The gypsy moth overwinters in egg masses attached to the bark of trees. The egg masses, usually about the size of a one dollar coin, are buff to tan in colour and may contain from 100 to 1,000 eggs. The severity of the infestation can be determined by the size of the egg mass. Egg masses tend to be smaller, about the size of a dime, when populations are on the decline. Larger eggs masses are a sign of stable or growing populations.

The eggs hatch into caterpillars (larval stage) when tree buds begin to open. This stage, lasting up to seven weeks, is when the insect feeds. It is, therefore, important to control gypsy moth infestations early in the growing season. Feeding ends by early July.

The pupal stage occurs after feeding and lasts 9-17 days. Pupal cases can be found in the same places as the egg masses. Adult moths appear in late July or August.