In the 1860s, artist and amateur scientist Leopold Trouvelot hatched an ill-conceived plan to create a new type of silk—a plan that included importing gypsy moth caterpillars (Lymantria dispar) from France to America. Instead of a better textile, Trouvelot created an ecological disaster. When some of his caterpillars escaped into the surrounding Massachusetts countryside, they thrived. Free from many of their native predators, they stripped trees bare as they munched their way across an ever-expanding territory. Nearly 150 years later, gypsy moth populations are still spreading, causing significant damage to deciduous forests wherever they go. (This past summer, caterpillar-induced defoliation was so extensive across New England and the Mid-Atlantic states that it could be seen from space.) But there’s hope for the trees. A virus that causes the caterpillars to melt into piles of goo is helping to keep the gypsy moth in check—and limiting its impact on countless U.S. forests.