A process-oriented model was developed to simulate the phenology of the western grapeleaf skeletonizer,Harrisina brilliansBarnes&McDunnough. Processes defined in the model include spring adult emergence, moth longevity, oviposition, and immature life-stage development. Model versions including discrete or distributed delays of life-stage development were compared. Model output was compared with pheromone trap count data collected in seven individual vineyards in 1988 and in three vineyards each year from 1989 through 1991. Model predictions and pheromone trap counts at 50% of cumulative catch were within 7 d of each other in 36 of 48 generational moth flight periods. Based on the level of variation in immature life-stage development that is characteristic ofH. brillians, simulation output differed little between discrete delay versus distributed delay model versions. Results indicate that model implementation has considerable potential for predicting the seasonal occurrence of damaging immature life stages of this vineyard pest.