Life table studies of European corn borer, Ostrinia nubilalis (Hübner), populations with and without inoculative releases of Trichogramma ostriniae (Peng & Chen) were conducted in sweet corn and field corn. Inoculative releases of T. ostriniae (at 72,000 females per hectare) provided relatively high parasitism of European corn borer eggs (≈37%) throughout the season. The increase in egg parasitism was not offset by compensatory changes in other mortality rates such as egg predation, eggs not hatching, and death of early instars after egg hatch. Early instar disappearance was a key mortality factor accounting for >70% of total mortality from egg deposition to established larvae. Egg parasitism by T. ostriniae was also a key mortality factor. Releases of T. ostriniae increased total egg and larval mortality of O. nubilalis from 61 to 92% in sweet corn and from 80 to 93% in field corn.