We had a major problem with gypsy moths in NH 25(?) years ago. Lasted about halfway through the summer. We wrapped hardwood trees with duct tape several inches wide, then put a strip of petroleum jelly in the middle. The tape was to protect the tree. The jelly was to prevent the caterpillars from climbing the tree to eat the leaves. You may have to replace jelly periodically due to melting and rain. More info:
http://www.planetnatural.com/pest-pr...-moth-control/