Description. Egg masses are light tan, and the eggs inside are black and pelletlike. Each mass may contain 400-600 eggs. The larval stage (caterpillar) is hairy, and a mature larva is 50-65 mm long with a yellow and black head. Behind the head on the thorax and abdomen are five pairs of blue spots (tubercles) followed by six pairs of brick red spots. The pupal stage is dark reddish-brown and is held in place to some object by small strands of silk. Male moths are dark buff and fly readily during the day. Females are white with black, wavy markings; they have robust abdomens and do not fly, and their wingspan can reach 5 cm.
Life Cycle. Egg masses deposited by females during July overwinter on trees, stones, and other substrates. Eggs hatch from late April through early May with most eggs hatching by mid-May. Small first instar larvae do not feed right after they hatch and can be dispersed by wind. Young larvae feed on foliage and remain on host plants night and day. In late May when about half-grown, larvae change their behavior and usually feed in the trees at night, and move down to seek shelter in bark crevices or other protected sites during the day.
Larvae reach maturity from mid-June to early July. Migrating caterpillars are often a nuisance during the last two weeks of June. Pupation takes place during late June and early July. The pupal cases may be observed attached to tree bark, stones, buildings, and other similar sites. Adults start emerging in late June with peak emergence in mid-July. This pest produces one generation a year in Pennsylvania.
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