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
Fraser Filter Fraser Filter A Fraser Filter, named after Douglas Fraser, is typically used in geophysics when displaying VLF data. It is effectively the first derivative of the data. If formula_1 represents the collected data then formula_2 is the average of two values. Consider this value to be plotted between point 1 and point 2 and do the same with points 3 and 4: formula_3 If formula_4 represents the space between each station along the line then formula_5 is the Fraser Filter of those four values. Since formula_6 is constant, it can be ignored and the Fraser Filter considered to be formula_7.