Where should I look for Spongy Moth (formerly Gypsy Moth) egg masses?

White oak is the preferred host for Lymantria dispar (formerly Gypsy Moth), but most other oak species (in the Northeast) are also highly susceptible, as well as many other deciduous species. This includes maple, birch, poplar, willow, apple, hawthorn, and many others. Conifers, such as pine and spruce, may also be attacked when the preferred host plants are in short supply. While many trees can survive a year of defoliation from Lymantria dispar if they are otherwise healthy, consecutive years of defoliation can highly stress host trees. These stressed plants then become more vulnerable to secondary pests, such as certain wood-boring insects and decay fungi.

Look for masses in early spring before trees fully leaf out.  Scrape off egg masses if possible especially on young trees.

Images of Egg Masses and more information on caterpillar control