Ash (Fraxinus) consists primarily of temperate, deciduous trees and shrubs, with plus or minus sixty species native to the Northern Hemisphere. Ash diversity is highest in China (twenty-two species) and the US (sixteen species). In eastern North America, six native ash species are under threat of functional extinction by an exotic insect pest, emerald ash borer (EAB; Agrilus planipennis), introduced from Asia to southeastern Michigan, probably in the 1990s. EAB adults feed on ash leaves, females lay eggs exclusively on ash, and larvae feed on cambial tissue in ash stems and trunks. There is no documented resistance to EAB among these six ash

species, and larvae commonly infest and kill healthy and stressed mature trees and juvenile saplings alike. This severely reduces opportunities for the evolution of increased tolerance to EAB and may hasten extinction.