The emerald ash borer, native to China, was transported to the United States in infested packing material in the late 1800s. It has since killed over 20 million ash trees across Michigan, Ohio, and Indiana by tunneling under the bark and feeding on the cambium layer. While oaks and aspens are most commonly infected, the emerald ash borer also threatens ponderosa and Coulter pine trees in western states, where tree loss from infestation is considered a normal ecological process.