Eggs hatch in about two weeks, depending upon temperature. Females lay an average of 100 eggs each. Nymphs are pale and wingless, with red eyes. They usually remain on the undersides of leaves and are quite active, often running sideways. With heavy infestations, leaves become yellow or brown, and drop.
Numbers are highest late in the summer, especially on end vines and border rows.
However, high populations are required to reduce fruit quality. Researchers in New York have created a conservative provisional action threshold of five nymphs per leaf before August 1, and ten nymphs per leaf thereafter. See the linked fact sheets on grape leafhopper from California and Colorado State . See monograph on leafhoppers on grape.