Conifers and hardwood trees are independently attacked by both insects and fungi; however, the focus of this review is tree mortality caused by insects and filamentous fungi that have developed symbiotic relationships to help overwhelm tree defenses and facilitate colonisation by both organisms. Several thorough reviews have been published that outline the mutualistic symbiotic relationships that exist between insects and fungi (i.e., Six and Klepzig Reference Six and Klepzig2004; Klepzig et al. Reference Klepzig, Adams, Handelsman and Raffa2009; Six and Wingfield Reference Six and Wingfield2011). The nature of the symbiotic relationship can vary; some insects, such as some bark beetles, all ambrosia beetles and Sirex Linnaeus (Hymenoptera: Siricidae) woodwasps require fungi for larval development (Klepzig et al. Reference Klepzig, Adams, Handelsman and Raffa2009). In other cases, such as Dutch elm disease, pathogens rely upon insects for long distance dispersal to host trees (Brasier Reference Brasier2001). Within the native ranges of host trees, co-evolved symbiotic relationships between insects and fungi have independently arisen multiple times (Six and Klepzig Reference Six and Klepzig2004; Klepzig et al. Reference Klepzig, Adams, Handelsman and Raffa2009). Typically damage is restricted to unhealthy trees, where the fungal symbiont has a role in overcoming defensive compounds that benefit the insect by concentrating nutrients that are made available to their larvae (Hulcr and Dunn Reference Hulcr and Dunn2011). Mycangia are structures of adult insects that carry the inoculum of symbiotic fungi for transmission to the tree host during insect attack and oviposition; these structures demonstrate the close evolutionary relationship between insects and fungi (Six and Klepzig Reference Six and Klepzig2004). Other mechanisms by which fungi are vectored by insects are within exoskeleton cavities or even on the surface of phoretic mites (Acari) that are associated with bark beetles (Coleoptera: Curculionidae) (Levieux et al. Reference Levieux, Lieutier, Moser and Perry1989; Hofstetter and Moser Reference Hofstetter and Moser2014). Upon inoculation, trees become colonised by these fungi that either provide nutrition to developing larvae, or are pathogenic to the host tree, or both.
Over the past century, through the increased movement of pests that has occurred via international trade, multiple urban and forest tree populations have been devastated by invasive pests, many of which are not problematic within their native range. Some of these invasive pests consist of a symbiotic complex that requires the introduction of both partners for effective establishment. Emerging threats to trees are new symbioses that have developed through the introduction of an insect or pathogen and the opportunistic development of a new symbiotic relationship with a native partner (Wooding et al. Reference Wooding, Wingfield, Hurley, Garnas, de Groot and Slippers2013). This can be considered as a threat because these new relationships may result in the initiation of a new evolutionary trajectory that could potentially provide an adaptive advantage to an insect or pathogen to cause elevated levels of damage or mortality to attacked trees.
In Canada, both established and recently developed symbiotic relationships are present, and due to range expansion, naïve hosts (i.e., tree species that are not recorded to have had contact with the insects or symbionts) are being exposed to long-established symbiotic partners. With climate change and the inadvertent introduction of new species through expansion of global trade, there are new risks posed to trees from insects and pathogens separately, and also through the emergence of new symbiotic partnerships due to host range expansion (Langor et al. Reference Langor, DeHaas and Foottit2009). In this review, the mountain pine beetle (Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae)), the invasive brown spruce longhorn beetle (Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae)), Sirex woodwasps, and Dutch elm disease are reviewed as examples of emerging symbiotic relationships that are either already causing damage, or are new combinations that may pose threats, to forests or trees within the urban and forested landscapes of Canada. These examples also all illustrate evolutionary mechanisms, such as vectoring of multiple fungal symbionts to facilitate adaptation during range expansion, fungal symbiont exchange between native and exotic vectors, the ability of pathogens to be vectored by closely related bark beetles, and the potential for interspecific hybridisation between native and exotic species. These outcomes may initiate new evolutionary trajectories and although they are less obvious than sudden tree mortality that can occur as a result of the establishment of non-native species, they provide further impetus for the actions of regulatory agencies that are responsible for the exclusion of exotic organisms.
Native insect-fungus symbioses
In Canada, the native mountain pine beetle has caused extensive damage to millions of ha of lodgepole pine (Pinus contorta var. latifolia Engelmann; Pinaceae) forest within its historical range in British Columbia (Safranyik et al. Reference Safranyik, Carroll, Régnière, Langor, Riel and Shore2010). In approximately 2002 the range of the mountain pine beetle expanded across the Rocky Mountains into northeastern British Columbia, likely via mass airborne dispersal (de la Giroday et al. Reference de la Giroday, Carroll and Aukema2012), and by 2006 the beetle had advanced eastward into the lodgepole pine×jack pine (Pinus banksiana Lambert (Pinaceae)) hybrid zone of boreal Alberta (Rice and Langor Reference Rice and Langor2009). The insect has since been confirmed to attack pure jack pine outside of the hybrid zone in Alberta, and it is considered a serious threat to jack pine across the boreal forest (Cullingham et al. Reference Cullingham, Cooke, Dang, Davis, Cooke and Coltman2011). The government of the Province of Alberta has aggressively responded to the threat posed by the mountain pine beetle through management actions directed at slowing the spread of the insect as outlined in their action plans and management strategy (Alberta Environment and Parks 2007). The government of Saskatchewan has also recognised the potential impact of this pest and since 2011 it has provided funds directly to the Province of Alberta to manage mountain pine beetle in eastern Alberta (Government of Saskatchewan 2015).
As demonstrated in vitro by Myrholm and Langor (Reference Myrholm and Langor2016), ophiostomatalean fungi are critical for larval development of D. ponderosae, and optimal larval development may be achieved with a specific fungal associate. Although a specific fungal associate may provide optimal larval nutrition and growth, the probability of insect survival is increased through the ability of the insect to vector multiple fungi. Three Ophiostomatales (Fungi) are vectored by D. ponderosae: Grosmannia clavigera (Robinson-Jeffry and Davidson) Zipfel, de Beer, and Wingfield; Ophiostoma montium (Rumbold) von Arx (Bleiker and Six Reference Bleiker and Six2009); and Leptographium longiclavatum Lee, Kim, and Breuil (Lee et al. Reference Lee, Kim and Breuil2005). As range expansion occurs, the ability to shift fungal associates to an optimal symbiont mixture is a mechanism that can facilitate survival in new environments on a new host. This was illustrated by Roe et al. (Reference Roe, James, Rice, Cooke and Sperling2011), who observed that the relative abundance of L. longiclavatum increased with latitude within D. ponderosae populations in Alberta and British Columbia, Canada. This is likely due to the lower optimal growth temperature of L. longiclavatum relative to other symbionts hosted by D. ponderosae, leading to competitive exclusion of the other symbionts (Roe et al. Reference Roe, James, Rice, Cooke and Sperling2011). Another hypothesis is that interspecific competition between the fungal associates based on fungal virulence to pine is a mechanism that may explain the differences in the abundance of the fungal associates; G. clavigera was found to be more virulent on lodgepole pine and lodgepole pine×jack pine hybrids than O. montium but there was no difference between the two fungi on jack pine (Rice et al. Reference Rice, Thormann and Langor2007a). A second inoculation study that included all three ophiostomatalean fungi found comparable virulence between all fungi and that lodgepole pine was less susceptible to the fungi than the hybrids with jack pine and pure jack pine, as assessed by lesion length (Rice et al. Reference Rice, Thormann and Langor2007b).
The eastward range expansion of D. ponderosae into the boreal forest of Canada represents a risk to forestry and the socioeconomic benefits associated with the forest industry (Safranyik et al. Reference Safranyik, Carroll, Régnière, Langor, Riel and Shore2010), and the ecosystem services provided by healthy jack pine forests. Range expansion into the boreal forest is a complex process that is mediated by abiotic conditions (climate), host availability, and the symbiotic relationship between D. ponderosae and the fungal symbionts that are necessary for larval development. The factors that drive mountain pine beetle outbreaks are well understood within its native range, but with the arrival of the insect and its fungal symbionts in the boreal forest, there is a level of uncertainty that exists making it difficult to predict exactly how devastating the beetle will be in this naïve system. Investigations in Alberta have shown that the beetle arrived in the boreal forest with its fungal symbionts; therefore, jack pine will be exposed to the insect in combination with its fungal symbionts for the first time in recorded history, providing an opportunity for researchers to study evolution in action.
Invasive insects with symbiotic partners: brown spruce longhorn beetle
Brown spruce longhorn beetle, Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae), an insect native to Europe where it infests declining Norway spruce (Picea abies (Linnaeus) Karst. (Pinaceae)), was confirmed to be present on red spruce (Picea rubens Sargent) in Point Pleasant Park, Halifax, Nova Scotia, Canada in 1999, in close proximity to a shipping container port. Re-examination of samples collected in 1990 indicated that the beetle has been present there since at least 1990 (Smith and Hurley Reference Smith and Hurley2000; Harrison and Smith Reference Harrison and Smith2013). When fungi were isolated from trees that were infested by T. fuscum, three species of ophiostomatalean fungi were commonly isolated: Ophiostoma piceae (Münch) Sydow and Sydow (Jacobs et al. Reference Jacobs, Seifert, Harrison and Hiristits2003), Pesotum fragrans Mathiesen-Käärik (Jacobs et al. Reference Jacobs, Seifert, Harrison and Hiristits2003; Jacobs and Seifert Reference Jacobs and Seifert2004a), and Ophiostoma tetropii Mathiesen (Jacobs et al. Reference Jacobs, Seifert, Harrison and Hiristits2003; Jacobs and Seifert Reference Jacobs and Seifert2004b), but only O. tetropii was strictly associated with P. rubens infested by T. fuscum (Jacobs et al. Reference Jacobs, Seifert, Harrison and Hiristits2003). Following the identification of T. fuscum in 1999, the Canadian Food Inspection Agency initiated an eradication campaign that was in place until 2006; in 2007 the campaign was shifted to a containment programme to slow the spread of the insect (Canadian Food Inspection Agency 2014). The exotic T. fuscum is morphologically similar to the native Tetropium cinnamopterum Kirby and both insects use the same mating pheromone, allowing trapping surveys to be conducted. In surveys conducted between 2008 and 2010, Rhainds et al. (Reference Rhainds, MacKinnon, Porter, Sweeney and Silk2011) determined that T. fuscum was relatively rare compared with T. cinnamopterum beyond 80 km from the site of introduction. The beetle is able to disperse via flight and by human-assisted dispersal through the movement of infested spruce products; therefore, the movement of high-risk forest products is restricted by ministerial order (Canadian Food Inspection Agency 2014). Further research on host preference by T. fuscum has shown that the beetle prefers stressed trees over healthy trees (Flaherty et al. Reference Flaherty, Sweeney, Pureswaran and Quiring2011, Reference Flaherty, Quiring, Pureswaran and Sweeny2013), thus lowering the risk profile associated with healthy red spruce, and this has had a bearing on the decision of Canadian Food Inspection Agency to shift to a containment strategy (Canadian Food Inspection Agency 2014).
The role of O. tetropii in the symbiosis in Canada has not been studied with the same level of detail as T. fuscum, although Jacobs et al. (Reference Jacobs, Seifert, Harrison and Hiristits2003) suggest that O. tetropii is not pathogenic to P. rubens. Inoculation studies of P. rubens seedlings and larger trees within the regulated area, using O. piceae and O. tetropii indicated that the native O. piceae is more aggressive than O. tetropii (Harrison and Smith Reference Harrison and Smith2013). In Europe, artificial inoculation of juvenile (14±1.4 cm diameter at breast height) Norway spruce, growing under natural conditions, with O. tetropii indicated that the fungus is pathogenic to Norway spruce (Sallé et al. Reference Sallé, Monclus, Yart, Garcia, Romary and Lieutier2005).
There is some uncertainty with respect to the historical presence of O. tetropii in Canada. The fungus was reported as present in the Muskoka region of Ontario in 1968 (Griffin Reference Griffin1968), although this could not be confirmed since the herbarium material was overgrown by an Aspergillus Micheli (Trichocomaceae) species (Jacobs and Seifert Reference Jacobs and Seifert2004b). In their initial study, Jacobs et al. (Reference Jacobs, Seifert, Harrison and Hiristits2003) tentatively suggested that O. tetropii was introduced into Nova Scotia with T. fuscum and in 2004 they confirmed the presence of O. tetropii in New York State, United States of America, associated with the spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae) (Jacobs and Seifert Reference Jacobs and Seifert2004b). It is possible that a population of O. tetropii was introduced into Nova Scotia with the introduction of T. fuscum from Europe and that another population of O. tetropii was also already present in North America and associated with D. rufipennis. A population genetics study of O. tetropii in North America would shed light on the relationship between the potentially introduced and native populations of this fungus and potential gene flow between the introduced and native populations. In their 2003 study, Jacobs et al. (Reference Jacobs, Seifert, Harrison and Hiristits2003) did not find O. tetropii in trees that were infested by the native T. cinnamopterum. It would be interesting to re-examine the fungal associates of the introduced T. fuscum and the native species of Tetropium Kirby in Canada, including T. cinnamopterum, throughout the range of T. cinnamopterum to assess the plasticity of the fungal symbionts and to answer the question “Can O. tetropii be vectored by T. cinnamopterum, and which ophiostomatalean fungi are now vectored by T. fuscum in Canada?”.
Tetropium fuscum has not dispersed with the same speed or impact as other invasive insects, such as the emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae). Within the city of Halifax, the initial eradication campaign severely impacted local stands of P. rubens; however, on a national scale the impact of T. fuscum and O. tetropii have not been as great as that of other invasive insects or fungi. The baseline studies that have been conducted on this system provide a solid foundation for future studies within this system that may generate knowledge that can be applied to other symbiotic invasive insects and fungi. This insect-fungus system should therefore be retained as a research focus and potential model system. Understanding invasion biology and the characteristics of these systems provides opportunities to develop strategies that can be put into place to prevent the establishment of other exotic species in the future.
Invasive insects with symbiotic partners: Sirex woodwasp
The European woodwasp, Sirex noctilio (Fabricius) (Hymenoptera: Siricidae), which attacks species of Pinus, was first discovered in southeastern Ontario in 2005 during a trapping survey that was conducted in response to the discovery of the woodwasp in New York State, United States of America, in 2004 (de Groot et al. Reference de Groot, Nystrom and Scarr2006). In Europe, where S. noctilio is native, it does not attack healthy trees (Ayres et al. Reference Ayres, Pena, Lombardo and Lombardero2014); however, in exotic locations S. noctilio is capable of attacking vigorous trees following establishment (Francke-Grosmann Reference Francke-Grosmann1963). Thus, S. noctilio is a species with a demonstrated ability to be invasive as it was established in New Zealand, Australia, South Africa, Uruguay, Argentina, Brazil, and most recently in North America (Ayres et al. Reference Ayres, Pena, Lombardo and Lombardero2014).
The fungus most often associated with S. noctilio is the basidiomycete Amylostereum areolatum (Chaillet ex Fries) Boidin (Amylostereaceae), which is carried by females in internal mycangia near the base of the ovipositor. This ensures inoculation of the fungus into the wood during the oviposition process, which creates a small wound in the bark (Slippers et al. Reference Slippers, Coutinho, Wingfield and Wingfield2003; Bergeron et al. Reference Bergeron, Leal, Foord, Ross, Davis and Slippers2011). The fungus plays an important role in the lifecycle of the woodwasp through decomposition of wood, which provides a nutritional resource and habitat to the larval stage (Gilmour Reference Gilmour1965; Boissin et al. Reference Boissin, Hurley, Wingfield, Vasaitis, Stenlid and Davis2012). The discovery of A. areolatum in Canada presented an opportunity to use population genetic tools to investigate the potential origin of the invasive S. noctilio-A. areolatum complex (Bergeron et al. Reference Bergeron, Leal, Foord, Ross, Davis and Slippers2011), which can inform decision makers about potential invasion pathways. In their assessment of the genetic diversity of A. areolatum, Bergeron et al. (Reference Bergeron, Leal, Foord, Ross, Davis and Slippers2011) discovered that the genetic variation in Canada is very low, with only two multilocus genotypes present here. One of these genotypes was shared with the southern hemisphere, while the other genotype, which composed 74% of the population, was unique to North America and of unknown origin.
Several species of Sirex are present in North America, including S. edwardsii Brullé (Bergeron et al. Reference Bergeron, Leal, Foord, Ross, Davis and Slippers2011) and S. nigricornis Fabricius (Wooding et al. Reference Wooding, Wingfield, Hurley, Garnas, de Groot and Slippers2013), which are associated with the basidiomycete Amylostereum chailletii (Persoon) Boidin (Bergeron et al. Reference Bergeron, Leal, Foord, Ross, Davis and Slippers2011). The natural history of the North American native species has not been studied as thoroughly as that of S. noctilio, although they are considered to be similar in many respects (Nielsen et al. Reference Nielsen, Williams and Hajek2009), including their preference for weakened trees (Stillwell Reference Stillwell1966). The establishment of an exotic insect and its fungal symbiont within the range of closely related native species has facilitated interactions between native and exotic species and their respective symbiotic partners.
The nature of the associations between S. noctilio and A. areolatum and the native S. nigricornis and A. chailletii has recently been studied, and it was discovered that symbiont switching can occur (Wooding et al. Reference Wooding, Wingfield, Hurley, Garnas, de Groot and Slippers2013) with A. areolatum sometimes in association with S. nigricornis and A. chailletii with S. noctilio. In a separate study, Nielsen et al. (Reference Nielsen, Williams and Hajek2009) found A. areolatum and A. chailletii within the mycangia of different Sirex species emerging from the same section of a tree. Symbiont sharing has important implications for the natural history of invasive organisms. Through switching symbiotic partners, an adaptive advantage may occur, resulting in increased virulence and subsequent damage to infected trees (Wooding et al. Reference Wooding, Wingfield, Hurley, Garnas, de Groot and Slippers2013). Another potential consequence of closely related fungal species growing within close proximity is the development of interspecific hybrids that can result in the generation of new species with altered virulence (Brasier Reference Brasier2001). The obvious threat posed by the establishment of the S. noctilio-A.areolatum complex in Canada is that it will behave as it has in other regions of the world; the more insidious threat is that symbiont switching or genetic recombination will create new genotypes that could facilitate greater impacts than anticipated.
Fungi that cause tree disease
Another type of symbiotic relationship occurs when insects serve as the long distance dispersal vectors for pathogenic fungi. This is particularly important because some pathogens, such as the Dutch elm disease pathogens, Ophiostoma ulmi (Buisman) Nannfeldt (Ophiostomataceae), and the more aggressive Ophiostoma novo-ulmi Brasier, that cause a lethal vascular wilt disease of elm, can be vectored by multiple species of bark beetles (Coleoptera: Culrculionidae: Scolytinae).
Dutch elm disease was first discovered in Canada in the province of Québec in 1944. The importation of elm nursery material and lumber from Europe was prohibited in 1928 and from the United States of America in 1934, and it is hypothesised that the pathogen was introduced into Sorel, Québec, as early as 1940, possibly on infected elm wood crate material that was used during the importation of machinery from Europe at the outbreak of World War II (Pomerleau Reference Pomerleau1961). The pathogen has also likely expanded its range into Canada from the United States of America through spread into southern Ontario from New York, Ohio, or Michigan, into New Brunswick from the Houlton area of Maine, and into Manitoba from the Red River area of Minnesota (Campana and Stipes Reference Campana and Stipes1981). In Canada, the native North American elm bark beetle, Hylurgopinus rufipes Eichhoff (Coleoptera: Curculionidae), can vector the disease, as can the introduced European elm bark beetle, Scolytus multistriatus Marsham (Coleoptera: Curculionidae) (Campana and Stipes Reference Campana and Stipes1981; Allen and Humble Reference Allen and Humble2002). The fact that a native insect vectors this exotic pathogen was likely an important factor in the establishment and spread of Dutch elm disease in Canada (Allen and Humble Reference Allen and Humble2002; Humble and Allen Reference Humble and Allen2006). An emerging risk for this disease is the potential new vector, Scolytus schevyrewi Semenov (Coleoptera: Curculionidae), which was introduced from Asia and is established in Colorado, United States of America (Loo Reference Loo2009). It was first detected in southern Alberta in 2006 (Langor et al. Reference Langor, DeHaas and Foottit2009), and it is now considered to be established in the prairie provinces (Veilleux Reference Veilleux2012).
The impact of Dutch elm disease in Canada has been extensive as all elm trees (Ulmus Linnaeus; Ulmaceae) in North America are susceptible to the disease (Campana and Stipes Reference Campana and Stipes1981). The American elm, Ulmus americana Linnaeus, which is widely planted in urban environments, has been particularly affected by the disease, and major Canadian cities such as Toronto, Ontario have lost a significant percentage of their elms (Hubbes Reference Hubbes1999). Management of Dutch elm disease has been a priority for cities such as Winnipeg, Manitoba, which lost more than 21 000 trees between 2009 and 2013 (Hintz et al. Reference Hintz, Carneiro, Kassatenko, Varga and James2013); the province of Saskatchewan has invoked regulations to prevent spread of the disease (Anonymous 2005); however, the disease is present in Saskatchewan and it threatens Alberta (Hubbes Reference Hubbes1999).
Although it is very unfortunate that Dutch elm disease was introduced into Canada, there have been two distinct benefits associated with its introduction: First, as a high priority pathogen, several important research programmes were launched in Canada to investigate topics such as population genetics of invasive pathogens (i.e., Hintz et al. Reference Hintz, Jeng, Yang, Hubbes and Horgen1993; Temple et al. Reference Temple, Pines and Hintz2006), pathogen gene expression and infection processes (i.e., Temple et al. Reference Temple, Horgen, Bernier and Hintz1997; Et-Touil et al. Reference Et-Touil, Rioux, Mathieu and Bernier2005), and alternative pathogen management strategies (i.e., Hubbes Reference Hubbes1999; Temple et al. Reference Temple, Pines and Hintz2006; Hintz et al. Reference Hintz, Carneiro, Kassatenko, Varga and James2013). Second, public awareness of the impacts of invasive alien species has been raised through the outreach activities of non-governmental agencies such as the Society to Prevent Dutch Elm Disease in Alberta, which promotes healthy urban forests. These organisations have served to raise the profile of invasive alien species in the urban environment through programmes to slow the spread of Dutch elm disease. Also, the networks and resources that these programmes have developed could be used to raise the awareness of other devastating invasive species, such as the emerald ash borer, which is currently present only in Ontario and Québec. If we can learn from this invasive organism, using techniques such as retrospective analysis, as conducted in the United Kingdom where researchers studied the Dutch elm disease outbreak of the 1970s to inform contemporary biosecurity policy (Potter et al. Reference Potter, Harwood, Knight and Tomlinson2011), and if we use existing outreach networks to prevent the spread of other invasive pests, perhaps other species of trees in Canada will have a better chance of survival.
Another instance of the introduction of Dutch elm disease occurred in New Zealand where exotic Ulmus procera Salisbury were discovered to be infected by O. novo-ulmi in 1990. Inspection of the diseased material revealed the presence of the insect vector Scolytus multistriatus. Surveys in Auckland discovered S. multistratus without O. novo-ulmi outside of the area affected by O. novo-ulmi, indicating that the bark beetle had been introduced before the pathogen and that two separate introductions occurred (Gadgil et al. Reference Gadgil, Bulman, Dick, Bain and Dunn2000). The ability of O. novo-ulmi to be vectored by multiple bark beetles, the fact that it likely spread widely in Canada on a bark beetle that is native to North America, and its ability to re-associate with its European vector after independent establishment of the vector and the pathogen in New Zealand, illustrate the risks associated with novel vector-pathogen relationships. These facts clearly indicate that there are multiple mechanisms that can facilitate the establishment and long distance dispersal of exotic pathogens, which must be considered for effective regulatory policy.
Canadian forests are not unique in experiencing damage from invasive organisms. Within many countries, forests and the ecosystem services that they provide have been negatively impacted by non-native organisms that have become established and spread following their arrival and the body of literature related to invasive organisms and their impacts is extensive (e.g., Liebhold et al. Reference Liebhold, MacDonald, Bergdahl and Mastro1995; Boyd et al. Reference Boyd, Freer-Smith, Gilligan and Godfray2013). Globalisation and increased trade have facilitated the international movement of insects and pathogens. Even 63 years after the formation of the International Plant Protection Convention in 1952 and the development of a set of International Standards for Phytosanitary Measures, the number of incursions of exotic organisms continues to rise (Evans Reference Evans2010). National quarantine policies and systems have been enacted by many countries to prevent the introduction of exotic insects and pathogens, yet the volume of trade and the impracticality of 100% inspection of imported goods has permitted the introduction of exotic organisms. The trade in live plants (Liebhold et al. Reference Liebhold, Brockerhoff, Garrett, Parke and Britton2012) and solid wood packaging material (Humble Reference Humble2010) have been recognised as two important invasion pathways for exotic insects and pathogens. The management of pathways, rather than individual high-risk organisms, leading to the strategy of “manage once, remove many”, can theoretically manage a larger number of species, including unknown and potentially damaging species, in an effective manner (Evans Reference Evans2010). The economic costs of eradication, if feasible, or ongoing management to mitigate losses from invasive insects or pathogens, are high and often borne by taxpayers (Hantula et al. Reference Hantula, Müller and Uusivuori2014). The evolutionary events triggered by the establishment of invasive organisms, such as new host associations and hybridisation events (Wingfield et al. Reference Wingfield, Slippers and Wingfield2010) and illustrated by the examples in this paper, demonstrate the adaptive capacity of exotic pests to respond to new environments. The economic costs and biological consequences of the establishment of exotic pests are high and mitigation of these impacts requires commitment from regulatory agencies that is supported by a solid foundation based in science and effective international cooperation.
These examples of both native and invasive insects and their symbionts in Canada illustrate the potential damage that can occur when an organism becomes established in a new region. They also demonstrate the flexibility and adaptive capacity that is inherent within symbiotic relationships that have allowed the survival of insects, such as the mountain pine beetle or the brown spruce longhorn beetle, in new environments. The example of symbiont exchange following invasion, as illustrated by Sirex woodwasps, clearly demonstrates how human actions can have potentially long-term evolutionary implications. The Dutch elm disease example illustrates the fact that exotic forest pathogens can be vectored by another species of insect that share the niche with its traditional symbiotic partner.
In the context of Dutch elm disease control, Campana and Stipes (Reference Campana and Stipes1981) stated that control of the disease is “At best limited, always expensive, always difficult, often frustrating, at times hopeless and never well understood”. This is because of the complex interactions that are involved between the tree host, the environment, the disease and the insect (Campana and Stipes Reference Campana and Stipes1981). Indeed, this sentiment can be extended to all invasive insect and pathogen combinations that have developed a new symbiotic relationship in a new environment where, as stated by Brasier (Reference Brasier2008) in the context of invasive plant pathogens, “Each imported pathogen is therefore an uncontrolled, potentially dangerous, open-ended experiment in evolution”.
Another direct consequence of the introduction of plant pathogenic fungi is the potential for interspecific hybridisation, which may result in hybrids with increased virulence (Brasier Reference Brasier2001). This type of hybridisation can occur when two closely related species are brought together through anthropogenic actions (e.g., inadvertent long distance dispersal facilitated by international trade) that place these species geographically close enough together. The introduction of exotic insects may also lead to interspecific hybridisation; as determined by Silk et al. (Reference Silk, Sweeney, Wu, Price, Gutowski and Kettela2007) and used by Rhainds et al. (Reference Rhainds, MacKinnon, Porter, Sweeney and Silk2011), the exotic T. fuscum and the native T. cinnamopterum share a common sex/aggregation pheromone. This leads to the question “What is the potential for intraspecific hybridisation between these species that respond to the same pheromone?”.
Range expansion by native and exotic organisms has resulted in the death of a large number of trees in Canada over an evolutionarily short time period. The long-term consequences of these human-mediated introductions, and the new associations and evolutionary trajectories that have been initiated through symbiont exchange and new vector relationships are even more of a concern. The evolutionary consequences facilitated by these processes are complex and the impacts on tree hosts are difficult to predict, but continued study of these systems has the potential to generate significant knowledge as we witness evolutionary processes unfolding before us. Efforts to prevent the introduction of additional exotic organisms into Canada through international trade are expensive in human terms, but these expenses need to be balanced by the impacts on ecosystem services and societal well-being that are afforded by healthy trees.
Funding for this manuscript was provided by the Canadian Forest Service. This manuscript was strengthened through reviews provided by David Langor, René Alfaro, and two anonymous reviewers.