Mild winter triggers early emerald ash borer emergence in Oregon

BEND, Ore. (KTVZ) -- The invasive emerald ash borer (EAB) pest has emerged weeks earlier than usual in King City, Ore., due to the region's extremely mild winter and warm spring days. Larvae of the pest matured faster, with the first adult confirmed in early May in King City, which is in Washington County.

This early emergence marks the first time the invasive pest has been seen inside King City, a community of about 5,000 people southeast of Portland.

Wyatt Williams, an Invasive Species Specialist for the Oregon Department of Forestry, confirmed the early emergence in early May after examining a Raywood ash tree in King City. In the four years since EAB was first found in Oregon in 2022, adults in northwest Oregon have consistently emerged in the first two weeks of June, continuing for several weeks into July. Updated models that accounted for warmer than expected temperatures this spring had pointed toward an earlier emergence.

The 2023-2024 winter in western Oregon was tied with 1934 as the warmest on record, followed by record-breaking or near record-breaking high temperatures this spring. Williams noted the rate of growth of EAB larvae is closely tied to outside temperatures. King City is within the existing Washington County EAB quarantine zone.

Oregon Department of Forestry Invasive Species Specialist Williams found a fully formed emerald ash borer adult that looked ready to emerge after peeling back bark from a section of a Raywood ash tree. "The very mild winter and the many warm spring days we've had in Oregon this year has allowed EAB larvae to develop more rapidly into adults," Williams said. "When we looked at the updated models that accounted for warmer than expected temperatures this spring, the models pointed toward an earlier emergence of adults. That's exactly what we're seeing," he added.

Williams came upon the adult EAB while following up on a hotline report made by Warren Tickner of Pacific Landscape Management. Tickner is contracted by King City to maintain publicly owned trees. He reported that Raywood ash trees in the city appeared to be declining, a classic indicator of EAB infestation.

During his inspection, Williams found several of King City's ash trees suffering dieback from verticillium wilt, a soilborne fungal disease. "Peeling back bark from a section of trunk of one of the trees, I found larval galleries and an EAB adult ready to take flight," Williams said. He also noted two ash trees had the characteristic twisting galleries made by EAB larvae under the bark.

Erin Carey, Public Information Officer for King City, acknowledged the discovery and the community's efforts. "we appreciate our residents' patience as we navigate the proper way to address and remove any trees infested with the emerald ash borer as soon as possible. We would also like to commend our landscape contractor for taking swift action when reporting his suspicion of the EAB to the appropriate authorities," Carey said.

Christine Buhl, an ODF Forest Entomologist, explained the broader implications of climate change on pest activity. "these changes in climate favor insect pests. It gives EAB adults more time to find a mate and then find an ash tree to infest with their eggs. At the same time, hotter temperatures and drought stress trees. This makes them less able to resist infestation and they may decline and die sooner," Buhl said.

Since the initial discovery, an adult EAB has been observed at a new site in Woodburn, less than a mile from Wyffels Park, where the pest was previously found last year.

The public is encouraged to learn to identify ash trees and monitor for signs of EAB infestation, such as dieback in the upper canopy. More information is available at OregonEAB.com.

If an ash or olive tree is suspected to be infested, reports should be made to the Oregon Invasive Species Council through their website, oregoninvasiveshotline.org/reports/create, or by calling one-866-INVADER (one-866-468-2337).