Parasitism studies from 1961 to 1974 of field-collected European corn borer (ECB),Ostrinia nubilalis(Hübner), larvae showed thatLydella thompsoniHerting (=L. stabulans grisescensRobineau-Defvoidy) was no longer present in Delaware. Weekly releases of this parasitoid were made in 1975 and 1976 in an attempt to reestablish it in the state. The first recoveries of the parasitoid were in 1978, and it was recovered from ECB larvae collected during the winter of 1979–1980 from corn fields throughout the state.The parasitism rate of ECB larvae byMacrocentrus grandiiGoidamich has noticeably increased during the last decade, and the data indicate thatNosema pyraustaePaillot is an important, consistent mortality factor of its host the ECB.