There is no preventive treatment to keep caterpillars from coming to your trees. Keeping trees and shrubs healthy is the first line of defense against gypsy moths. The second line of defense is making your yard less pleasant for gypsy moths. While the caterpillars will eat the leaves of hundreds of species, they do have some favorites and some that they avoid. Keep this in mind when choosing new plantings for your yard.
Caterpillars prefer oak, crab apple and apple, willow, aspen, birch, basswood, linden, and tamarack.
Caterpillars avoid ash, cedar, locust, balsam fir, pine, maple, nut trees, beech, spruce and hemlock. Color photos may help you identify the different life stages of gypsy moths. A caterpillar earlier than May or later than mid-July. An adult moth earlier than July or later than August. A white moth that flies.