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The Douglas-fir Bark Beetle
SOURCE: Field Guide to Diseases and Insect Pests of Idaho and Montana Forests, US Department of Agriculture Forest Service 1987; PN# R1-89-54

Douglas-fir Beetle

Dendroctonus pseudotsugae Hopkins

Hosts.--Douglas-fir. Western larch may be attacked but seldom, if ever, is brood produced in those trees.

Distribution.--Wherever Douglas-fir is found in Idaho and Montana.

Damage.--Adults and larvae feed in phloem layer of inner bark, girdling tree, and usually killing it. Strip attacks may be made. Tree is inoculated with blue stain fungi. Trees less than 12 inches d.b.h. are seldom attacked.

Identification.--Evidence that a Douglas-fir has been successfully attacked is the red-orange boring dust in bark crevices (fig. 01) or on the ground around the tree. Attacks are often high on the tree's bole, so careful inspection may be required to determine if beetles are present. Sometimes the most evident sign of infestation is the clear resin exuded from entrance holes high on the stem near the upper limit of infestation (fig. 02). These "pitch streamers" are often visible from a considerable distance.

In the phloem, egg galleries are parallel to wood grain and are commonly 8 to 10 inches in length. Eggs are laid alternately along opposite sides of alleries (fig. 03). Larvae mine outward from the egg gallery and later eed toward the outer bark. Most broods overwinter as adults in the outer bark. The primary attack period is from mid-April to early June. Beetles that overwintered as larvae attack in midsummer. Adults are about one-fourth inch long, and are black with red-brown wing covers.

Similar damages.--Attacks by secondary bark beetles may produce boring dust in bark crevices. It is usually finer than that made by Douglas-fir beetle. Gallery pattern will distinguish Douglas-fir beetle.

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