From Now thru mid-April: Destroy egg masses.
Gypsy moths cause defoliation of trees; the caterpillars feed on more than 300 species of deciduous and evergreen trees. The female lays all her eggs in a single teardrop-shaped mass 1 – 1.5 inches long and covers it with yellowish-tan colored hairs from her own body. Each female lays only one egg mass and each egg mass can hold up to 600-1000 eggs. The egg masses are laid in August and can be found on the underside of branches, picnic tables, trailers, RV's, wood piles, swing sets.... pretty much anywhere.
You can be proactive and destroy the egg masses now to prevent them from hatching this spring. Spraying them with a horticultural oil (labeled for gypsy moth egg masses) when the temperature is above 40 degrees will suffocate the eggs so they don't hatch in April. At any temperature, you could scrape them into a container and cover them with soapy water for two days. Then just discard them in the trash. If any part of the egg mass is still on the tree, or falls onto the ground, they will survive and hatch. For more information go the Gypsy Moth Website.