In North America, the emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), continues to spread, and its egg parasitoid, Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae), is being released for emerald ash borer biocontrol well beyond their endemic climatic ranges in China. We developed a multiple cohort rate summation model to simulate O. agrili F₀, F₁, and F₂ generations, and emerald ash borer oviposition for examining host–parasitoid synchrony across a north–south gradient from Duluth, MN (latitude 46.8369, longitude –92.1833) to Shreveport, LA (latitude 32.4469, longitude –93.8242). Temporal occurrences of critical day length for O. agrili diapause induction were integrated into the model. We used O. agrili and emerald ash borer trapping data from south central and northwestern Lower Michigan for model validation. Simulations demonstrated that 1) F₀ adult emergence consistently occurred 2–5 d before emerald ash borer oviposition began; 2) F₁ adult emergence was most synchronized with peak emerald ash borer oviposition compared with other generations; and 3) emerald ash borer oviposition was complete, or near so, when F₂ adult emergence was predicted across the north–south gradient. Comparison of O. agrili trap captures with model simulations demonstrated that primarily two adult O. agrili generations (F₀ and F₁) emerged per year in Michigan and almost all F₂ larvae entered diapause despite day lengths longer than critical day length in south central Michigan. Critical day length varied temporally across the north–south gradient during emergence of O. agrili generations. Determining day lengths perceived by O. agrili larvae in the field should improve model realism for examining spatiotemporal variation in O. agrili population dynamics.