In the year since the emerald ash borer was detected in Oregon, questions have been pouring in about what to do if an ash tree becomes infested.
Currently, the emerald ash borer has been found in trees only in Washington County, where a quarantine prohibits moving ash (Fraxinus) or fringe tree (Chionanthus) wood in or out of the area. Dave Shaw, a forest health specialist with the Oregon State University Extension Service, encourages homeowners to not take down their ash trees before they are infested or unless the insect has been detected nearby.
It could be years before emerald ash borers find your tree, Shaw said, which then take several years to die after being infested. Observations from the eastern United States, where the beetle was first detected in 2002, suggest that trees in areas where the infestation has started take longer to die than in areas where the insect is well-established. When emerald ash borer is confirmed, though, the tree needs to be removed before it becomes a hazard.
Homeowners have the option of using a systemic pesticide to keep the insects at bay, but once the beetles arrive, pesticides won't kill them, Shaw said. And applications are expensive and must be repeated every two to three years, he said, comparing the use of chemicals to using a Band-Aid.
"If you just have one tree and can afford it and don't mind the pesticides, they are available, but the most effective treatments must be applied by licensed professionals and both the product, and the labor add up quickly," he said. "There's also the issue of pollinator health. Once a systemic pesticide is applied it travels through the entire tree and pollinators can take it up."
If a homeowner or woodland owner chooses to use a pesticide, a list can be found in the Extension article "What to do about emerald ash borer: Recommendations for tree protection in EAB-infested areas."
Symptoms of an emerald ash borer infestation are a dying crown, sucker shoots growing from the trunk or base of the tree, D-shaped holes in the bark, splitting back and S-shaped areas underneath the bark. Often, you'll notice holes in the trunk made by sapsuckers trying to get to the insects' larvae. Many other pests cause the same symptoms, but in summer, adult insects will be visible for identification.
Public agencies and landowners are determining what trees can be planted to replace native trees. Homeowners can start to prepare by researching what tree to plant as a replacement.
Heather Stoven, OSU Extension horticulturist, said the first thing to think about is "right tree, right place." Choose wisely by considering your site's sun exposure, soil, nearby structures and overhead power lines. Ask nursery professionals or the Master Gardeners in your area for recommendations. If you're planting in a parking strip, it's important to obtain a permit from the city where you live, Stoven said.
Many trees would be a fine replacement for ash, Stoven said. Her top 10 concentrates on mid-size shade trees.
- Dogwood (Cornus): For its stunning white, pink or red flowers that cover the tree in spring. Some, like Japanese dogwood (Cornus kousa), have nice fall color. Oregon's native Pacific dogwood is an understory tree and needs some shade so won't be appropriate in many situations. Hardy to Zone 5.
- Ginkgo: For its fan-shaped leaves, buttery yellow fall color and drought tolerance. Hardy to Zone 3.
- Hornbeam (Carpinus): For its blazing fall colors in orange and red, drought tolerance and attractiveness to wildlife. Hardy to Zone 3.
- Magnolia: For its beautiful, fluted flowers in white, pink or yellow. Some are fragrant and some have attractive fall color. Hardy to Zone 5.
- Maple (Acer): For its brilliant fall color, distinctive foliage and some are drought tolerant. Native vine maple and Rocky Mountain maple are small, understory trees and may not work as an ash alternative. Hardy to Zone 4.
- Oak (Quercus): For its familiar lobed leaves, vivid fall color, wildlife value and some are drought tolerant. Native Oregon white oak and California black oak are two possibilities, though they can get up to 80 feet so be sure you have the room. Hardy to Zone 5 or 6, depending on species.
- Snowbell (Styrax): For its graceful pendulous bell-shaped white flowers that cover the tree in May and June. Hardy to Zone 5.
- Tupelo (Nyssa): For its striking red fall color, bird habitat and a tough demeanor that tolerates pollution, salt and poor soils. Hardy to Zone 4.
- Yellowwood (Cladrastis): For its showy fragrant, white or pink flowers and clear yellow fall color. Hardy to Zone 4.
- Zelkova: For its distinctive fall color in shades of yellow, orange, deep red to reddish purple. It's also tough, drought-tolerant and has gray-brown bark that often exfoliates to expose an orangish inner bark. Hardy to Zone 5.
The Oregon departments of agriculture and forestry are monitoring the Forest Grove discovery site and trapping insects to monitor how far and fast it is spreading. Properly identifying emerald ash borer is essential to help track and slow its spread. If you see one, report it to the Oregon Invasive Species Hotline. OSU Extension Emerald ash borer resources include photos of the beetle and articles on protecting trees, identifying ash trees, FAQs, alternatives to ash in their native habitat and more.
When emerald ash borers spread across Oregon, their effect on ash trees will be devastating, Shaw said. Based on what's been observed in the eastern United States, he said it's likely that 99% of Oregon's ash trees will disappear. That includes both the trees that enhance urban and suburban areas, but also the native ash that grow in riparian areas along waterways and cool and filter the water for fish, Shaw said.
The insect has destroyed over 100 million ash trees in the eastern U.S. Research is underway in many areas of the country to find a biological or chemical application to halt emerald ash borer, but so far there's no cure for a tree infested by the beetle. It can take up to six years for the tree to die, but eventually it will be killed, Shaw said.
"We have a small region where it is concentrated," Shaw said. "ODA is trying to keep it in that zone. They've ringed Forest Grove with trap trees - trees you girdle to cause high stress. These beetles are more attracted to stressed trees. If they find any, they'll destroy them. That's one way we can try to control the population. They are also releasing parasitoid wasps that feed on emerald ash borer. We hope these practices help. We're not going to roll over."
