Seasonal flight of the lilac borer,Podosesia syringae(Harris), was monitored with pheromone traps during 1980–1982, and cumulative emergence was related to physiological time (i.e., degree-days [DD]). A base temperature of 10°C yielded the lowest coefficient of variation for DD summations to first catch, and was therefore selected for use in the linear heat unit system. First capture of lilac borer males in Kentucky ranged from 13 April to 6 May, and corresponded to a mean DD accumulation from 1 January of 168.9 in Lexington and 189.9 in Louisville. Flight in both cities peaked from late May to early June and ended by mid-July. Of three forecasting methods, average DD summations or projected DD requirements from a regression model were more reliable than average calendar dates for predicting optimum treatment dates in Louisville.