Watkins & Gorman: Ash borer eats its way into Western Oregon
Guest writer Susan Watkins owns a small woodland southwest of McMinnville with her husband. An urban pencil-pusher for most of her career, she jumped headfirst into forestry to become a master woodland manager, past member of the statewide Committee for Family Forestlands and co-creator of Ties to the Land, a succession-planning guide for resource landowners. She also sits on the board of the Yamhill County Small Woodlands Association.
Co-writer Alex Gorman serves as the OSU Extension Service’s forestry and natural resources agent for Yamhill County. In that capacity, he works closely with the county’s small woodland owners. He combines a background in mycology and forest pests with a passion for forestry, education and applied research. He helps landowners make wise decisions in managing their resources. He also serves on a statewide task force investigating the EAB invasion. He is open to fielding questions at alexander.gorman@oregonstate.edu.
A shiny green insect is making big — and devastating — news in Oregon.
The dreaded emerald ash borer has completed its two-decade trek west to our state, making its first appearance in stands of ash in Forest Grove in neighboring Washington County. En route, it has killed millions of ash trees in the U.S.
First spotted near Detroit, Michigan, in 2002, it feeds on a tree’s cambium layer, creating s-shaped galleries just beneath the bark. This severs the root-to-shoot connection by girdling the tree, causing mortality in one to five years.
“EAB has become the most destructive and costliest forest pest ever to invade North America,” according to Wyatt Williams, an invasive species specialist with the Oregon Department of Forestry.
The pest has “spread to 35 states and five Canadian provinces, killing up to 99 percent of the ash trees in some locations,” he said. Along the way, it has claimed “hundreds of millions of urban and wild ash trees.”
Ash is the predominant species in riparian areas in the Willamette Valley, making up 65% to 70% of riparian forest stands. It is common in McMinnville and environs. But Williams expects most of the valley’s ash trees to be “dead or dying” within 10 years.
“Ash is an ecologically vital tree that shades water, keeping it cooler for fish,” Williams said. “The roots stabilize streambanks, reducing erosion. And lots of animals, birds, and insects eat the seeds and leaves.”
That being the case, he said, “Losing it will likely have a huge impact on those ecosystems.”
Several salmon species could be severely impacted if the stream cover that the ash provides is lost. Moreover, ash is not the only vulnerable species. Olive trees belong to the same family and are also susceptible.
At least one third of Ramsey McPhillips’ 575-acre farm along the South Yamhill River, just south of McMinnville, is planted in ash. That leads him to brand the emerald ash borer “my biggest fear.”
“My old growth ash is magnificent,” McPhillips said. “The survey corner marker trees from the 1850s are Oregon Ash. My home is built from ash.”
Ironically, McPhillips has already experienced first-hand the devastation the ash borer can cause. That’s because he owned woodland in the Catskill Mountains of New York when the pest invaded there.
“We logged as quickly as we could to save the wood,” he said. “Then we used the logging money to plant maple and oak and spruce. It was horrible.”
An infested tree will begin to lose leaves from the top of its canopy. Eventually, the entire tree will succumb.
Experts expect the ash borer to go through a complete metamorphosis on a one-year cycle in Oregon. That cycle consists of four life stages: egg, larva, pupa, and adult.
Eggs are laid in bark crevices in the summer. The larvae burrow through the bark into the cambium layer to feed on the phloem, etching s-shaped galleries as they mature.
Mature larvae overwinter in their galleries until late spring, when they pupate and transform into adults. The metallic green adults survive only a few weeks, emerging from the tree to mate and lay eggs during brief windows from late May through early September.
Mated females are attracted to volatile chemical compounds released by stressed ash trees. They will fly more than 10 kilometers to find suitable trees.
The Department of Forestry considers ash trees within 15 miles of a known infestation to be at immediate risk. McMinnville and Forest Grove lie just 22 miles from each other — much too close for comfort.
The Oregon Department of Agriculture is taking the lead in the effort to slow the borer’s spread in the Willamette Valley.
In Forest Grove, it is being assisted by the Department of Forestry, Oregon State University Extension Service, Tualatin Soil & Water Conservation District and Clean Water Services. They are joining to train forestry and nursery personnel in identification of the emerald ash borer and infested trees.
Their immediate goal is to determine the current limits of the infestation, termed “the area of concern,” then develop a plan to slow the invasion. Transportation of ash out of the area of concern will be prohibited.
Unfortunately, it’s difficult to tell that a tree has been infested until two to three years after the ash borer first lays its eggs.
The first thing to look for is the characteristic D-shaped exit holes left by adults boring their way out from under the bark. The holes may be only 1/8 of an inch long, and the D’s may be sideways, so it requires close inspection.
Unfortunately, by then infection is already well underway below the bark layer.
Alternatively, you can look for flaking bark caused by woodpeckers finding and feeding on borer larvae, and for s-shaped galleries visible through cracks in the bark or patches where it has crumbled away. Infested trees may also sprout epicormic limbs from their trunks.
In urban settings, quick removal of stressed and infested trees may help limit spread. But state ag and forestry officials are not suggesting, at least at this time, that susceptible trees be removed from forests. In fact, removing trees may be counterproductive.
Research suggests female borers may fly farther in search of new hosts in areas where few ash trees exist close to an already infested site, thus spreading an infestation more widely and more quickly.
If you opt to cut down ash trees, you can burn the wood with appropriate permits and timing or retain it for personal use as firewood. But infested wood should not be moved off the property.
Knowing the infestation was coming, Oregon agencies prepared their initial EAB Readiness and Response Plan in 2018. They also began gathering ash seeds from trees across Oregon in hopes of eventually preserving genetic diversity through replanting.
The agencies are also investigating possible biological control tools. They are committed to do everything they can.
What can you do to help?
As the emerald ash borer moves through the Willamette Valley, some landowners may wish to protect certain trees on their property. Several effective insecticide treatments exist, and in situations where dead trees would have to be removed, using insecticide before they become infested has proven cost-effective.
In woodland settings, protecting all the ash is not practical, but treating legacy trees may be worth the effort to some landowners.
A single treatment can protect a tree for up to three years. Insecticides must be applied while the tree is still healthy, however, as infested trees lose the ability to carry water —and consequently, insecticide — up the trunk to the canopy.
Landowners are advised to wait to apply treatments until the ash borer has been identified in their county. As of this writing, that has not yet been the case in Yamhill County.
In addition, you can assist in being on the lookout for signs of infection and reporting them to authorities.
Oregon State University is offering a free forest pest detection course online at workspace.oregonstate.edu/course/Oregon-Forest-Pest-Detector?hsLang=en.
You can progress at your own pace.
Suspected infestations may be reported online at oregoninvasiveshotline.org.
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