The adults are strong flying wasp-like insects. Widespread outbreaks seldom occur due to scattered distribution of hosts. Populations can become high and remain high on individual trees, resulting in considerable defoliation year after year. Few natural parasites are reported but high larval mortality has been reported with heavy rain and winds. A thorough spraying of the foliage after the leaves are fully expanded will control this insect.
Mountain ash is susceptible to several diseases and many insects infest it.
Mountain ash sawfly is among the more common annual insect pests. Edited in February 1997, based on material developed in 1983. Lawrence K. Forcier, Director, University of Vermont Extension, Burlington, Vermont. University of Vermont Extension and U.S. Department of Agriculture, cooperating, offer education and employment to everyone without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status. Last reviewed April 6, 1998.