none Parasitism studies from 1961 to 1974 of field-collected European corn borer (ECB), Ostrinia nubilalis (Huebner), larvae showed that Lydella thompsoni Herting (= L. stabulans grisescens Robineau-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 by Macrocentrus grandii Goidamich has noticeably increased during the last decade, and the data indicate that Nosema pyraustae Paillot is an important, consistent mortality factor of its host the ECB. Animals and man; Control; Biological control; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Zoogeography; Dispersal; Means of dispersal; Land and freshwater zones; Nearctic region; North America; USA Nosema pyraustae (Microsporida ); Macrocentrus grandii (Braconidae ); Lepidopteran hosts; Ostrinia nubilalis, increased larval parasitization rate; Prevalence; Ostrinia nubilalis (Lepidoptera), increase; Delaware; Lepidopteran host, prevalence increase Lydella thompsoni (Tachinidae); Control of; Lepidopteran hosts; Ostrinia nubilalis; Prevalence following reintroduction; Prevalence; Ostrinia nubilalis (Lepidoptera), following reintroduction; Introduction, control of lepidopteran pest, establishment; Delaware; Control of lepidopteran host, establishment following reintroduction Ostrinia nubilalis (Pyralidae); Control by; Lydella thompsoni (Diptera); Dipteran parasites; Lydella thompsoni; Prevalence following reintroduction; Hymenopteran parasites; Macrocentrus grandii; Protozoan parasites; Nosema pyraustae, increased larval parasitism rate; Prevalence; Protozoan, & dipteran & hymenopteran parasite increase; Delaware; Dipteran, hymenopteran & protozoan parasites