Degree-day phenology models for the yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer), were developed from data sets collected in infested plantations of black spruce, Picea mariana (Mill.), and white spruce, P. glauca (Moench) Voss, in New Brunswick and Quebec, Canada, between 1995 and 1999. The models describe the relationships between degree-day accumulation (above −1 °C, from 1 April) and cumulative adult emergence, capture in pheromone traps, the dates of appearance of first adult, egg, and larva, and the relative frequency of successive larval stages. The models predict adult emergence with a precision of ±2 days and male catch in pheromone traps with a precision of ±1.6 days. The first adult, first egg, and first larva occurred after 527 ± 42, 660 ± 52, and 725 ± 18 degree-days above −1 °C, respectively, and the dates of these events are predicted within ±1.8 days. The dates of 50% occurrence of the successive instars are predicted within 4.5 days of observed dates, and the date of peak 2nd instar is predicted within ±3.6 days.