Gypsy moth eggs overwinter and hatch during warm weather in spring, mostly soon after oaks begin to leaf out, but a few days later than many native spring caterpillars do. There is only a single annual generation in all parts of the range. First instar larvae usually disperse via the wind on warm, sunny days and commence feeding once they find an acceptable plant. The great majority of hatching is probably in a period of about a week (Schweitzer, pers. obs.; Doane and McManus, 1981, p 184-6), but it can spread over a month with egg masses in the sunniest places hatching earlier. Larval development is slower than most other spring feeding caterpillars (Schweitzer, pers. obs.) such as cankerworms (Alsophila, Bistonini), Malacosoma, Xylenini, most Catocala, spring Lycaenidae, Tortricidae. Most larvae hatch about early May and mature in late June or July in southern New England. Larval growth is substantially asynchronous, due largely to the variety of plants eaten, and a few larvae may still be found in late July. The pre-pupal and pupal stages combined last about 2 weeks, so moths are generally seen at least throughout July and early August in southern New England. All stages occur earlier southward (depending on altitude) with both sexes often near peak by the end of June in Cumberland and Cape May Counties, New Jersey. Males were well represented in blacklight samples from the State College, Pennsylvania pine barrens collected in September 1985 (Schweitzer, pers. obs.). Females usually lay their eggs in one mass on their first or second day and soon die. Females usually mate only once, and since males can mate several times and live a few days, males are effectively in surplus supply. Depending largely on the nutritional quality of larval food females lay from about 100 to1000 eggs. Fecundity is much lower in outbreaking populations, especially in areas of heavy defoliation, than at low densities.
Egg masses are generally within a few meters of the female's cocoon. They are often placed on tree trunks or the underside of limbs. They may be placed in bark furrows or behind loose or shaggy bark if available. They are also often laid on boulders, outdoor furniture, in or on outbuildings etc. Most are less than 5 meters off the ground. While most are somewhat sheltered many are not. Egg masses are generally fairly easy to find and egg mass counts are used to index the outbreak potential. Roughly a count of 500 or more per acre indicates a high potential for an outbreak that spring, unless they are small due to previous defoliation stress on the larvae. Be careful to count only current year egg masses.