The nymphs secrete a sticky substance known as honeydew as they feed. A black sooty mold fungus forms on this honeydew and, besides its unsightly appearance, it may cause damage by interrupting the normal process of photosynthesis. On the fruit a roughening or 'russet' of the skin occurs. If they are abundant, they may also be found under leaves on the ground. Most of the eggs will have hatched by the time the flower petals fall.
Young nymphs migrate to the axils of leaf petioles and of forming fruit.
As these sites become overcrowded, the nymphs move on to the undersides of the leaves. There are three to four generations per year in most of New York State. Females of the later generations will deposit most of the eggs along the leaf midribs. One female pear psylla may deposit up to 500 eggs. Eggs in the late dormant to bud burst are found singly or in rows on spurs and twigs, or around bud scales.