Landowners and municipalities should be prepared to report sightings and deal with the aftermath of an aggressive borer insect recently confirmed to be in the Pacific Northwest, which could have dire consequences for Washington’s native and ornamental ash tree species.
Emerald ash borer (EAB) is an invasive pest that causes high levels of mortality of North American ash species. It was accidentally introduced to North America in Michigan in the 1990s. Its first known occurrence on the West Coast was confirmed in northwest Oregon in July 2022. The Pacific Northwest region can expect significant losses of the native Oregon ash (Fraxinus latifolia) as well as the ornamental ashes commonly found in urban forests and home landscapes in communities around the region.
The introduction of EAB has changed forests and communities in the eastern United States by practically eliminating ash species from the landscape. Many states and communities have tried to mitigate the spread and impact of EAB, but its recent spread to Oregon demonstrates of the difficulty of containing this aggressive pest. Land managers and communities are encouraged to prepare for the seemingly inevitable spread of EAB into Washington.
Oregon ash is an important tree for ecosystems and communities in the Pacific Northwest. It is endemic to the west coast where it is distributed from central California to the Puget Sound area. It is a dominant component of riparian and wetland areas where it helps protect water quality and provides habitat for salmon and other aquatic species. Oregon ash and ornamental ash trees are also important components of urban forests where their large canopies provide shade and other ecosystem services for communities. The loss of ash trees in the Pacific Northwest will diminish ecosystem functions in sensitive areas. It will also cause a loss of valuable ecosystem services and require the expensive removal of hazardous dead trees in urban landscapes.
Land managers and municipalities should identify vulnerable areas and prepare for the arrival of EAB. Property owners and municipalities should avoid planting ash species. In natural areas, alternatives to Oregon ash include black cottonwood, willows, red alder, and western redcedar. There are numerous non-ash options for urban forests. Existing high value ash trees in urban forests can be protected by biennial stem injections of emamectin benzoate. These injections must be done by pesticide-licensed tree care professionals. The use of certain parasitoid wasps as biocontrol agents is a promising long-term control option for both urban forests and natural areas, but biocontrols will not stop initial outbreaks. More specific recommendations will be available soon from WSU Extension.
Individuals and municipalities are encouraged to report signs of EAB infestations in ash trees such as adult insects, larval galleries, or D-shaped exit holes in the bark. Symptoms of EAB infestations include crown dieback, wilting, leaf chlorosis, early leaf drop in autumn, and extensive woodpecker activity. Anyone who observes these symptoms on ash trees is encouraged to check for and report signs to the Washington Invasive Species Council or Washington State Department of Agriculture via the Washington Invasives mobile application. Local WSU County Extension offices can help concerned citizens confirm signs and symptoms. Non-ash species are not affected by EAB.
Researchers are working to develop planting stock of North American ash species that is genetically resistant to EAB. Researchers are also continuing to investigate biocontrol options and develop next-generation insecticides. WSU Extension will continue to share updates with the public as information becomes available.
In the meantime, WSU Extension will be hosting two evening workshops in southwest Washington (Ridgefield and Olympia) in early October. Participants will be trained in proper identification of ash trees, EAB symptoms, and the beetle itself. Implications for management will also be discussed. To learn more, go to https://forestry.wsu.edu/sw/events/eab/. Registration closes at noon on Oct. 4.
- Emerald Ash Borer Information Network
- Emerald Ash Borer Identification Guide
- USDA APHIS Emerald Ash Borer website
- Biology and Control of Emerald Ash Borer (USDA Forest Service Publication):
- Insecticide Options for Protecting Ash Trees from Emerald Ash Borer
- Emerald Ash Borer Biological Control Release and Recovery Guidelines