Populations of mimosa webworm, Homadaula anisocentra Meyrick, and its parasitoids Elasmus albizziae (Burks) and Parania geniculata (Holmgren) were observed on thornless honeylocust, Gleditsia triacanthos variety inermis L., at the West Lafayette, IN, campus of Purdue University between 1998 and 2001. Mimosa webworm populations were quantified by visually estimating percentage of browning of each tree canopy. Average browning per tree varied from 10.1 to 13.7% in the first generation and from 19.7 to 32.5% in the second generation. Average parasitism per tree ranged from 9.3 to 15.2% in the first generation and from 28.1 to 30.2% during the second generation. During the course of the study 71.5% of all parasitized mimosa webworm pupae were attacked by E. albizziae. The remaining 28.5% were attacked by P. geniculata. Total numbers of E. albizziae collected per tree in a season were positively correlated with the percentage of canopy turned brown by the second generation of webworms. Annual estimates of E. albizziae abundance were positively correlated with year-to-year declines in second generation webworm injury when average estimates per tree declined by 8.1 and 10%, but absent when average injury declined by 4.7% or increased by 5.7%. The apparent numeric response of these parasitoids and their delayed negative density dependence suggest that these parasites can occasionally contribute to decline of mimosa webworms in an urban forest.