Spruce beetle

Distribution

Canada

The insect occurs throughout the range of spruce in British Columbia and Yukon.

Micro-habitat(s)

Trunk, Woody debris, Stump, Bark

Damage, symptoms and biology

The spruce beetle, Dendroctonus rufipennis, is the most destructive pest of mature spruce trees in British Columbia. Outbreaks usually last 2-5 years, and these may severely deplete the large-diameter spruce (Piceaspp.) component of forest stands.

Windfall, freshly cut logs, stumps and shaded slash are nearly always infested, but standing trees may also be attacked when conditions are favourable. At low population levels, the spruce beetle prefers weakened or decadent trees and downed host materials such as windfall, logs, and logging residue. Outbreak populations - which may develop when such food and breeding materials are ample - can then surge into apparently healthy trees and stands.

Boring dust is an important means of detection during the first year following attack, although it becomes less conspicuous toward fall and winter through the action of rain and wind. It is light brown and about as coarse as sawdust from a handsaw.

Pitch tubes are occasionally formed by the tree's resin flowing out of the entrance holes made by the attacking beetles. If the beetles are successfully repelled by the tree, the pitch tubes are usually whitish; if the beetles are able to continue their gallery, the pitch becomes intermixed with boring dust and the pitch tubes are reddish brown.

Flaking of the outer bark scales by woodpeckers is a conspicuous but not infallible indicator of bark beetle attack. Trunks of trees worked over by woodpeckers are reddish-purple instead of the normal gray.

Dying and dead spruce do not assume the bright red colour common to most other dying conifers. Fading of the foliage to a yellowish green may be noticeable during the winter following attack, particularly in the lower crown. By the second autumn, most of the needles may have been lost and, for a year or two, the tree will have a reddish appearance from a distance due to the colour of the small, bare twigs. When these twigs fall, the trees are less conspicuous. In general, foliage discoloration and loss are not apparent until a year or more after attack.

In British Columbia, the spruce beetle usually has a 2-year life cycle. Some populations (or parts of them) may mature in 1-3 years, according to geographical location, elevation, and variations in the mean temperature of the spring and summer months. In the most exposed host materials such as stumps and log decks and along stand edges, broods often develop as mixtures of 1-year and 2-year cycle individuals, depending on the amount of solar radiation on infested bark.

Description of the spruce beetle
Egg: Pearly white, oblong, 0.75-1.0 mm long. Eggs are laid in galleries about 13 cm in length.

Larva: Stout, cylindrical, legless, and wrinkled. The head is pale tan and the body creamy white. When fully grown, the larva can be 6-7 mm long.

Pupa: Creamy white, becoming pale tan near maturity. Pupae have wings, legs and antennae that are visible, and are about 6 mm long.

Adult: Robust and cylindrical, about 6 mm long. Immature adults range from pale yellowish to medium brown; mature adults are all black or have a black head and prothorax and reddish brown wing covers.

Life cycle (East of the Rockies)

Main host(s)

Black spruce, Engelmann spruce, red spruce, Sitka spruce, white spruce