City staff: aphid problems on linden street trees likely to require removal and phased replacement

Phil, a city staff member working with urban forestry, told the committee the city has 534 linden street trees—about 5% of the city’s street-tree inventory—and the trees generate many resident complaints because aphids feeding on the trees produce sticky honeydew.

“We've got 534 of them, which represents 5% of our street tree population,” Phil said. He summarized past steps: a moratorium on planting linden trees about ten years ago, trial treatments that failed, and legal and practical limits on effective pesticides.

Phil explained that recent and forthcoming state rule changes have tightened use of certain insecticides. “There’s a new RCW on the use of certain insecticides that kill aphids because they often also kill honeybees,” he said, noting that some effective products are neonicotinoids that are increasingly restricted because of impacts on pollinators. He said viable treatment options such as soil drench or trunk injection are not currently available and that canopy sprays—where neonicotinoids historically were used—raise stormwater and pollinator concerns.

Staff described removal and replacement as the practical solution after other options failed. Phil said the city has replaced three streets to date and has about 111 left in a higher-priority corridor (SR 1). At current funding levels, a complete replacement program across all streets would take many decades. “The current replacement timeline, with current funding levels, it’s going to take us about a hundred years,” he said.

Phil described a cost driver and a pilot project to reduce expense. Tree replacement costs are dominated by soil excavation, hauling and importing new soil; the tree itself is a small fraction of the cost. In a recent pilot, crews used an 18-inch excavation and a custom imported soil/biochar mix on one side of a street and a full-spec three-foot excavation on the other to compare results. The city will evaluate tree performance over several years before deciding whether the lower-cost approach is acceptable at scale.

Committee members asked whether quick removals without replacement were feasible. Phil said removal is easy but cautioned against mass removals because losing canopy would reduce shade, increase urban-heat impacts and could raise pavement maintenance costs. He said the city typically removes a tree only immediately before replacement to avoid denuding neighborhoods.

The committee directed staff to reprioritize the urban forestry replacement schedule to give linden trees higher weight so they can be replaced sooner; staff said they will return in January with the reweighted priority list and the proposed 2026 street-tree replacement project details.

Ending: Staff recommended a phased replacement program informed by the pilot soil/specification test, reweighted priorities to address linden complaints, and a January follow-up to present the reprioritized list and budget implications.