We tabulated flowering plant species visited by 151 species of parasitic Hymenoptera. These data were extracted from records of C. Robertson who collected >15,000 insect visitors of flowering plants in central Illinois during a 33-yr period. Species diversity of parasitoids was highest on plant species of the Apiaceae and Asteraceae. The most abundant parasitoids were 2 tiphiid and a scoliid species. Parasitoid species in 6 dominant families (Ichneumonidae, Braconidae, Chrysididae, Tiphiidae, Pteromalidae, and Eucoilidae) showed similar preferences for 10 plant species. Most parasitoid species visited a limited range of host plants, which may have implications for conservation biological control and conservation biology.