Gypsy Moth caterpillars eat the leaves of shade and ornamental trees. Older caterpillars eat more leaves and can cause defoliation. There are several trees preferred by the Gypsy Moth caterpillar. Oak is the main host species, followed by aspen, beech, birch, willow and white pine. There are also trees that the Gypsy Moth caterpillars do not like to eat. Examples include ash, juniper, locust, walnut, cedar, dogwood and catalpa. When caterpillar populations are low, feeding is usually confined to preferred species. When populations are high and food becomes scarce, Gypsy Moth caterpillars will eat leaves from a much wider variety of trees, often anything in sight. When planting for the future, select plant species that are not preferred by the Gypsy Moth.

more info: http://www.ent.msu.edu/gypsyed/docs/forest.html