Evolving symbioses between insects and fungi that kill trees in Canada: new threats associated with invasive organisms

Abstract

Abstract-Symbiotic relationships between insects and fungi are known to cause tree mortality either through direct damage by larval feeding that can be facilitated by symbiotic fungi, or through insects vectoring pathogens directly to healthy trees. Within their native ranges, the impacts of many insect-fungus symbioses are restricted to weakened and declining trees; however, within the last century tree mortality caused by globally invasive insect-fungus associations has had a devastating impact on trees in both urban and natural forest ecosystems. Unfortunately, Canadian forests have been seriously affected by invasive organisms and an emerging threat is the expansion of a native bark beetle into the boreal forest of Alberta. This paper reviews the symbiotic relationships between selected invasive insects and pathogens that cause tree mortality within the urban and forested landscapes of Canada; it uses these case studies to illustrate potentially damaging new evolutionary trajectories.

Pathologists and entomologists must join forces against forest pest and pathogen invasions

“…Most alien insect pests and pathogens that cause damage in invaded areas were not known as causes of damage, or even described, in their area of origin (Roques et al 2015;Burgess and Wingfield 2015). Moreover, many insects and fungi can hardly be identified at species level on the basis of morphology alone, making it difficult to distinguish a potential introduced organism from a closely-related native species, as exemplified by Hymenoscyphus fraxineus, the causal agent of ash dieback (Gross et al 2014) or the brown spruce longhorn beetle (Tetropium fuscum) which was not recognised as an exotic in Canada, because of morphological similarity to the native Tetropium cinnamopterum (Ramsfield 2016). It is, therefore, essential to develop molecular tools that will allow detection and identification of potentially invasive alien species to be able to set up measures to eradicate them at an early stage (McTaggart et al 2016).…”

“…Insect infestation can predispose trees to attack by fungal pathogens, increasing damage caused by the pathogens and enabling weaker pathogens to attack hosts (Xi et al 2018). Some forest insects are known to carry various species of fungi that they use as symbionts for larval development (Ramsfield 2016), "cultivate" and use for food in the galleries they form in the tree (e.g. ambrosia beetles) or to overcome the induced defences of colonised trees (e.g.…”

Atmospheric change as a driver of change in the Canadian boreal zone1

“…Mountain pine beetle (Dendroctonus ponderosae Hopkins, 1902; MPB) has recently spread eastward from British Columbia (with its fungal symbionts; Ramsfield 2016) and is now present in the western Boreal Plains, northern Montane Cordillera, and southern Boreal Cordillera of the western boreal (see fig. 10 in Langor et al 2014).…”

The contribution of genetics and genomics to understanding the ecology of the mountain pine beetle system

“…We are seeing increasing pressure on our forest systems by both native fungal and insect species in response to changes in climate (Gauthier et al 2014) and invasive pests as a result of international trade (Tobin et al 2013;Ramsfield 2016). From our experiences, the strength of this project has stemmed from co-collection of the interacting species, allowing us to develop a multi-taxa approach to understanding both individual species parameters and interactions among the species.…”

“…Moving forward, a better understanding of Dutch elm disease (Ophiostoma ulmi (Buisman) Nannfeldt (Ophiostomataceae), Ophiostoma novo-ulmi Brasier) in Canada could be gained from a genetic-genomic approach. Genetics could address questions regarding the role that the two insects (native elm bark beetle (Hylurgopinus rufipes Eichhoff) and an invasive bark beetle (Scolytus schevyrewi Semenov)) (Ramsfield 2016) play in vectoring the fungi, as well as what contributes to the spatial distribution of the fungi.…”