Christmas tree pests and weeds of different pests, most of which cause cosmetic damage to the trees, important in the Christmas tree industry. The balsam woolly adelgid ("Adelges picae") are small soft-bodied insects which attack the Fraser fir. The insects appear as white, woolly spots on the tree and are fatal to the fir if left untreated. The woolly adeligid was native to central Europe where it affected the silver fir; it was introduced to the United States, where it has no known native predators, before 1900. Balsam twig aphids ("Mindarus abietnus") are another type of tree pest which affects fir and spruce trees in
Tree of virtues and tree of vices (c. 1310-1340). The nodes of the tree-diagrams are the Cardinal Virtues and the Cardinal Vices, respectively, each with a number of secondary virtues or secondary vices shown as leaves of the respective nodes. While on a tree of virtues, the leaves point upward toward heaven, on a tree of vices the leaves point downward toward hell. At the root of the trees, the virtues of "humilitas" "humility" and the vice of "superbia" "pride" is shown as the origin of all other virtues and vices, respectively. By this time, the concept of showing hierarchical concepts of medieval philosophy in diagrams also