This study was undertaken to determine the impact of insecticides on the seasonal abundance of the insect and spider fauna associated with European corn borer (ECB), Ostrinia nubilalis (Hubner), adult mating and resting sites (i.e., action sites). A total of 24,497 ground-dwelling insects was collected during 1983 and 1984. Coleoptera, Orthoptera and Homoptera accounted for 98% of the total insect fauna identified from the ground strata. Three ground-dwelling predator species abundant in ECB action sites were Evarthrus alterans Casey, Pterostichus chalcites (Say) and Harpalus pennsylvanicus DeGeer. Carbaryl or fenvalerate applied for adult ECB control had minimal impact on the ground-dwelling insect community;A total of 99,519 foliage-dwelling insects was collected during this two-year study. The major herbivore species identified from the foliage of ECB action sites were Athysanus argentarium Metcalf and Draeculacephala anticox (Walker). Coleomegilla maculata (DeGeer) and Hippodamia convergens (Guerin-Meneville) were the most abundant predators inhabiting the upper grass canopy of ECB action sites. A. argentarium was the only foliage-dwelling insect species reduced when carbaryl or fenvalerate was applied to the foliage. Foliage predator populations were unaffected by the application of insecticides;A hunting guild, primarily Lycosidae, Thomisidae and Gnaphosidae, best characterized the ground-dwelling spider community. Gnaphosa parvula Banks is a new state record for Iowa. The foliage-dwelling spider community was characterized as belonging to both orb-web and hunting guilds. Tetragnathidae, Araneidae, Salticidae, Thomisidae, and Oxyopidae accounted for 89% of the foliage-inhabiting spiders. The application of insecticides primarily directed at the upper foliage canopy has minimal impact on the ground-dwelling spiders. Only the foliage-inhabiting Tetragnathidae spiders were reduced for 28 days in treated action sites (1984). The role of action sites as a source of predators for Iowa cornfields is also discussed.