Severe dieback on black oak trees on New York's Long Island was first apparent in the late 1980s. A twig- galling cynipid wasp and a fungal canker were associated with crown dieback. Symptoms were more severe in residential and urban park "ecotypes" than forested areas. Residual gall wasp populations from within twigs were compared among ecotypes and three crown categories (low, moderate and severe dieback). Insect populations were significantly greater on trees with severe crown dieback in residential areas and urban parks. In forested areas, insect numbers were similar among crown categories. Radial growth differed significantly between trees with severe and low crown dieback prior to 1980 in one residential area, one forested area and both urban parks. High numbers of gall wasps associated with urban areas, combined with previous stressors, are likely to have contributed to the greater severity of decline in parks and residential areas than forested areas.