WSU Extension


Caption: Pacific flatheaded borer
Photo by: K. Grey
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Plum, Prune (Fresh) : Pacific flatheaded borer
(revision date: 5/20/2014)

The Pacific flatheaded borer typically attacks weakened, injured, or stressed trees. The larvae feed between the bark and the sapwood. They can weaken and girdle trees. Injured bark may show dark depressions on the surface or cracks through which the sawdust-like frass can be seen. The larvae are white or cream-colored with broad, flattened heads. They can reach up to 1" in length. The adult borer is a dark, coppery-brown beetle. The broad, flattish adults are about 1/4" to 1/2" long. Adult females lay eggs in bark crevices on the trunk below the lowest branches. The larvae burrow into the bark after hatching. The borer prefers to feed in and lay eggs on wood that is exposed to sunlight. This insect feeds on many species of trees and shrubs.
Management Options

Non-Chemical Management
  • Provide proper culture. Vigorous, healthy trees are less likely to be attacked and more likely to survive attacks.
  • Birds, carpenter ants, and parasitic wasps help control borer populations. Avoid use of broad-spectrum insecticides which kill beneficial insects.
  • Remove weeds and debris around trees to make it easier to detect borer activity.
  • Use barriers or sunshades on trunks to prevent egg-laying.
  • Borers cannot be controlled after they enter the bark.
Select non-chemical management options as your first choice!

Chemical Management

None recommended

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Caption: Pacific flatheaded borer
Photo by: K. Grey