Most of the yearly cycle of gypsy moth is spent in the egg stage. Although weather can influence the exact time of egg laying and spring hatch, the egg stage in Michigan usually runs from August to April. There is only one generation of gypsy moth per year. Most eggs are laid within 24 hours of mating, but the process may take several days. Because the female cannot fly, the egg mass is usually laid within a few feet of the female pupa casing. The eggs are covered by a dense coating of hairs from the abdomen of the female. As a result, the egg mass in the late summer and early fall appears to be a dark tan to light brown. As the eggs mass weathers, it looks more light tan to gray in color. An individual egg mass may contain anywhere from 100 to 1,200 eggs. Healthy populations can average 750 eggs per mass while unhealthy populations average 300 eggs per mass.

Hatch of the larvae is influenced by spring warming trends and, once started, most eggs hatch within a week. However, egg masses in cool, shaded areas, higher elevations or in close proximity to the Great Lakes can hatch over an extended period of time. The larval stage occurs in early May into July. Larvae are approximately 1/8 inch at hatch and end up about 2 to 3 inches long before pupation. Newly hatched larvae will sit on an egg mass in wet, cool weather. In some cases the egg mass is completely covered with the small bodies of hundreds of larvae. Eventually they move up into the tree producing a thin line of silk behind them. Wind picks up the thread and young larvae and moves them to another location. This process is called ‘ballooning’ and is responsible for the natural spread of the gypsy moth.

Young larvae feed during the day, mainly in the morning and then again in the late afternoon. When resting, the young larvae can be found on the underside of the leaf along the leaf’s mid-rib. As they grow older, feeding becomes a nocturnal activity and they rest during the day. This results in daily migration up the tree as evening approaches and down the tree in the early morning. The migration involves the search for a cool, shaded location in which to spend the heat of the day.

Larvae are referred to by their instar or periods between molts. A first instar larva is newly hatched. A second instar larva has gone through one molt, a third instar larvae has completed two molts and so on. Males usually have 5 instars, 4 molts, while females complete 6 instars or 5 molts. This extra feeding time is needed by the female for the nutrient demanding process of producing eggs.

When the larval stage is completed the caterpillar sheds its last larval ‘skin’ and within an hour the pupa turns from white with a greenish cast to a brown, tear drop shaped structure. This structure is attached to whatever the larvae was on at the time pupation began. The process of changing into a moth takes approximately 2 weeks (16 – 17 days for the female). In Michigan pupation occurs in July and early August.

The adult emerges in Mid-July to early August and contains mature eggs or sperm so mating can occur immediately. The male is a mottled brown color with black wing markings. The female is white to cream colored with black markings. One other distinctive characteristic is the broad, feathery antennae of the male. These antennae are used to locate the female moth, which gives off a chemical, the sex pheromone, to aid the male in finding the female and thus assisting in reproduction. The adult form does not feed and usually lives only about a week.