Douglas-fir Engraver Beetle (Scolytus unispinosus)
The Douglas-fir engraver beetle is a small (< 1/8” long) beetle that attacks small diameter trees or branches and makes larval galleries in their host’s cambial layer (under the bark). These beetles tend to mate in late spring and early summer, and are generally more active in trees that are already experiencing stress such as drought or attacks from more aggressive pests or pathogens. Trees that have been attacked by the Douglas-fir engraver beetle show classic crown symptoms of beetle attack, including rapid individual branch, top, or whole tree decline and red needles in the affected area/s. Exit holes (smaller than the diameter of a grain of rice) may be present on the trunk or branches, and boring material may persist on the outside surface of the bark. One of the more reliable ways to distinguish between this beetle and other bark beetles such as the Douglas-fir pole beetle is to examine the larval galleries. The Douglas-fir engraver beetle makes a one to three inch long main gallery with larval chambers coming off continuously along the main gallery. At one end of this main gallery, there is also a short nuptial chamber. The beetle itself is dark brown to black, and has a flat posterior relative to its head and to other beetles when viewed from above.
Unknown, but native throughout the North Coast.
This beetle is a secondary stressor and is unlikely to kill healthy trees. Therefore, the best management strategy is to promote tree health by thinning, controlling other pathogens such as root disease, and avoiding general stress where possible. Being aware that drought can reduce tree vigor and resin production may also help in planning for upticks in beetle-related mortality post-drought.
Pests and Pathogens with Similar Symptoms
Douglas-fir Pole Beetle (Pseudohylesinus nebulosus): This beetle causes the same kinds of crown symptoms and can be managed the same way. However, if you’re curious about which one you have, take a look at the larval galleries under the wood and read about the differences in gallery styles here.
Douglas-fir Beetle (Dendroctonus pseudotsugae): This beetle causes similar crown symptoms but can kill large, healthy trees. Additional signs and symptoms of this beetle include orangish boring material in bark crevices and (sometimes, depending on tree vigor) pitching along the trunk. The most definitive way to distinguish this beetle from others is by the organization of its larval galleries, which is compared to common, less aggressive Douglas-fir inhabiting beetles here.
Flatheaded Fir Borer (Phaenops drummondii): This beetle causes similar crown symptoms but can kill large, healthy trees. However, unlike other beetles listed here, the flatheaded fir borer makes 1/8” to 1/4” oval-shaped exit holes without boring dust. Additionally, the flatheaded fir borer does not make galleries in the wood like bark beetles do. Instead, the larvae of these beetles make pupal chambers on the inner side of their host’s bark.
Differences between Douglas-fir engraver beetle and Douglas-fir pole beetle and management strategies:
Differences between Douglas-fir engraver beetle, Douglas-fir pole beetle, and Douglas-fir beetle:
Evaluating tree status during/after a beetle attack: