A comparison of the seasonal biology of Thymelicus lineola at Normandin and Joliette, Quebec from 1974 to 1976 showed marked differences between regions for the same year, and within regions for consecutive years. These data together with constant temperature development information were used to develop a predictive phenological model for this insect, which was then validated using independent data sets collected between 1976 and 1981. Good agreement between observed and predicted values were obtained. The usefulness of the model in predicting optimal spraying dates was tested at Amqui and Normandin, Quebec in 1982. Accurate predictions were obtained at least 2 weeks in advance, sufficient for use in a management programme.