The gypsy moth, Lymantria dispar, is a devastating pest introduced into North America from its native habitat in Europe and Asia. E. Leopold Trouvelot brought European specimens into Boston in 1868 or 1869, and within a decade outbreaks of the gypsy moth began in the area. Since that time the moth's range has steadily expanded, and it can now be found in many areas of North America, though the largest concentrations are in the Appalachian Mountains, the Ozarks, and around the Great Lakes.
Gypsy moths eat the foliage of hundreds of species of plants, but its favourite foods - or hosts, as the researchers call them - seem to be oak and aspen. While smaller populations of moths do not seriously threaten forests, large populations can completely defoliate an area. A few years of defoliation can result in tree death. Attempts to wholly eradicate the gypsy moth have failed, though with careful pesticide spraying local infestations can be controlled.
I got most of this information from the extensive gypsy moth information website at
http://www.fs.fed.us/ne/morgantown/4557/gmoth/