none The effects of weather on the spruce budworm parasitoid, Apanteles fumiferanae Vier., are examined. a phenological model based on temperature-dependent rates of development and longevity is developed and validated with field data. The model is used to explore the effects of age-specific mortality on phenological behaviour of the parasitoid and the seasonal synchrony between the parasitoid and its host. The results show that the parasitoid adult ecloses well before the host reaches an age susceptible to parasitism but that the egg maturation period and the longevity of the parasitoid diminish the consequences of the apparent asynchrony. The historical data reveal that the relative phenological characteristics of A. fumiferanae and its host vary little from year to year. In the second part of the study, temperature is shown to have to strong effect on adult parasitoid activity and on the rate of oviposition.