The ghosts in this story are eastern and Carolina hemlock trees. Hemlocks provide valuable ecosystem services in Appalachian forests, including cover for wildlife and cooling shade along waterways. But they are being killed in increasing numbers by an exotic invasive insect, the hemlock woolly adelgid. Native to Asia, the hemlock woolly adelgid is transported through forests by animals, wind and, accidentally, by people. Often called “gray ghosts” because of their pale, skeleton-like appearance, the dead hemlocks are obvious across the mountain landscape. Using a forest monitoring tool known as ForWarn, scientists are able to see just how devastating the hemlock losses have become across the southern Appalachians, where the hemlock woolly adelgid thrives in the warmer temperatures.