Diseases that are 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 (DED), have been extensively researched, whereas others are poorly understood; significant data gaps exist for the ecology, epidemiology, and management of most of these diseases. The emergence and unexpected importance of these problems are discussed. An underlying factor in most of the interactions is the absence of a coevolved history between the so-called “naive” 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 during this session as “black swan events.”
