Ash Tree-Killing Insect Now in Forest Grove

Twenty years have passed since that awful day in Michigan when people started to notice something was wrong with their ash trees. They were dying.

Closer inspection revealed that a pest new to North America had arrived – the emerald ash borer. Only half an inch long and an eighth of an inch wide, this metallic green insect has already killed tens of millions of ash trees, pushing five native species – green, white, blue, black and pumpkin – to critically endangered status.

Over the past two decades it spread from the Detroit area to 35 states and five Canadian provinces.

This summer it was reported for the first time in Oregon in Forest Grove, just 21 miles west of Portland. Our city will never be the same.

The adult Emerald Ash Borer is striking in appearance and smaller than a dime. Photo Credit: Leah Bauer, USDA Forest Service Northern Research Station (bugwood.org).

Ravenous killer

When it comes to emerald ash borers, it’s the babies that do the killing. Adult females seek out ash trees and deposit their eggs under the bark. When they hatch, they start chewing up the living inner bark (phloem and xylem). The feeding channels the larvae excavate cut the tree’s canopy off from the roots, so they cannot pull up water and send back nutrients made in the leaves during photosynthesis. Within three to four years this can kill a large ash tree.

Emerald ash borer most likely came to North America on wooden pallets on imported goods from eastern Asia, where it native. There, it mostly attacks stressed ash trees. But in the U.S. and Canada it kills stressed and healthy ash alike. All North American and European species are vulnerable, including Oregon’s one species of native ash – the Oregon ash (Fraxinus latifolia).

Within three to 10 years of arriving in a new area, emerald ash borer has infested almost every ash tree. Mortality of ash trees has reached over 99 percent in many areas, causing ash to disappear from forests where it was a prominent part of the ecology. About 12,000 street trees in Portland are ash vulnerable to emerald ash borer.

Potential impact on the Linear Arboretum

In the latter half of the 20th century, ash became popular replacements for elm trees lost to Dutch elm disease. New cultivars further increased ash trees’ popularity, such as:

·       Autumn Purple (a white ash introduced 1956 with reddish-purple fall color)

·       Summit (a seedless green ash introduced 1957 with yellow fall color)

·       Autumn Applause (a white ash introduced 1975 with red fall color)

·       Raywood (a narrow-leaf ash that became widely available from 1979 with purple fall color)

Dozens of Raywood ash were planted by the City of Portland in the Ainsworth median in the 1980s. As they grew larger, their weak roots became more apparent. One or two keel over in windstorms or ice events almost every autumn or winter.

There are still 20 Raywood ash left in the median along with two green ash. Raywood’s long, narrow leaves give the tree an airy, dainty look. In October they stand out for their wine red to purple fall color. The hue gave rise to their other common name - the claret ash. Nine of these trees are east of NE 23rd (with two new ones in the planting strip in front of a house facing Alberta Park). Another 13 are west of NE 29th. The entire block of trees between NE 30th and 31st are Raywood ash. They make a pretty picture. With the arrival of emerald ash borer, they represent the ecological folly of planting rows of identical tree clones.

How soon can residents along Ainsworth expect to lose these ash trees? No one knows for certain. But it is known that no state has succeeded in eradicating emerald ash borer once it’s turned up. Females are capable of flying up to about 10 miles in a year. The trees in Forest Grove might have been infested for a few years before anyone noticed and reported them to the state. So potentially these pests could already have silently slid into Portland undetected.

Early stages of infestation are hard to see as the bugs lay their eggs at the top of the tree. Succeeding generations move farther down the trunk to humans’ eye level, which is when their distinctive D-shaped exit holes are usually first noticed. Look for signs of canopy that is thinning or declining at the top but not farther down on a tree, or for sprouting of shoots on the trunk as the tree desperately tries to survive. The adults emerge through one-eighth-inch wide holes in the bark from May through July.

SIghting and Reporting

Anyone sighting an emerald ash borer in a new location is asked to report it online at the Oregon Invasive Species Hotline at https://www.oregoninvasivespeciescouncil.org/report-an-invader. Scroll to the bottom of the page and click on the box that says “Report An Invasive Species.” . You’ll need to give the exact street address and the nearest ash tree as well as a photo. Emerald ash borer are easily mistaken for several harmless green native insects, so take closeup photos and compare these to a sheet of look-alikes at https://www.oregon.gov/oda/programs/IPPM/SurveyTreatment/Documents/EABLookAlikes.pdf before filing a reported sighting

Some cities in the central U.S. wanting to save particularly high-value ash specimens have found that certain systemic insecticides targeted to emerald ash borer injected into the tree or used as a soil drench can fend off infestation if applied before the insect has started attacking a tree. But the chemicals must keep being reapplied and the cost mounts up. The chemicals may also be harmful to bees and other pollinators. Where, when and if they are to be used requires careful thought.

Replacement for New Growth, Diversity and Resistance

Removal of trees once they become infested and replacement with other species is usually the most practical option. Given that several of the Ainsworth ashes are leaning and show signs of stress during drought years, replacing them as they die with different species will help reduce the risk of wholesale loss of trees from new pests.

A good example might be the Kentucky coffeetree planted this spring in the median at NE 35th Place. Like an ash, it has compound leaves but few known pests or diseases and is not overplanted in Portland.

Gymnocladus dioicus, or Kentucky Coffeetree. Credit: Jim Gersbach.