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Destructive Tree Diseases Associated with Ambrosia and Bark Beetles: Black Swan Events in Tree Pathology?

Diseases associated with ambrosia and bark beetles comprise some of the most significant problems that have emerged on trees in the last century. They are caused by fungi in the Ophiostomatales, Microascales, and Hypocreales, and have vectors in the Scolytinae (ambrosia and bark beetles) and Platypodinae (ambrosia beetles) subfamilies of the Curculionidae (Coleoptera). Some of these problems, such as Dutch elm disease, have a long history, have been extensively researched, and are fairly well understood. In contrast, other similar diseases developed recently and are poorly or partially understood. The emergence and unexpected importance of these tree diseases are discussed in this article. An underlying factor in most of these interactions is the absence of a coevolved history between the so-called “naïve” or “new encounter” host trees and the pathogens and/or beetles. For the ambrosia beetles, these interactions are associated with susceptibility to what are typically benign fungi and atypical relationships with healthy trees (ambrosia beetles favor trees that are dead or stressed). Interestingly, the pathogens for both the ambrosia and bark beetle–associated diseases often have symbiotic relationships with the insects that are not based on phytopathogenicity. Some of the most alarming and damaging of these diseases are considered “black swan events”. Black swan developed as a metaphor for a supposed impossibility that is contradicted with new information. Today, Black Swan Theory focuses on unexpected events of large magnitude and consequence.