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Flatheaded fir borer (Phaenops drummondii)

Trees Affected
Flatheaded Fir Borer Larvae and Galleries. Source: Dave Powell, USDA Forest Service (retired), Bugwood.org

Common Hosts: Douglas-fir, true firs

Occasional Hosts: Spruce, western hemlock, pines


Adult Flatheaded Fir Borer. Source: Bill Schaupp, USDA Forest Service
The flatheaded fir borer is a wood-boring beetle that lives across North America. Adults are about a quarter to a half inch long and feed on the needles of their conifer hosts. However, most of the damage to host trees is caused by larvae, which feed on the cambium just below the bark. Unlike bark beetles, flatheaded fir borers do not create boring dust on the outside of the trunk, which makes it challenging to detect this beetle until significant damage is done. However, affected trees may be missing bark due to woodpeckers foraging for borer larvae, and 1/8” to 1/4” oval-shaped exit holes are indicative of fir borer presence. Additionally, trees may experience branch death and/or full crown decline over the course of one or two years. Although attacks and symptoms may be confined to the top of the tree, it is also possible for these beetles to kill their host tree. This is especially likely when trees are drought-stressed or are otherwise unable to produce enough resin to defend against attacks. 

Local Distribution
Flatheaded Fir Borer Trunk Damage. Source: Donald Owen, California Department of Forestry and Fire Protection, Bugwood.org


Management Strategies

Woodpeckers and parasitic wasps are natural predators of the flatheaded fir borer, so providing suitable habitat for these animals may be helpful. Management strategies vary by site, but avoiding additional stressors such as soil compaction and backfilling over roots is generally recommended. Since water-stressed trees are less able to produce enough resin to defend against insect attacks, reducing water stress via thinning or other methods can be another viable strategy.

Pests and Pathogens with Similar Symptoms

Douglas-fir beetle (Dendroctonus pseudotsugae): This beetle causes similar crown symptoms and is the other beetle that can kill large, healthy Douglas-firs in this region. However, unlike the flatheaded fir borer, Douglas-fir beetle exit holes are often accompanied by orangish boring material. Additionally, the Douglas-fir beetle makes galleries in the wood itself instead of on the inner side of the host’s bark.

Further Reading

Overview of biology, damage, and management: