Spruce beetle
Last updated on January 26, 2024Dendroctonus rufipennis
The spruce beetle is a species of bark beetle native to Canada. Spruce beetle is the most damaging pest of mature hybrid white spruce in British Columbia.
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Description
Adults are hard, stout-bodied cylindrical insects, with black-brown or black reddish wing covers. They are tiny, averaging four to seven millimetres in length (about half a centimetre).
Host tree species
Spruce beetle primarily attacks hybrid white spruce (white spruce and Engelmann spruce hybrid) in British Columbia. Large diameter, mature spruce trees are attacked.
Spruce beetle also attacks several other species of spruce:
Life cycle
The spruce beetle has a one to three-year life cycle. Historically, the two year life cycle has been the most common. When a one-year cycle occurs due to favourable weather, beetle populations can increase rapidly.
Spruce beetles must overwinter once to mature fully. Mature adults emerge and attack new host trees from late May to early July. Some spruce beetle immature adults may emerge from the tree in the fall to overwinter in the soil at the base of the tree.
Adult spruce beetles bore approximately 12cm long galleries under the bark, and lays about 100 eggs per gallery. The larvae hatch and feed under the bark forming fan-shaped galleries.
Life cycle details
A brief synopsis of the spruce beetle's life cycle is as follows:
- Adult females emerge when ambient air temperature exceeds approximately 16* C, find new suitable host material, and emit aggregating pheromones to initiate mass attack
- Males join the females and each pair construct an egg gallery under the bark, parallel to the grain
- The sapwood is inoculated with spores of a blue stain fungus as the egg gallery is built
- Eggs are laid and hatch into larvae which feed on the phloem in feeding channels constructed at right angles to the egg gallery
- After four instars, larvae pupate and develop into adults under the bark
- Young adults pick up blue stain fungal spores while in the pupal chamber
The timing of life-cycle events for any species of bark beetle will vary from year to year and from location to location due to variations in climate and local weather.
Damage symptoms
Fading of the foliage to a yellowish-green is usually not noticeable until 18 months or longer following attack, particularly on rich sites. By the second autumn, most of the needles have usually turned brown and are shed by the following spring. Green needles on the ground or on the leaves of ground cover beneath infested trees may appear before any evidence is visible in the crown itself.
Adults construct galleries in the phloem. Light brown to red-brown boring dust, or frass, will be present on the bark or around the base of infested trees. Pitch tubes are occasionally formed by resin flowing out of entrance holes.
Sometimes flaking of the bark by woodpeckers is a sign of infestation.
The action of the larval feeding in the phloem and fungal colonization of the sapwood completely blocks water uptake and kills the infested tree. In some cases only one side of a tree will be successfully attacked (strip attack); this tree will survive unless living portions are re-attacked in subsequent years.
Spruce beetles normally infest downed trees or logging debris, but when beetle populations are large, they will attack and kill living trees, causing widespread damage.
Similar damage by other insects
Attacks by secondary bark beetles may produce boring dust in bark crevices.
Ips beetle species, sometimes known as engraver beetles, can be distinguished by the gallery patterns and the lack of frass in galleries. The adult ips beetle is different from the spruce beetle because it has a rear concave depression lined with spines. Spruce beetle larvae can be distinguished by the presence of two rear brown hatch marks.
Identification images
Woodpecker bark scaling of an attacked tree
Woodpecker bark scaling of an attacked tree.
Adult spruce beetle
Adult spruce beetle.
Trees fade to yellowish-green and then grey within a year
Trees fade to yellowish-green and then grey within a year following an attack.
Egg and larval galleries are up to 13 cm long
Egg and larval galleries are up to 13 cm long. Frass is usually present in the galleries.
Larvae
Larvae. Note the anal shields.
Fast facts
Appearance: Dark brown-black with reddish wings; cylindrical; 4-7 mm long
Life cycle: Typically two years; warmer climate can require only one year
Overwintering stage: Year 1: Larva; Year 2: Adult
Flight period: May-June
Hosts: Weakened standing large diameter, mature spruce trees or freshly fallen/windthrown trees and stumps