The development, flight activity, and density of the native eulophid,Sympiesis marylandensisGirault, a major parasitoid ofPhyllonorycterspp., were examined in apple orchards, forests or both at two Connecticut sites between 1985 and 1988. In orchards, S.marylandensishad two or three generations per generation ofP. crataegella(Clemens), for a total of six or seven per year. The proportion of parasitoid pupae in diapause steadily increased from September to November. Males numerically dominated the pupal samples. In apple orchards, adults ofP. crataegellacaptured on yellow sticky traps had three major periods of activity, whereasS. marylandensishad three or four that corresponded to periods of adult parasitoid emergence. The trap catch ofS. marylandensisin June to early July was relatively low in a commercial orchard, but not in an unsprayed orchard. Autumn activity was the greatest in years when an insecticide for leafminers was not applied. Parasitoid abundance on traps was lower in forests than in orchards, and higher in spring and autumn than at other times in forests. From September to November of 1988, the density ofP. crataegellaimmatures in a sprayed orchard was negatively correlated with the density ofS. marylandensis. The effect ofS. marylandensisis greater than was suggested previously.