How to Cope with Emerald Ash Borer

If you live in the Willamette Valley, you may have heard the warnings of the dreaded Emerald Ash Borer (Agrilus planipennis), which was recently found in Oregon. The first documented sighting was in July of 2022 in a parking lot in Forest Grove. This is the first confirmed Emerald Ash Borer (EAB) infestation west of Colorado. EAB is a pest that has decimated ash trees in the eastern parts of the US since its initial detection in 2002. And now it has found its way here.

EAB is native to Asia, and it is thought to have been introduced here via wood shipping crates. It infests all species of ash trees, where it lays eggs, which then feed on the inner bark and phloem of the tree, causing rapid crown decline and, ultimately, death of the entire tree. Larvae overwinter in the bark of the ash tree and emerge as adults in late spring. The exit holes of the adults are distinct in that they leave a small “D” shape in the bark of the tree. From May to October, the adults fly up to seven miles to find other host trees nearby.

Common Concerns and Questions

Should we stop planting ash trees in Oregon?

Not necessarily. While there is a risk of spending a lot of money on ash trees that may likely be killed by the EAB, we are also facing the threat of losing our native ash species entirely if we don’t try to continue to repopulate and save seeds of the trees for future plantings. EAB predates on ash trees that are 10 to 15 years old. It is important to protect surviving Ash trees that are outside of this age range, to continue the genetic spread of this tree despite the mortality from EAB. There have been success stories of young ash trees that were planted after the EAB infestation on the East Coast that are still surviving today. Native ash trees may be able to build resistance to the pest, or outlive the infestation, if we continue to plant them.

What is the best tree species to replace native ash?

Although there are no trees that exactly mimic Oregon’s native ash, Fraxinus latifolia, alternative trees can provide similar benefits to the riparian ecosystems where Oregon Ash are most commonly found. It is always important to plant a diverse selection of native trees, shrubs, and forbs for best success.

• For shade and wildlife support: white alder, big-leaf maple, black cottonwood, chokecherry, Scouler’s willow, quaking aspen.
• For tolerance to dry and wet conditions: Oregon white oak, cascara, Willamette Valley ponderosa pine, Pacific madrone.
• Assisted migration candidates to tolerate a changing climate: incense cedar, California bay, and California black oak.

What is the impact of losing ash trees to EAB?

The loss of this native tree species may lead to a number of impacts on environmental and human health.

Environmental impacts

• Loss of shade in streams and rivers, raising temperatures of water.
• Loss of biodiversity in riparian zones, leaving natural spaces vulnerable to establishment of invasive plant species.
• Ash connects landscapes through riparian corridors. The loss of this connectivity will cause stress on various wildlife, soil, and water quality.

Human impacts

• Loss of greenways in urban areas can lead to higher rates of depression and obesity.
• Loss of shade in urban areas can lead to heat islands, increasing the summer temperatures to fatal levels.
• Dead ash trees in urban areas can pose a risk of falling on structures and homes.

What Can Be Done?

Treatment Methods for Dead Trees

• Chip infected ash trees into 1″ by 1″ chips and leave to decompose. The EAB will be cooked in the thermal decomposition of chip piles.
• Burn infected woody debris at 170 degrees for 1 hr.
• To process, debark and remove 1″ sapwood for mill.

Prevention and Slowing the Spread

Prevention is possible in some cases. Here are some methods to try to save ash trees:

• Buy It Where You Burn It – It is recommended not to move or transport ash wood from May 1st to October 15th when adults are active. To slow the potential spread of EAB and other forest pests, leave firewood at home – do not transport it to campgrounds or parks. Use firewood from local sources. Learn more at Oregon Invasive Species Council.
• SLAM (Slow Ash Mortality) – This project in Michigan has been shown to slow the spread of EAB. The ash tree is girdled and a section of bark is removed, to attract adult females looking to lay their eggs. The tree is then injected with insecticie, killing adults. The “trap tree” is removed and destroyed to prevent the larvae from completing their life cycle.
• Biological control – 3 species of wasp from the native range of EAB in China have been studied as a biological control for EAB. These wasps provide predation on larvae and adults of EAB and have been showing success in slowing the spread in 30 states since the release of eight million wasps in 2007.
• Pesticide injection – can prevent EAB infection for up to three years. In some pesticide injection into ash trees can prevent and slow the spread significantly and improve the health of infested trees. For more information, view the Multistate EAB Insecticide Fact Sheet.

For more information, read “Alternatives to Ash in Western Oregon” from OSU Extension Service.

In Conclusion

It is important to always plant for biodiversity in any natural or urban area to provide resilience against pests and disease that disproportionately impact a particular species. This helps the ecosystem bounce back from disturbance and allows for wildlife to better adapt.

Research into the prevention, treatment and restoration of ash species has been constant since EAB’s US introduction in 2002. The East Coast states that have faced the brunt of this invasion have also been able to provide the most recent science informing us about the impacts of EAB. Now that the pest is here in Oregon, we have an abundance of resources to look to for best management practices to help us protect our native ash trees.