A biotype of the green peach aphid.Myzus persicae(Sulzer), from Yakima, Wash. (46°34'N) produced gynoparae and males in response to daily scotophases that were 1.0–1.5 h shorter than those needed to induce the production of these morphs by a biotype ofM. persicaederived from a more southern latitude (40°N) in Spain. Predictions regarding the production of these morphs under fall conditions in their regions of origin are consistent with field observations in these regions.