The influence of degree–day accumulations and photoperiodic induction of diapause on the phenology ofErythroneura Comes (Say) was investigated. In growth chamber experiments, nymphs reared at photoperiods of710 degree days (DD) before 1 August are required to produce a 2nd generation ofE. comesunder climatic conditions in New York.Variationsin temperature–driven development are hypothesized to strongly influence year–to–year variability in leafhopper abundance. Degree–day accumulations by 30 June correlated well with degree–days by 1 August and may provide a useful early–warningsystem for predicting years in which leafhopper population densities are above average.