Spruce Beetle in Alaska Forests

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The Spruce Beetle in Alaska Forests (PMC-10060)

by Richard A. Werner, written 2006

The spruce beetle is a bark beetle that attacks Lutz and Sitka spruce trees in South central and White spruce trees in Interior Alaska. Bark beetles kill trees by boring through the bark and feeding and breeding in the phloem -the thin layer of soft living tissue directly beneath the bark. The phloem is vital to trees, as it transports food manufactured in the needles down to the roots. If the phloem is girdled, the tree will die.

Spruce beetle-killed trees in Alaska.
Spruce beetle-killed trees in Alaska.

Small populations of beetles are always present in spruce forests. Most of the time, the number of beetles is kept low by parasites and predators of the insect, but when conditions are right, spruce beetles may suddenly increase to epidemic numbers. The right conditions include an abundance of breeding material accompanied by an extremely dry summer. Beetles attack and breed in fresh windthrown trees, felled trees, injured trees and large diameter logging slash. When the beetle population outgrows the supply of dead and injured trees, they move into nearby living trees, particularly mature stands of Lutz, Sitka, and White spruce.

Of all the insects that affect spruce in Alaska, the spruce beetle causes the most damage. Close to 900,000 acres of ongoing infestation and newly infested spruce were detected in 1995.

Signs That Beetles are Present

The primary indication that beetles are attacking a tree is reddish-brown dust which accumulates on the bark, in bark crevices, and on the ground beneath the attacked tree.

Globules of resin or pitch tubes at the entrance hole into the bark are another sign of beetle attack. Entrance holes are usually found in the roots (both exposed and underground) and lower fifteen feet of the trunk. Early detection requires close examination of trees from early June to mid-July. To determine if spruce beetles are present, remove the bark around an entrance hole to locate the adult and larval tunnels.

Resin flow on newly infected trees

Beetles that attack healthy, vigorous trees are usually trapped in a mass of resin and "pitched out" of the entrance hole.

Bark removed by woodpeckers searching for beetles.
A spruce beetle-attacked spruce with bark removed
by woodpeckers.

Beetle infested trees are often sought out by woodpeckers. Pieces of bark chipped away by the woodpecker accumulate on the ground beneath the trees. This is especially noticeable in the winter when the bark accumulates on the snow.

Bark removed by woodpeckers searching for beetles

A change in foliage color is another indication of spruce beetle attack. Needles begin to fade from dark green to pale yellowish-green one year after attack and to reddish-brown the following summer.

In some cases, needle discoloration may not be noticeable until one year after the attack and sometimes not until after the beetles have left the tree. By mid-summer, one year after initial attack, many needles have dropped and the tree turns reddish-brown. Three to five years following attack, the trees appear silvery-gray and remain that way for many years.

Life History

The spruce beetle in Alaska has a one- or two-year life cycle. Adult beetles emerge from infested trees from mid-May to mid-June, and their flight to fresh host materials lasts until mid-July. When the female beetle finds a suitable host, she bores into the bark and constructs and egg gallery in the phloem parallel to the wood grain and usually above the entrance hole. After mating occurs, the female lays whitish-yellow eggs in clusters on either side of the gallery. Eggs hatch into white grub-like larvae which feed in the phloem cross-wise to the egg gallery. Larvae do not enter the wood but may score the outer surface.

Under section of bark showing larval and adult galleries and pupal chambers (one-half normal size)

Adult beetle laying eggs in phloem (twice normal size)

One-year life cycle beetles develop from egg to pupae the first summer. New adults spend the winter under the bark at the base of the infested tree.

Two-year life cycle beetles spend the first winter as larvae beneath the bark. In spring they resume development and eventually transform into white pupae for a short time and then to adult beetles. The Adults then migrate to the base of the dead or dying tree where they overwinter.

Guidelines for Reducing Beetle Infestation

Various activities which disturb the environment of spruce contribute to spruce beetle attack and epidemic outbreaks. These activities include timber harvest; land clearing related to road, seismic line, pipeline, powerline, or building construction; severe winds which cause windthrown trees, and wildfire.

Spruce beetle attacks may be prevented or reduced by following these guidelines:

Proper Management of Spruce Forests

  1. Maintain spruce stands in a healthy and vigorous condition by removing overmature, slow-growing, diseased, injured, and dying trees.
  2. Remove damaged or windthrown trees from spruce stands under management.
  3. Establish a stand rotation age (harvest age) of less than 100 years.
  4. Timber sale size and orientation of cutting areas are important in creating stands that can with- stand high winds. Leave-strips between clearcut or shelterwood cutting areas should be more than 100 feet wide. Timber sales should not be located along ridgetops where shallow-rooted spruce are highly susceptible to high wind.

Timber Harvest

  1. Overmature, slow-growing trees should be removed from forest stands as they are highly susceptible to spruce beetle attack.
  2. Windthrown trees, particularly in recently logged areas, should be removed.
  3. All logs cut after March should be removed and utilized prior to beetle flight the following May. Logs cut during the summer months should be removed shortly after cutting.
  4. All slash and cull logs four inches in diameter and larger should be disposed of by burning, burying, chipping, or peeling.
  5. Stumps should be cut as low as possible.
  6. Whole tree logging will eliminate most of the breeding material usually left in the forest and concentrate it at the logging landing where it can be destroyed.

Infested spruce trees

Rights-of-Way Construction

  1. Timber along rights-of-way for roads, seismic lines, pipelines, and power lines should be cut in the fall and the logs utilized before the next spring. Slash should be treated as described earlier. Trees next to the right-of-way should be examined for beetle attacks in late summer following cutting. If trees are infested, they should be removed.
  2. Care should be taken to avoid scarring trunks with mechanical equipment, severing roots, altering drainage patterns, or severely compacting the soil.

Proper slash disposal along a powerline right-of-way Improper slash disposal along a powerline right-of-way

Home Construction

  1. Trees removed for home construction should be properly disposed of or utilized. If stockpiled for firewood or used for construction, the bolts or logs should be peeled. Mechanical damage to standing trees should be avoided and damaged areas should be cleaned with a knife and allowed to heal over.
  2. Excess soil should not be placed on top of or removed from the area over the root zone. Trees breathe to some degree through the roots and the addition or removal of soil can cause suffocation
  3. Avoid soil compaction around the base of trees and do not surface these areas with rock, concrete, or asphalt. Sewage drainage fields should be located away from trees because excess water can create stress conditions in adjacent trees.
  4. Insecticides can be used to protect live trees from beetle attack. Water solutions of chemicals should be applied with a pressurized sprayer to the trunks of trees before beetle flight. Your local Alaska Cooperative Extension office can provide additional information. Observe all precautions and restrictions when using pesticides.