What's happened to our fir trees? Mystery solved!

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Adult flatheaded wood borer

The flatheaded fir borer is a wood-boring beetle that feeds beneath the bark, similar to a bark beetle. While most wood boring beetles in western
conifers infest dead or dying trees, the flatheaded fir borer can attack and kill healthy trees, acting as a primary pest. It is particularly aggressive, targeting Douglas-fir trees at lower elevations and dry sites, especially during droughts. It can also infest and kill western larch trees. This beetle is distributed across North America and has been reported in Europe.

Flatheaded wood borer larvae

The life cycle of the flatheaded fir borer lasts about a year, but it can be longer depending on the quality of the host tree. Adult beetles emerge in spring and feed on conifer needles before finding a suitable host. They lay eggs in bark crevices, and the hatched larvae immediately burrow into the inner bark. Larvae primarily feed on cambium and some phloem without entering the sapwood. In late summer or early fall, larvae move to the outer bark to construct pupal cells and overwinter. Adult beetles emerge the following spring.

The flatheaded fir borer commonly infests felled or weakened trees, such as those affected by drought, fire, defoliation, or mistletoe. The beetle can infest the entire tree, including exposed roots. Attacks often lead to tree mortality, with patterns concentrated along edges, ridges, or drainages. Infested trees can defend themselves by producing resin, but if unsuccessful, the tree may die. Detection of infestation is challenging as there are no external indicators, although woodpecker activity on the bark can sometimes indicate infestation.

Management strategies include replanting with appropriate tree species, avoiding root damage, removing fire-damaged trees, and promoting tree vigour. Insecticides have not proven effective against the flatheaded fir borer. Natural regulation of populations occurs through parasitic wasps and woodpeckers that prey on the larvae.

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Douglas fir trees that have been infected by flatheaded wood borers