Douglas-fir beetle

Distribution

British Columbia

Micro-habitat(s)

Damage, symptoms and biology

The first obvious evidence of attack in a stand is the presence of trees with discoloured foliage. However, this discoloration may not occur until a year following attack when the beetles are ready to leave (in some cases, may have already left) to attack new host material. This discoloration may be observed from the air or from high vantage points. Confirmation that the damage is due to Douglas-fir beetles may be obtained by removing bark and observing the typical gallery systems.

The gallery system is in the inner bark next to the sapwood. The egg galleries made by the parent adults are parallel to the grain of the wood, usually with a slight hook or curve at the beginning. They are usually 20 to 25 cm long but may be 75 cm long and 5 to 6 mm wide. The larval galleries diverge from the egg groups in fanshaped groups that alternate from side to side of the egg gallery. Most of the egg gallery and all of the larval gallery are packed with boring dust. The larval mines frequently disappear from the inner surface of the bark since the larvae, when nearing maturity, often bore into the inner bark. The pupal cells are constructed at the end of the larval mines.

Brood survival in standing trees is usually higher and more concentrated in the middle portion of the infested stem. In downed trees, egg galleries tend to be uniformly successful but more dense on the shaded underside.

Trees are usually not infested above a top diameter of 15 to 20 cm. Other bark beetles often occur in the top of the stem, especially Scolytus tsugae (Swain) and Pseudohylesinus nebulosus (Le Conte).

The work of adults and larvae eventually girdles the tree and, along with an associated fungus, results in the tree’s death. Foliage discoloration, from green to pale yellow-green to red, occurs a few months to a year after attack, depending on seasonal weather, locality, date and intensity of infestation, and elevation. The red foliage remains on the tree for an average of 2 years. Occasionally the needles may drop without discoloration. Often conks of the pouch fungus, Cryptoporus volvatus(Peck) Shear, form on the outer bark the year following attack.

The egg is elliptical, pearly white, and 1 to 1.2 mm long. The larva is a white, legless grub with a pale brown head, about 6 mm long when mature. The pupa is white to light tan, about 6 mm long, with adult features (legs, wings, etc.) visible. The adult is a stout, brown to blackish-brown beetle with reddish elytra, 4 to 7 mm long.

The duration of the life cycle is approximately 1 year and two broods may be produced each year.

The Douglas-fir beetle overwinters primarily as young adults or as mature or nearly mature larvae. The adults typically fly and attack susceptible trees in the spring, shortly after daytime temperatures exceed 18⁰C. The major flight period usually occurs in May and June. Larvae that have overwintered complete their development and emerge in July and August. The parent beetles occasionally emerge the same summer for a second flight or “summer flight”, and then attack fresh material and establish a second brood.

Main host(s)

Douglas-fir, Rocky Mountain Douglas-fir, western larch