Blue-Stain Fungi

Eleven isolates of blue stain fungi including seven geographic strains of Ceratocystis coerulea were found to have temperature and maxima ranging from 29 to 39 °C (Lindgren, 1942).

From: Wood Microbiology (Second Edition) , 2020

Fungi

Taina K. Lundell , ... Kristiina S. Hildén , in Advances in Botanical Research, 2014

2.4 Blue stain

Blue stain fungi are Ascomycota species not destructing wood xylem lignocellulose. These fungi (e.g. species of the genera Ophiostoma and Grosmannia) are frequently disseminated as spores by wood-inhabiting beetles. Their hyphae extend in living wood trunks—thus either being pathogenic by growing in living ray parenchyma or phloem cells or being more saprobic by decomposing wood resins and waxes while growing in the resin ducts of conifers (Ballard, Walsh, & Cole, 1984; DiGuistini et al., 2011). Typical for blue stain species is the generation of dark-coloured melanins on their hyphal cell walls for protection against light, drought and host tree resistance factors. The wood-staining saprobic species Ophiostoma piceae is able to grow on triglycerides and oleic acid, and CAZy glycoside hydrolases are expressed on these substrates (Haridas et al., 2013). Since the blue stain species are not decomposing the main wood lignocellulose components (cellulose, hemicellulose and lignin), they are considered to be less important for wood organic carbon cycling.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123979407000112

ENTOMOLOGY | Bark Beetles

M.L. Reid , in Encyclopedia of Forest Sciences, 2004

Management Options

Bark beetles that kill mature trees have many negative economic impacts. If the tree had been intended for timber, it remains usable for only a year or two after death before it becomes fractured. Discoloration by blue-stain fungi reduces the value of the wood for esthetic purposes. Penetration into sapwood by ambrosia beetles can reduce the structural and esthetic value of the affected area of wood. When outbreaks result in millions of trees being killed simultaneously, increased salvage harvesting may depress prices, and disrupt harvesting plans and expected future yield. The potential loss of individual trees valued by people also prompts management actions.

Management of bark beetles affecting trees includes three approaches. These are: (1) killing beetles directly; (2) manipulating beetle movement using semiochemicals (pheromones and kairomones); and (3) stand and landscape management to prevent increases in beetle populations.

Killing bark beetles is difficult because most of their life cycle is spent within plant tissue. For individual beetle-infected trees, it is possible to kill beetle broods by applying insecticides that are conducted through the tree's vascular system to the developing broods (e.g., monosodium methanearsenate). An interesting biological approach is to attract less aggressive but faster-developing bark beetle competitors into trees colonized by pest species. However, these individual tree treatments are not practical on a large scale. Small groups of trees may be felled and either debarked or burned. Infested stands may be harvested or burned with prescribed fire although, as noted above, fire may not kill most beetles. When stand removal is prescribed, beetles can be lured into the stand using semiochemicals to maximize the number of beetles removed. A difficulty with any plan to remove beetles in trees is that the presence of beetles may be hard to detect, as trees may not show obvious signs of attack until broods are well developed or already emerged. Consequently, experienced surveyors are required on the ground to assess beetle populations.

For some species of bark beetle, large numbers of beetles can be removed by using traps baited with semiochemicals, especially pheromones. To minimize the number of predators that are also captured, small discrepancies in the chemicals that are maximally attractive to bark beetles and their predators can be exploited. Mass-trapping is simple and inexpensive to implement once the baits have been developed, but it is difficult to assess how much the population is reduced, since many dispersing beetles fail to establish in trees anyway. In addition, the baits may attract high densities of beetles into a local area, increasing the risk that trees around the baited traps will be successfully attacked (spillover). Consequently, it is often recommended that the baited traps be placed far from host trees. An alternative approach is to use baited trees as traps (trap trees); these tend to be more attractive than baited traps initially, but then become unattractive once saturated with beetles, minimizing the risk of spillover. More effort is required to dispose of the trap tree to prevent beetle emergence than for pheromone traps.

Manipulation of beetle search behavior is an approach that takes advantage of bark beetles' reliance on chemical cues for host selection and mate finding. Beetles can be deterred from settling on trees, or even in stands, by conspecific antiaggregation pheromones, pheromones of competitor bark beetle species, or nonhost volatiles. For species that require high densities of beetles to overcome tree defenses, even some deterrence might allow trees to defend against beetle attacks.

A preventive approach to bark beetle control is to manage stands and landscapes to prevent the development of large beetle populations. However, by definition, pest species use trees that people want, so any plan to make host trees difficult for beetles to find will usually compromise the economy of harvest. Indeed, many bark beetle species have become pests because their host plants have been planted in monocultures, reducing dispersal mortality. It is possible to manage the risk of beetle attack by predicting when a stand is likely to be at risk, and taking action at that time. Risk and hazard rating systems are based on stand conditions (e.g., tree size, age, density, physiography) and on current beetle population size. Beetle population size can be assessed by surveying the number of trees recently killed in the area, by assessing the success of broods, and by monitoring baited traps.

A preventive method widely used to control mountain pine beetles (D. ponderosae) is stand-thinning. The mechanisms by which this method works are unclear, but may include increased vigor of remaining trees and a less favorable microclimate of thinned stands (warmer and windier). Some studies of thinning, focusing on other bark beetle species, have found no effect or a positive effect of thinning on beetle populations. If thinning is conducted on mature stands, costs of this approach include increased tree damage due to wind sway and wind throw, as well as the requirement to enter the stand multiple times. Thinning is therefore not an approach to be implemented indiscriminately.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B0121451607000302

Pine Bark Beetles

C. Tittiger , G.J. Blomquist , in Advances in Insect Physiology, 2016

2.1 Symbiotic Contributions

Earlier studies suggested that bark beetle pheromones could be produced by gut tract bacteria (Brand et al., 1975) or fungus (Brand et al., 1976). Endosymbiotic yeast resident in mountain pine beetle (D. ponderosae) alimentary canal can produce verbenone, an antiaggregation semiochemical, from trans-verbenol (Davis et al., 2013). The mutualistic fungus, Grosmannia clavigera, which is carried in the mycangia of D. ponderosae likely supports pheromone component biosynthesis indirectly by providing nutrients (Bentz and Six, 2006). For the spruce bark beetle (Ips typographus), the symbiotic blue stain fungi G. penicillata and G. europhioides can synthesize the aggregation pheromone component 2-methyl-3-buten-2-ol de novo (Zhao et al., 2015). While these data suggest that bark beetles respond to symbiont-produced semiochemicals, it is also clear that major components are produced in insect tissues. All enzymes that have been characterized in pheromone biosynthetic pathways are eukaryotic, as indicated by the presence of poly(A) tails, alignments with genomic DNA sequences (Keeling et al., 2013) and expression studies, including qRT-PCR and in situ hybridizations, that localize mRNAs to bark beetle tissues (rev. in Blomquist et al., 2010). In addition, it is difficult to reconcile JH III-regulated pheromone production with a bacterial or fungal source of pheromone. With the exception of a recent study by Zhao et al. (2015), published evidence for symbiant contributions to pheromone biosynthesis is indirect at best; to date, there is little evidence that symbiants or associates contribute significantly to monoterpenoid pheromone components.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/S0065280616300030

Susceptibility and Response of Forests to Disturbance

Richard H. Waring , Steven W. Running , in Forest Ecosystems (Third Edition), 2007

2 Bark Beetles

Different species of bark beetles attack a wide variety of conifers. Female beetles select susceptible trees based on the presence of terpenes that are generated by the conifers in increasing amounts as temperatures rise (Christiansen et al., 1987). Bark beetles deposit their eggs in galleries excavated in the phloem, cambium, and sapwood of trees. Successful brood production is contingent on the death of these tissues. Most species of bark beetles can only breed in trees that exhibit severe decline or are already dead, and so they merely promote decomposition and mineralization. A few species, however, are able to attack and kill living, sometimes quite healthy trees. Epidemic outbreaks by these "aggressive" species may greatly alter the state and function of forest ecosystems over large areas.

Aggressive species have developed three ways of conquering living trees: (1) by having the first attacking beetles produce chemical attractants to bring other beetles, (2) by tolerating resin secretions, and (3) by inoculating trees with a pathogenic fungus that kills by halting water transport through the sapwood (Christiansen et al., 1987 ). The degree to which trees can defend themselves successfully is based on the extent to which they can produce resins and mobilize carbohydrates to wall off areas in the phloem and sapwood where beetles have introduced fast-growing strains of blue-stain fungi. Stored reserves are generally insufficient by themselves to protect trees against mass beetle attacks ( Christiansen and Ericsson, 1986). In cooler climates where only one beetle population develops in a year, attacks are synchronized with the expansion of new growth. Some variation exists, however, because budbreak is controlled by soil temperature, which constrains water uptake, more than by air temperature to which insect development is closely keyed (Beckwith and Burnell, 1982). Genetic variation also exists in both tree and insect populations.

In another well-replicated experiment, synthetic pheromones were released to attract mountain pine beetles (Dendroctonus ponderosae) to various thinning and fertilization treatments in 120-year-old forest of lodgepole pine (Pinus contorta) (Waring and Pitman, 1985). Treatments included (1) N fertilizer, (2) N fertilizer combined with a reduction in canopy LAI of about 80%, (3) additions of sugar and sawdust to limit mineralization by microorganisms, and (4) untreated plots. At the start of the experiment, tree growth efficiency averaged less than 70 g wood produced per square meter of leaf area per year with a stand average LAI of 4.7. As beetles killed trees and foliage was shed a year later, more light, nutrients, and water became available to surviving trees.

Within 2 years of the application of fertilizer, surviving trees increased their efficiencies by more than 55% to values above 100 g wood m−2 leaf area year−1 (Table 6.2). Surviving trees in untreated stands also increased their growth efficiency by over 40% after 2 years. Only in the sugar and sawdust treatment did tree mortality not result in a significantly improving residual tree growth efficiency. When 100 trees sustaining different levels of attack were compared, tree mortality, measured by the proportion of sapwood observed with blue-stain fungus, was accurately predicted by noting when the ratio of bark beetle attacks (square meter of bark surface) to growth efficiency (grams of wood produced annually per square meter of foliage) exceeded 1.2 (Fig. 6.6). At values above 100 g wood m−2 leaf area year−1 no successful bark beetle attacks were recorded. The same relationship was demonstrated in other thinning experiments with ponderosa pine (Larsson et al., 1983) and lodgepole pine grown at different densities (Mitchell et al., 1983). A comparable response has also been reported for European spruce bark beetle (Ips typographus) cited by Christiansen et al. (1987).

TABLE 6.2. Growth Efficiency (Wood Production per Unit Leaf Area) under Various Treatments a

a
Means (n = 12) connected by brackets are significantly different at p = 0.05. From Waring and Pitman (1983).

FIGURE 6.6. Growth efficiency provides an index to the density of bark beetle attack on lodgepole pines. Filled or partly filled circles represent the proportion of sapwood killed on attacked trees. Open circles represent trees able to survive all beetle attacks before any conducting tissue was killed. The dashed vertical line indicates the boundary above which beetle attacks are unlikely to cause tree mortality.

 (From "Modifying lodgepole pine stands to change susceptibility to mountain pine beetle attack," by R. H. Waring and G. B. Pitman, Ecology, 1985, 66, 889–897. Copyright © 1966 by the Ecological Society of America. Reprinted by permission.)

In areas where bark beetle outbreaks occur, thinning may improve the resistance of residual trees if sufficient time is allowed to raise their growth efficiency to a safe level, as demonstrated in a photograph taken after a bark beetle epidemic swept through an even-aged pine forest that had been partially thinned (Fig. 6.7). Thinning alone, however, may not be sufficient to prevent subsequent mortality if other abiotic or biotic factors constrain water, nutrient, and CO2 uptake. Annual growth efficiency may prove an inadequate index if conditions are highly variable from year to year or if beetle attacks extend throughout the growing season (Lorio, 1986).

FIGURE 6.7. Mountain pine beetles attacked and killed blocks of old-growth lodgepolc pine in eastern Oregon, except where the forest was previously thinned. (Photograph provided by John Gordon, Yale University, School of Forestry and Environmental Studies, courtesy of Boise Cascade.)

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123706058500128

The Genus Tomicus

François Lieutier , ... Massimo Faccoli , in Bark Beetles, 2015

8.3 Tree Mortality

Tree mortality occurs when bark beetles and associated fungi are abundant enough to overcome the resistance in the trees under attack. In contrast to truly aggressive bark beetles in the genera Dendroctonus, Ips, and Scolytus (Chapters 8, 9, and 12, respectively), pine shoot beetles are generally not capable of overwhelming the resistance of healthy pine trees. The main reason for this is probably the lack of powerful aggregation pheromones leading to successful mass attacks (Byers, 2004 ). Another factor may be the lower virulence of the blue-stain fungi associated with the pine shoot beetles than that of more aggressive bark beetles ( Lieutier, 2004), but this issue is discussed elsewhere both in this and in other chapters. The topic of host resistance and tree defensive reactions is very complex and covered in Chapter 5.

Considering that healthy pine trees are not available for stem attacks by the pine shoot beetles unless their resistance is substantially reduced, major tree mortality caused by pine shoot beetles are fairly rare episodes and always preceded by some predisposing factor(s). The role of predisposing factors to damage by T. piniperda was discussed in Section 6.2. There is one reported case of heavy tree mortality caused by T. piniperda in North America, which occurred in southern Ontario (Scarr et al., 1999). Surveys of eight damaged pine stands (Scots, white or jack pine) revealed almost total tree mortality in one Scots pine stand, none in an adjacent white pine stand, and up to 38% mortality in the other stands. As was mentioned earlier, fallen shoot numbers ranged from 4 to 12/m2. In Eastern Europe, including Russia, there are many reports on the damage caused by T. piniperda: Estonia (Voolma and Luik, 2001), Poland (Gidaszewski, 1974; Borkowski, 2001), Romania (Drugescu, 1980; Mihalciuc et al., 2001), and Russia (Agafonov and Kuklin, 1979; Bogdanova, 1998; Kolomiets and Bogdanova, 1998; Gninenko and Vetrova, 2002).

Damage by T. destruens has been frequently reported from nearly all countries in southern Europe, e.g., Portugal (Ferreira and Ferreira, 1986, 1990), Spain (Amezaga, 1996; Fernández Fernández and Salgado Costas, 1999), France (Carle, 1975), Slovenia (Jurc, 2005), Greece (Kailides, 1964; Markalas, 1997; Avtzis and Gatzojannis, 2000), and Italy (Masutti, 1969; Triggiani, 1984; Boriani, 1998), where the beetle is listed among the most aggressive pests of the Mediterranean pine forests (Nanni and Tiberi, 1997). In addition, T. destruens is considered to be a major pest in Turkey, where the number of infested P. brutia has greatly increased in recent years (Sarikaya and Avci, 2010). In Tunisia, large and increasing damage to maritime pine forests has been recorded since 1972 (Hamza and Chararas, 1981) and is still occurring (Ben Jamâa et al., 2000). Serious damage is reported also in Israel (Halperin, 1978; Halperin et al., 1982; Mendel, 1987), Algeria (Chakali, 2003, 2005), and Morocco (Ghaioule et al., 1998).

There are many Chinese papers reporting on T. yunnanensis damage. Most of these papers are published in Chinese, but starting with Ye (1991) there is an increasing number of papers in English (for references see Långström et al., 2002 and Lieutier et al., 2003). The damage situation in China differs drastically from that in Europe with T. piniperda, in that there has been large-scale tree mortality due to T. yunnanensis during the last decades (Ye, 1991; Lieutier et al., 2003). Although these trees may suffer some drought stress from time to time (Ye, 1992), a more important explanation for this outbreak is that the intensive shoot damage itself may render the trees susceptible to further stem attacks, leading to a vicious self-perpetuating cycle (Lieutier et al., 2003; Section 4.5). The extent of the damage is the largest ever observed for pine shoot beetle species, and the outbreak seemed to coincide with the maturation of large plantations of Yunnan pine that were established in the 1960s (for references see Långström et al., 2002). This outbreak is discussed in more detail below.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780124171565000101

Nematodes and other worms

R.A.J. Taylor , in Taylor's Power Law, 2019

Pine trees

The pinewood nematodes Bursaphelenchus xylophilus and B. mucronatus are phoretic on cerambycid (longhorn) beetles Monochamus carolinensis and M. saltuarius, respectively. In addition to the direct damage to trees by the beetles, the nematodes cause pine wilt disease, which can be fatal. Warren and Linit (1992) asked if the presence of M. carolinensis was necessary for B. xylophilus infestation, other than for transport. They compare the distribution and abundance of B. xylophilus in Scots pine (Pinus sylvestris) bolts infested with and without M. carolinensis. Blue-stain fungus was introduced to paired bolts from each of seven healthy pine trees to enhance nematode population growth and nematodes were introduced 1 week later. One bolt of each pair was exposed to beetles for 3 days to permit oviposition. The seven pairs of pine bolts were incubated for 60–70   days until beetles began to emerge. Each bolt was sampled for nematodes and nematode counts analyzed by TPL and Green's coefficient (Chapter 3). The nematode distributions were aggregated in both sets of bolts (Appendix 7.C1), with super-aggregated TPL slopes not significantly different (P  >   0.18) in beetle-infested and uninfested bolts (common b  =   3.10   ±   0.389). However, the pattern of density within the bolts differed with the radial distribution more heterogeneously in the noninfested bolts than the infested.

Many Bursaphelenchus species are obligate fungus feeders, while B. xylophilus and B. mucronatus feed on both pine wood and fungus growing in the pine. The relationship between B. xylophilus and the fungi cohabiting Japanese black pine (Pinus thunbergii) was examined by Sriwati et al. (2007) who isolated 18 species of fungus from black pine trees. In a laboratory experiment, they investigated the relationship between the 18 fungi and B. xylophilus. Isolates of each fungus were cultured and then inoculated with 300 B. xylophilus. After inoculation, nematodes were extracted from the fungus cultures and counted. Sriwati et al. provide the log of population density (#/plate) and SD for the 18 fungi. The grey mold Botrytis cinerea, a preferred food of the B. xylophilus culture was included as a control. Nematodes were counted 5, 10, and 15   days post inoculation. The association between fungus and nematode population growth ranged from negative through neutral to very positive, providing a range of population means suitable for TPL analysis. Fig. 7.14 (Appendix 7.D1) shows the ensemble TPL identifying the three time frames over seven orders of magnitude of mean. The three B. xylophilus treatments fed B. cinerea (circled) are all below the best fit line, indicating lower variation on the preferred food source. The three time periods do not differ in either intercept or slope (P  >   0.55). As the nematode populations grew on the fungus cultures, the variance mean relationship remained constant. The longest time period spanned the entire seven orders of magnitude of mean and the 5 day period spanned 1.5 orders. This experiment resulted in significantly lower slopes than Warren & Linit's experiment probably as a result of the differences in setup and sampling procedures.

Fig. 7.14

Fig. 7.14. Mixed-species ensemble TPL (NQ  =   5, NB  =   36) of Bursaphelenchus xylophilus growing on 19 fungus cultures recorded at 5, 10, and 15   days post inoculation. Each point is nematode density (#/plate) on a different fungus culture with the control fungus Botrytis cinerea (circled). Temporal TPL (NQ  =   2–14, NB  =   37) of phoretic B. mucronatus released from longhorn beetle Monochamus saltuarius over a 70   day period.

 Data from Table 2 in Sriwati et al. (2007) and from Table 1 in Jikumaru et al. (2001).

The related pinewood nematode B. mucronatus carried by the longhorn beetle M. saltuarius was investigated by Jikumaru and Togashi (2001). They recovered 43 emerging adult M. saltuarius from dead Japanese red pine (Pinus densiflora), which were transferred to individual containers with red pine twigs. Every 5 days, the beetles were transferred to new containers and nematodes extracted and counted: nematodes washed from the old containers were also counted. At the end of the experiment, the nematodes still on the beetles and washed from the containers were added to the nematodes recovered from the twigs to estimate the total nematode load and from that the transfer rate for each beetle. Transfer rate was divided into nematode departure efficiency and nematode transmission efficiency, both of which were highly variable. Jikumaru & Togashi record the number of nematodes recovered from the twigs exposed to each beetle during each 5-day period up to 70   days. Of the 43 beetles, four died in the first 10 days and failed to transmit any nematodes and two were uninfected. The mean and variance of nematodes recovered per 5-day exposure period from NB  =   37 beetles provide for a temporal TPL with NQ  =   2–14 (Fig. 7.14; Appendix 7.E1). Two factors contributed to the range of means over nearly four orders of magnitude: the beetle survival, which spanned 2–120   days with half the beetles surviving 50 of 70   days and the nematode transmission efficiency, which ranged from 0% to 50% and averaged 11%. Both variables are skewed and their combination contributed to the TPL's steep slope (b  =   1.85   ±   0.039), which is not significantly different (P  >   0.35) from the estimate obtained from Sriwati et al.'s (2007) experiment with B. xylophilus.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128109878000070

Chemical Ecology and Phytochemistry of Forest Ecosystems

Axel Schmidt , ... Jonathan Gershenzon , in Recent Advances in Phytochemistry, 2005

Methyl Jasmonate Application to Saplings in the Laboratory

When methyl jasmonate was sprayed on the foliage of 1-2 year-old P. abies saplings from a uniform genetic background, this treatment triggered a dramatic increase in terpene levels. 23 There was a more than 10-fold increase in monoterpenes and a nearly 40-fold increase in diterpenes in wood tissue. In contrast, in the bark there was a much smaller increase in monoterpenes and no significant change in diterpene levels. Curiously, the response to methyl jasmonate took much longer than previously-observed inductions of plant defenses with this elicitor. Significant increases were not seen until 15   days after application. 23 Examination of the anatomy of the treated saplings revealed that methyl jasmonate had stimulated the formation of a ring of new resin ducts (traumatic resin ducts) in the newly-formed xylem (Fig. 1.1). Franceschi and co-workers had previously shown that wounding of P. abies or infection with Ceratocystis polonica, a blue-stain fungus vectored by the bark beetle Ips typographus, could induce the appearance of traumatic resin ducts over a 36   day period. 19 Apparently, this response also occurred with methyl jasmonate. 24 A change is triggered in the developmental program of the cambium whereby some of the xylem mother cells become resin duct cells rather than tracheids. The fact that traumatic duct formation requires the differentiation of entirely new cells explains why terpene induction requires such a long interval after methyl jasmonate application. Formation of traumatic resin ducts represents a major investment for P. abies and puts heavy demands on limited resources. Careful anatomical studies have shown that both height growth and stem growth is reduced by about 50% in 2-year-old plants after traumatic ducts are induced by application of 100   mM methyl jasmonate externally to the stem bark (Krokene P. et al., unpublished results).

Fig. 1.1. Induced anatomical defense responses in Norway spruce. (A, B) Formation of a ring of new, traumatic resin ducts (TD, arrowheads) in the xylem of 2-year-old Norway spruce saplings after application of methyl jasmonate. A large cortical resin duct (CD) can be observed in the phloem, but these ducts do not appear to respond to methyl jasmonate treatment. (C) Normal phloem and sapwood anatomy of an older tree, with concentric rings of polyphenolic parenchyma cells (PP) in the phloem above the cambium (X) and normal wood below. (D) After treatment with methyl jasmonate or fungal infection the PP cells increase greatly in size and traumatic resin ducts (arrowheads) forms in the wood.

Methyl jasmonate treatment not only triggers a dramatic change in terpene quantity, but also causes changes in terpene composition. 23 For example, of the two major monoterpenes in the wood, α-pinene and β-pinene, the proportion of α-pinene to β-pinene changed from about 1:1 in control saplings to 1:2 after methyl jasmonate treatment, with increases in the relative amounts of the (-)-enantiomers in relation to the (+)-enantiomers of both compounds. Among the diterpenes, levopimaric acid increased over 5-fold after methyl jasmonate treatment in comparison to a 2.5-fold increase in most of the other major diterpene acids.

Methyl jasmonate spraying also induced some increases in monoterpene and sesquiterpene levels in needles, but these were only 2-fold. 25 More significant was that methyl jasmonate application led to a 5-fold increase in the emission of terpenes from the foliage, and emission had a pronounced diurnal rhythm, with the maximum amount released during the light period. The composition of the emitted volatiles also shifted dramatically from a blend dominated by monoterpene olefins, such as α-pinene and β-pinene, to one in which the major compounds were sesquiterpenes, principally (E)-β-farnesene and (E)-α-bisabolene, as well as the oxygenated monoterpene, linalool. These compounds are of particular ecological interest, as they have been reported to attract natural enemies of herbivores or repel herbivores directly in other plant species. 25 Recent work has shown that methyl jasmonate treatment of large Norway spruce trees reduces both the attack rate and colonization success of the spruce bark beetle in the field (Krokene, P. and Christiansen E., unpublished results).

The dramatic increase in terpene formation, accumulation, and emission in P. abies in response to methyl jasmonate is consistent with the effect of methyl jasmonate or jasmonic acid on many defense compounds in angiosperms. 22 In conifers, jasmonates had been previously shown to promote the formation of heat shock 26 and defense signaling proteins, 27 to enhance resistance to pathogenic fungi, 28 and to promote colonization with ectomycorrhizae. 29 In relation to terpenes, jasmonates had been shown to promote formation of an oxygenated sesquiterpene, todomatuic acid, and an oxygenated diterpene, paclitaxel (taxol), in cell cultures, 30,31 but had never before been reported to enhance terpene accumulation in intact plants. The finding has been corroborated by a study including small plants and larger trees of Norway spruce, where methyl jasmonate induced increased resin flow and other defense reactions when applied externally to intact bark. 32

The concentrations of methyl jasmonate found to be effective in spraying P. abies saplings in our work (maximum effect at 10   mM) were relatively high compared to those typically used on angiosperm foliage: 10   μM – 1   mM. 33-35 This may only be a consequence of the need for higher concentrations to penetrate the thick cuticle of conifer needles. More recently, we have shown that a 100   μM spray of methyl jasmonate is effective in inducing terpene accumulation in P. abies saplings when formulated as a 0.5   % solution in Tween 20 detergent (Schmidt, A., unpublished results).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/S0079992005800024

PLANT DISEASES CAUSED BY NEMATODES

GEORGE N. AGRIOS , in Plant Pathology (Fifth Edition), 2005

Symptoms

The foliage of infected branches or whole trees suddenly becomes grayish-green, and the trees stop exuding resin from their wounds. The foliage then becomes yellowish green, and at first some, then all, of the needles turn brown (Figs. 15-35A and 15-35B). Within 4 to 6 weeks from the appearance of symptoms, the tree or branch has totally brown foliage and appears wilted, although sometimes the needles are retained without obvious droop. In many affected trees blue stain in wood is heavy (Fig. 15-35E). Infected trees invariably die (Fig. 15-35B).

The Pathogen: Bursaphelenchus xylophilus

This pathogen, also known as the pinewood nematode, is about 800 micrometers long by 22 micrometers in diameter (Fig. 15-35C). It develops and reproduces rapidly, completing a life cycle within four days during the summer. Each female lays about 80 eggs, which hatch and produce the four juvenile stages and the adults. While the tree is still living, the nematodes feed on plant cells, but after its death they feed on fungi that invade the dying or dead tree. In late stages of infections, a different form of third-stage juveniles, called the dispersal stage, appear. These have large amounts of nutritional reserves and a thick cuticle and they molt to fourth-stage dispersal juveniles. The latter are especially adapted to survive in the respiratory system of certain cerambycid beetles, by which they are transmitted to healthy trees. Bursaphelenchus xylophilus is mycophagous, i.e., it feeds and can complete its life cycle feeding on many kinds of fungi, e.g., Alternaria, Fusarium, and the blue stain fungi (Ceratocystis spp.).

Development of Disease

The pinewood nematode overwinters in the wood of infected dead trees, which also contain instars (larvae) of cerambycid beetles such as Monochamus alternatus. In early spring, the instars excavate small chambers in the wood in which they pupate. As the adult beetles emerge from the pupae later in the spring, large numbers of fourth-stage dispersal juveniles enter the beetles and more or less fill many of the tracheae of the respiratory system (Fig. 15-35D). The emerging adult beetles bore their way out of the wood, each carrying with it an average of 15,000 to 20,000 fourth-stage dispersal juveniles, and fly to succulent branch tips of healthy trees where they feed for several weeks. As the beetles feed by stripping the bark and reaching the cambial tissues (Fig. 15-35F), the fourth-stage dispersal juveniles emerge from the insect and enter the pine tree through the wound. Once in the plant, the dispersal juveniles undergo the final molt to produce adult nematodes, which then reproduce. The nematodes migrate to the resin canals, where they feed on the epithelial cells lining the canals and cause their death as well as the death of the surrounding parenchyma cells. The nematodes move quickly through resin canals in both the xylem and the cortex, reproduce rapidly, and, within a few weeks, build up enormous populations in the host.

The destruction of the resin canals stops all resin flow from wounds within about 10 days of inoculation. In the next three weeks, transpiration by the foliage declines rapidly and stops as the foliage loses color and the tree suddenly wilts. Nematode populations reach a maximum level after the death of the tree, about one month after inoculation. In later stages of the disease, as the condition of the tree deteriorates, nematode populations decline. At the same time, however, there is a gradual increase in the proportion of the dispersal third-stage juveniles in relation to the total population of the nematode in the wood. The third-stage dispersal juveniles are the resting stage of the nematode.

In the meantime, the adult Monochamus beetles, the vector of B. xylophilus, after they have fed on tender pine twigs for about one month, look for and deposit their eggs under the bark of stressed and dead pine trees, including trees showing symptoms or dying from infection by the pinewood nematode. The first two instars of the insect feed under the bark, but the third penetrates the wood, where, after a molt, it produces the fourth instar, which overwinters in the wood. In early spring, the fourth instar excavates a chamber in the wood, in which it pupates, and attracts numerous third-stage nematode juveniles all around it. The latter molt to produce fourth-stage dispersal juveniles, which infect the adult insect as soon as it emerges from the pupa, and thus the cycle is completed.

In some temperate regions, primarily pine trees stressed by various diseases and insects are attacked by the pinewood nematode but typical wilt symptoms are not usually produced.

Control

Control measures involving insecticide treatment to control the beetles, and early removal and burning of dead and dying pine trees to eliminate the breeding habitat of the nematode and of the beetle, are only moderately effective and practical only in restricted localities. Neither of these controls is possible in large forests. Affected susceptible pine species planted as shade trees should be replaced with more resistant pine species or with other types of trees.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B978008047378950021X

Responses and modeling of southern pine beetle and its host pines to climate change

Carissa F. Aoki , ... Kamal J.K. Gandhi , in Bark Beetle Management, Ecology, and Climate Change, 2022

4.1 The past and present of predictive modeling and outbreak dynamics

With southern pine beetle's recent range expansion into the mid-Atlantic and New England states, interest in predictive population modeling related to understanding outbreak dynamics—particularly with regard to increasing temperature due to climate change—has increased. This coincides with the increasing availability of spatial data related to climate variables, as well as more advanced statistical methods for data analyses. At the same time, southern pine beetle research has a rich history, with questions about what might drive outbreak cycles already being asked nearly a century ago (Craighead, 1925). The historical trajectory of southern pine beetle research has reflected both a more applied component, beginning with the development of "economic entomology" in the 1920s (Kingsland, 1995), followed by theoretical approaches based in mathematical population ecology, and the more recent inclusion of methods combining population dynamics with spatial data related to climate. Here we briefly review the history and development of southern pine beetle research related to outbreak dynamics and identify some critical issues in current modeling efforts, particularly with regard to the use of climate data. As research on the effects of temperature and precipitation having been discussed above for both beetles and host trees, we focus here on population approaches, and recent efforts to use regional or continental scale climate data in conjunction with beetle population data.

4.1.1 Population approaches

Unlike insects whose outbreaks are cyclical, southern pine beetle outbreaks occur at irregular intervals and are thus challenging for both management planning purposes and ecological understanding. Early population modeling efforts focused on trying to understand the patterns and processes of these outbreaks. Decades of previous research investigated the role of climate variables, and water balance in particular, on host susceptibility and consequent likelihood of beetle outbreak. Water deficit (Craighead, 1925; Kroll & Reeves, 1978; Rudinsky, 1962) and water surplus (Lorio, 1968, 1986; Reeve, Ayres, & Lorio, 1995) were both shown to have an influence on oleoresin exudation in host trees. By the late 1980s-early 1990s, bottom-up forces were commonly thought to be the primary factor affecting the occurrence of beetle outbreaks. Studies utilizing methods in population dynamics were thus presented in opposition to the accepted wisdom of the time.

These efforts demonstrated a delayed density-dependent regulation on southern pine beetle populations (Turchin, Lorio Jr., Taylor, & Billings, 1991) and identified southern pine beetle's primary clerid predator, Thanasimus dubius (Fabricius) as a potential driver of outbreak dynamics (Reeve et al., 1995). Further work described the numerical response of T. dubius to southern pine beetle (Reeve, 1997) and demonstrated a one year lagged response, supporting the theory that outbreak cycles were due to delayed density dependence related to predator-prey dynamics (Reeve & Turchin, 2002; Turchin, Taylor, & Reeve, 1999). Cycles at this time were generally thought to occur at approximately 7–9 year intervals.

Later modeling and experimental work found that these predator-driven cycles were not as comprehensive an explanatory mechanism as initially thought and that various other factors can also play a strong role in outbreak dynamics. These included indirect effects of mites phoretic on southern pine beetle and an accompanying blue-stain fungus (Hofstetter, Cronin, et al., 2006; Hofstetter, Klepzig, et al., 2006); new evidence that temperature might play an important role (Friedenberg, Sarkar, Kouchoukos, Billings, & Ayres, 2008); and support for an alternate attractors model that suggested that competitors, in addition to predators, might drive the occurrence and decline of outbreaks (Martinson, Ylioja, Sullivan, Billings, & Ayres, 2013). Refuting previous work, clerid predators were also found not to lag annual southern pine beetle abundance but rather to respond instantaneously (Weed, Ayres, Liebhold, & Billings, 2017). In addition, the late 20th and early 21st centuries brought long periods of endemic population levels, in contrast to the 6- to 10-year intervals previously documented (Clarke et al., 2016). With different competing hypotheses for outbreak occurrence still on the table, and an apparently new pattern of outbreaks across time, the need to find methods that would accommodate variation across space and time has become apparent.

4.1.2 Integrating space into population modeling

The studies of the 1990s and early 2000s reflected population dynamics approaches, with beetle abundances considered as a population, but not in a spatially explicit manner. Meanwhile, throughout the latter half of the 20th century, methods in biogeography and landscape ecology gained prominence. Along with the simultaneous rise of GIS and remote sensing technologies, and the subsequent increase in the availability of broad spatial data sets, the analysis of patterns in space came increasingly into use as another way of gaining understanding in ecological questions. These methods and approaches have enabled the asking of broader questions about the effects of climate change on forest disturbance processes, including those related to southern pine beetle and its distribution.

In an early, pre-GIS study, Kroll and Reeves (1978) used climate variable data from a single location to develop a regression model predicting number of southern pine beetle infestations in that location. GIS mapping enabled later workers to ask similar questions on a regionwide scale. Ungerer et al. (1999) used data interpolated from 33 weather stations, along with physiological data documenting the lower lethal temperature for southern pine beetle to ask whether its northern range limit might be determined by the latter. Their results showed support for this hypothesis and demonstrated that beetle populations might be expected to move northward under climate change scenarios that included a rise in minimum winter temperatures. Williams and Liebhold (2002) built on this work using discriminant function models to predict the northward movement of both host trees and of beetle outbreaks under several climate change scenarios. In a rare analysis of potential climate change effects on southern pine beetle populations across the southwestern USA, Waring et al. (2009) used degree day modeling to analyze bark beetle generation times under climate change, finding a predicted increase in the number of generations across the region.

Recent work seeks to address the complexity of climate change variables over space and time. Lesk et al. (2017) used spatially variable climate change scenarios to predict northward range expansion, rather than the uniform scenarios utilized in previous studies. Munro et al. (2021) incorporated temporal variation as well, including daily, monthly, and annual averages across variables. Climatic variables were used to assess historical and future spatiotemporal changes in southern pine beetle trap catches for their native range in southeastern USA. This work found that prior year fall (August) and current year winter (January–March) temperatures influence the number of trap catches in spring. Additionally, Munro et al. (2021) were able to build on prior work to reduce and quantify prediction uncertainty for each pixel in space and time, a component rarely addressed in prior southern pine beetle modeling work.

One potential hindrance to predictive modeling over large scales of space and/or time is the availability of long-term southern pine beetle monitoring data, the scale at which such data are collected, and potential mismatches with the scale at which climate variables are modelled in space. Researchers and managers of southern pine beetle have long benefited from the southwide spring trapping process implemented by the Texas Forest Service beginning in 1987. Each spring, pheromone-baited traps are placed in the field by cooperators from states across the south. Trap captures of southern pine beetle and clerid predators are then used to predict summer outbreak (Billings, 2011a, 2011b; Billings & Upton, 2010). Due to its operational nature, traps are not extensively placed—usually one to three traps per participating county or ranger district per season—but the availability of a >   30 year data set has afforded numerous research opportunities that would not otherwise have been possible. These data have been used by forest managers over the last several decades to help manage their local forest resources, and the data are obtained at the scale of interest for that endeavor, which is to say, stands within forests, and perhaps within counties and/or National Forests. Analyzing these data along with climate data, which are often at a scale measured in kilometers or more, presents challenges both in understanding how biological processes might differ across scales, and how uncertainty at these different scales should be considered (Fig. 4). Consideration of variables that change over different time scales (e.g., tree growth over years versus species composition over decades to centuries) only adds to the challenge.

Fig. 4

Fig. 4. Scales of time and space as applied to southern pine beetle, its occurrence at individual, community/population, and range scales (top), and the scales of selected variables of consideration in predicting the effects of climate on beetle populations (bottom). The dotted black box indicates the inference space of climate data; the dotted grey box indicates the inference space where climate data and management scale data (forests to counties) are brought together despite their difference in scales; and the dotted arrow indicates the use of finer scale data to produce the predictions for this inference space. Integrating models over time as well as space poses a particular challenge.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128221457000155

Microbiome of forest tree insects

Juliana A. Ugwu , ... Fred O. Asiegbu , in Forest Microbiology, 2021

5.3 Microbiomes of Coleoptera (beetles)

Some of the most devastating forest tree diseases that emerged during the last century resulted from the interactions between fungi and wood-boring beetles (Curculionidae; Coleoptera) in the subfamilies Scolytinae (bark beetles and ambrosia beetles) and Platypodinae (ambrosia beetles) (Six, 2003; Hulcr and Dunn, 2011; Ploetz et al., 2013). The bark beetles and ambrosia beetles differ in their feeding preferences. Bark beetles feed on the host tree phloem tissue (the vascular tissue responsible for the transport of sugars) while most ambrosia beetles bore deeper into the xylem (sapwood and heartwood) and rely on fungi as their sole source of nutrition in this otherwise nutrition-poor niche (Batra, 1966, 1967; Beaver, 1989; Farrell et al., 2001). Although bark beetles may have access to more readily available nutrients, many species also feed on symbiotic fungi as a supplement in their diets (Six and Paine, 1998; Ayres et al., 2000; Bleiker and Six, 2007).

Bark beetles include many aggressive (primary) tree pests that can cause significant economic losses to forestry and forest ecosystems. However, the majority of bark beetle species infest mainly already dead, dying, or stressed trees in their native environments and are thus usually harmless to healthy living trees. They are abundant and important components of forest ecosystems (Martikainen et al., 1999). A characteristic that has fascinated researchers is their widespread association with microorganisms, including fungi, bacteria, and metazoans (mites and nematodes). The first reports of associations between bark beetles and fungi and their roles in timber staining were recognized already in the 19th century (Schmidberger, 1836; Hartig, 1844, 1878). It was not until the early decades of the 20th century, with the expansion and mechanization of forestry and forest product industries as well as modern globalization, that the economic importance of bark beetles and the fungi they carry became evident and were recognized as serious risks to forest health.

Due to their economic and ecological importance as well as scientific curiosity to understand symbiotic interactions and their evolutionary histories, research on the bark beetle microbiome (Fig. 18.1) has been active during the past century. It has also strongly been focused on certain wood-inhabiting fungi. Bark beetles are found in association with diverse fungi, mainly ascomycetous species that are members of the fungal orders Hypocreales, Ophiostomatales, Microascales, and Saccharomycetales (Ploetz et al., 2013 ). The most investigated of these microorganisms are the so-called blue-stain fungi, commonly known also as "ophiostomatoid fungi" (the term used later in the text to refer to the assemblage of morphologically similar fungi adapted for arthropod dispersal) ( Wingfield, 1993). Many ophiostomatoid fungi have pigmented hyphae that colonize freshly exposed sapwood and cause grey, black, or brown discoloration of wood, downgrading the value of timber and resulting in economic losses (Uzunovic and Byrne, 2013). The damage to wood is cosmetic in contrast to the structural damage of wood caused by rot fungi (Seifert, 1993). Research on ophiostomatoid fungi has also been fueled by the fact that some fungal species are aggressive tree pathogens when accidentally introduced into new environments, and examples such as Dutch elm disease have caused major losses to forestry and greatly impacted natural forest ecosystems worldwide (Ploetz et al., 2013).

Since the earliest studies, controversy has surrounded the taxonomic placement of ophiostomatoid fungi and the role they potentially play in bark beetle lives and the tree killing processes of primary bark beetles. The difficulties in providing correct species identification and confusion over the taxonomy of these fungi are because the species share similar, minute, and overlapping morphological characteristics; the simultaneous presence of various life stages; and the sharing of the same or similar ecological niches in beetle galleries. The typical morphological features of ophiostomatoid fungi are their spore-forming structures, which are considered adaptations for dispersal by arthropod vectors (Malloch and Blackwell, 1993). The spore-forming structures of both the asexual and sexual states of these fungi are typically long stalks or necks, which bear spores in their apices in slimy masses that provide a mechanism to reach and attach to the bodies of passing arthropod vectors for transport to new host trees. The ecological and morphological similarities of these fungi have evolved more than once during evolution and are thus examples of convergent evolution (Malloch and Blackwell, 1993). Phylogenetic analyses have shown that ophiostomaid fungi are members of Sordariomycetes (Ascomycota) that reside in two distinct orders, the Ophiostomatales and Microascales (Hibbett et al., 2007; De Beer et al., 2013; Ploetz et al., 2013). In the single family Ophiostomataceae, 10 genera are currently included (De Beer et al., 2013; Bateman et al., 2017; van der Linde et al., 2016). Microascales is comprised of five families, of which the Gondwanamycetaceae, Graphiaceae, and Ceratocystidaceae include ophiostomatoid fungi (De Beer et al., 2014, 2017; Mayers et al., 2015; Nel et al., 2017). In the Ophiostomatales, all the ophiostomatoid fungi reside in the Ophiostomataceae (De Beer and Wingfield, 2013). Considering their importance as forest tree pathogens, only the families Ceratocystidaceae and Ophiostomataceae include important tree pathogens, the majority of which reside in Ceratocystis, Endoconidiophora (Microascales, Ceracystidaceae), Leptographium, Ophiostoma, and Raffaelea (Ophiostomatales, Ophiostomataceae) (Jacobs and Wingfield, 2001; Harrington et al., 2010; Ploetz et al., 2013; Seifert et al., 2013; De Beer et al., 2014). The advances in molecular genetic tools, especially DNA sequence comparisons, have greatly enhanced the accurate and reliable identification of these fungi while accelerating the taxonomic work, delineation of species boundaries, and understanding of the true species diversity. The scientific research has been active, resulting in the discovery of numerous species novel to science and taxonomic revisions. In many cases, several changes in the nomenclature of these fungi have occurred. If you are not a specialist in the field, it is advised to check the synonyms and confirm the currently valid name, especially when dealing with quarantine or phytosanitary issues.

Unlike bark beetles, ambrosia beetles usually infest only dead or stressed trees and their fungal associates are tree pathogens only in rare cases, typically connected to their introduction into new environments (Ploetz et al., 2013). An example of a recent such event is the damage caused by the fungal symbiont Fusarium euwallaceae together with its invasive host, the polyphagous shot hole borer (PSHB) (Paap et al., 2018). Interactions between ambrosia and bark beetles and ophiostomatoid fungi are among the most intensively investigated insect-fungi relationships in forest ecosystems. A range of different types of associations exist, varying from mutualistic interactions that benefit both the fungus and the beetle to occasional relationships that likely do not have importance for the beetle, but benefit the fungus as a means of facilitated transport to a new host tree. There is increasing evidence that some fungal symbionts can facilitate beetle colonization success and amplify insect damage (Wadke et al., 2016; Zhao et al., 2018). In some cases, the fungi are obligate nutritional symbionts of the ambrosia beetles (Batra, 1966, 1967; Beaver, 1989; Farrell et al., 2001; Ploetz et al., 2013) while the others are saprotrophs inhabiting the beetle galleries on wood. The associations also vary depending on the family to which the fungi belong. The majority of ophiostomatoid fungi in Ophiostomataceae are typically more specific with certain beetle species compared to species of Ceratocystidaceae (Harrington et al., 2010; Jacobs and Wingfield, 2001; Kirisits, 2004; Paine et al., 1997). Ceratocystis and Endoconidiophora species also attract various other insect vectors than beetles by producing strong aromas to attract their vectors (Kile, 1993).

While most studies have focused particularly on fungi associated with beetles, rather little is known about other microbes involved in these interactions. Yeasts have been recognized as constant components in bark and ambrosia beetle galleries (Siemaszko, 1939; Davis, 2015), but they have probably been overlooked in the majority of the previous collections that are mainly based on the culturable fraction of fungal diversity. Particularly common seem to be ascomycetous yeasts, which are more dependent on vectors to move to new host trees compared to the basidiomycetous species (Kurtzman et al., 2011). Molds (e.g., Penicillium, Aspergillus, Mucoromycetes) are also abundant and likely overlooked (Davis, 2015; Silva et al., 2015; Kasson et al., 2016; Li et al., 2018; Hofstetter et al., 2015).

Other organisms are also involved in these interactions, and increasing evidence indicates that bark beetle-associated mites are important vectors of fungi present in beetle galleries (Moser et al., 1989; Chang et al., 2017; Vissa and Hofstetter, 2017). The mites can carry fungal spores in their bodies or specialized structures called sporothecae (Moser, 1985). Some mite species feed on fungi and can thus promote the growth of certain fungi in beetle galleries (Hofstetter and Moser, 2014). Nematodes are also common associates of beetles, found in beetle galleries and on the bodies of beetles. Some of them are beetle parasites and have also been studied as potential biological control agents of pest beetles (Grucmanová and Holuša, 2013). Nematodes consume other microorganisms, including fungi and bacteria (Yeates et al., 1993; Ledón-Rettig et al., 2008). As for other insects, bacterial symbionts are also common and diverse, including enterobacteria as the most prevalent ones (Dohet et al., 2016).

In summary, microbiomes associated with bark and ambrosia beetles are very diverse and complex. Only a few have been extensively studied, and generally, our knowledge of these associations is still limited and biased toward forest ecosystems in parts of Europe and North America. Although research has been active, it has remained in the discovery phase for a long time, at least partially because the vast majority of microbes associated with bark and ambrosia beetles remain uninvestigated. The development of high-throughput sequencing methods has provided tools to explore whole microbial communities, but only a few studies thus far have focused on those associated with bark and ambrosia beetles (Kostovcik et al., 2015).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128225424000188