Arborvitae (Thuja)-Borer

Flat-headed borers (Chrysobothris nixa)
Shothole borers (Phloeosinus punctatus)

Pest description and crop damage Flatheaded borers are pests of many different trees and shrubs. The 0.5 inch adults are either brown to metallic gray or black with red or orange markings, depending on species. Larvae are whitish to pale yellow and about 0.5 inch long when fully developed. Just back of the head is a broad, flat enlargement giving a "flat-headed" appearance. Individual plant damage can be one-sided depending on the site of feeding. Most damage occurs near the base of the plant, commonly at the soil line. Flatheaded borers tunnel extensively in the shallow, cambial tissue. Shothole borer adults are brown beetles less than 0.08 inch long. Larvae feed beneath the bark and may girdle the trunks and branches of trees. The larvae of the shothole borer feed beneath the bark on the trunk, leaving characteristic "shothole" symptoms (tiny holes in the bark) when they emerge. Larval feeding also may be associated with weeping sap on the trunk. Young, recently planted trees are most susceptible.

Scouting and thresholds Watch for depressions in the bark or cracks through which frass may be seen. Entry holes and borer damage greatly affect marketability; action thresholds are very low. Prevention is key.

Management-cultural control

As the borer is attracted to weakened trees, the best control is to keep trees healthy. Avoid wounding plants, and follow good pruning, watering, and fertilization practices. Keeping weeds, grass, and trash from the base of the trees makes it easier to detect and remove borers. Rogue and destroy infested plants.

Management-chemical control


Chemical management is rarely economic except for high value hosts. Apply as a protective trunk spray prior to adult flight in the summer.

For more information

PNW Nursery IPM: Flatheaded cedar borer (