Sawfly Outbreaks in the Willamette Valley

Dave Overhulser, Entomologist, Oregon Department of Forestry

Several reports of sawflies defoliating Valley pine in Lane and Yamhill counties were received in May of 2003. This sawfly is a native species of Neodiprion whose outbreaks are of short duration and cause little damage. The larvae feed gregariously on the older needles of ponderosa pine in the spring causing the foliage to turn brown and landowners to be concerned.

With damaged brown needles present and the new shoots and needles not completely expanded, trees appear to be suffering significant damage. However, sawfly outbreaks in ponderosa pine last only one or two years and do not kill trees. In the worst case, sawfly defoliation only reduces growth. Usually sawfly outbreaks collapse quickly from the combined effects of predators, parasites and disease on the population. The use of insecticide to control sawflies is not warranted under most circumstances.

Reports of sawfly defoliation of pine in the Willamette Valley were quite rare ten years ago. Undoubtedly Neodiprion populations cycled up and down in the past, but with so few host trees present the outbreaks went largely unnoticed. As thousands of acres are planted to ponderosa pine, sawfly outbreak cycles will become increasingly visible to landowners and foresters.


Last Updated 02/24/08