal Divers 22: Leach, J.G., Orr, L.W., and C. Christensen The interrelationships of bark beetles and bluestaining fungi in felled Norway pine timber. J Agric Res 49: Lim, Y.W., Kim, J.J., Lu, M., and C. Breuil Determining fungal diversity on Dendroctonus ponderosae and Ips pini affecting lodgepole pine using cultural and molecular methods. Fung Div 19: Molnar, A.C Pathogenic fungi associated with a bark beetle on alpine fir. Can J Bot 43: Montana Department of Natural Resources and Conservation: Forestry Division. Mountain Pine Beetle. Helena, MT. Accessed Negron, J.F., Witcosky, J.J., Cain, R.J., LaBonte, J.R., Duerr, D.A. II, McElwey, S.J., Lee, J.C., and S.J. Seybold The banded elm bark beetle: a new threat to elms in north America. Am Entomol 51: Negron, J.F. and J. B. Popp The flight periodicity, attack patterns, and life history of Dryocoetes confusus Swaine (Coleoptera: Curculionidae: Scolytinae), the western balsam bark beetle, in north central Colorado. West N M Naturalist 69: Pain, T.D., Raffa, K.F. and T.C. Harrington Interactions among Scolytid bark beetles, their associated fungi, and live host conifers. Ann Rev Entomol 42: P a g e

13 Reid, R.W Moisture changes in lodgepole pine before and after attack by the mountain pine beetle. Forest Chron 37: Six, D.L. and B.J. Bentz Temperature determines symbiont abundance in a multipartite bark beetle-fungus ectosymbiosis. Microbial Ecol 54: Six, D.L. and M.J. Wingfield Phytopathogenicity of bark beetle-fungus symbioses: a challenge to the classic paradigm. Annu Rev Entomol 56: Whitney, H.S Association of Dendroctonus ponderosae (Coleoptera: Scolytidae) with blue stain fungi and yeasts during brood development in lodgepole pine. Can Entomol 103: Wood, S.L The bark and ambrosia beetles of North and Central America (Coleoptera: Scolytidae), a taxonomic monograph. Great Basin Nat Print. Yamaoka, Y., Hiratsuka, Y., and P.J. Maruyama The ability of Ophiostoma clavigerum to kill mature lodgepole pine trees. Eur J Forest Pathol 25: P a g e