A field study was conducted in Alberta to examine the dynamics of diapause recruitment and survival among overwintering horn flies, Hematobia irritans (L.). Cohorts of eggs were reared in experimental manure pats created at weekly intervals from July into September in three successive years. Nondiapausing adults emerged 16–47 d after pats were created in July and August, but few emerged when pat temperatures fell below 15°C. Survival to fall emergence varied with date, but was only weakly related to pat temperature. The proportion of pupae in diapause reached 1.0 in August each year. Overwintering survival of diapausing pupae varied from 0 to 85% among dates and years, and showed a curvilinear relationship with temperature during the fly’s diapause sensitive period. Thus, flies that emerged in a given spring arose from oviposition during a 3- to 6-wk interval in the preceding fall.