Periodical Cicada nymphs remain between 2 and 18 inches below the surface. Warm weather may accelerate their emergence by a week or so; conversely, cold weather may delay it. Soil temperature is the determining factor; the temperature in the nymphs' exit tunnels must exceed 64 degrees F. Yes, they drink sap from tree roots, but they are not known to harm the trees. Think about it: These guys don't move around much, which means they're most likely feeding on a single tree for 13-17 years. That wouldn't be possible if the tree were harmed by it.