Spruce beetle management

Feature Article:

Management of Spruce Beetle in Minnesota:
Guidelines for Professional Land Managers

M.A. Albers, and S.J. Seybold

Spruce beetles are bark beetles native to Minnesota that attack white spruce trees. In Minnesota, they prefer trees 12 inches in diameter and larger. They will, however, occasionally attack smaller diameter trees. Spruce beetles have been found attacking and killing spruce trees predominately along the northern half of the North Shore of Lake Superior and within several miles of the lakeshore.

Pitch tube oozing from bark of a spruce tree infected with spruce beetle.
Pitch tube

Spruce tree killed by spruce beetle.
Spruce tree killed by spruce beetle

Spruce beetle.
Spruce beetle

To minimize spruce beetle damage you must learn to identify the signs of spruce beetle attack. The best evidence of attack includes reddish boring dust at the base of the tree, and secretions of resin (pitch tubes) on the bark surface of the tree stem. Fallen trees or trees that have more advanced infestations may only have boring dust, but no pitch tubes. Removal of a small piece of bark will reveal the galleries and mines of the adult and larval beetles. The needles of heavily attacked trees will eventually become tan or red and the tree will die. However, it may take six to twelve months or longer for the needles to turn color. A tree with just a few attacks may not die during the first year, but it may continue to be attacked and eventually die in the following year or years. Infested trees are a source of more beetles that are likely to threaten nearby healthy white spruce trees.

Management of spruce beetles is based on three key strategies:

  • Maintain healthy and vigorous trees
  • Remove or destroy fallen trees and tree parts (i.e. high tree stumps) capable of producing large numbers of adult beetles
  • Manage and control spruce beetle populations

The first strategy is to make your trees as healthy and vigorous as possible. This will make them less attractive to the beetles and better able to defend themselves if they are attacked. The second strategy involves sanitation and removal of breeding material to prevent large populations of spruce beetles from developing on the property that you manage. These small (3/16th by 1/4th inch) beetles must attack trees in large numbers to successfully kill the trees. Removal of material in which they can breed and develop reduces the available habitat capable of producing large numbers of adult beetles that can attack nearby healthy trees. Fresh windthrown trees can produce 5X to 10X the number of beetles that a standing spruce tree can produce. The third strategy, managing and controlling spruce beetle populations, involves removing or destroying infested trees to reduce the size of local beetle populations and/or protecting your uninfested trees from future attack.

Listed below are suggestions for reducing damage and mortality to white spruce by spruce beetles:

  1. Improve and maintain the health and vigor of your trees. Fast growing, healthy trees are more resistant to spruce beetle attacks than slow growing, unhealthy trees. Spruce beetles prefer trees weakened by factors such as defoliation, drought, disease, root and stem wounds, and overcrowding (high stand density). Weakened trees are more susceptible and not as capable of defending themselves against spruce beetle attacks as are healthy trees.

    1. Water trees during periods of drought. Large trees require substantial amounts of water. Place a hose on the ground under the trees and allow water to soak the area under the crown out to the edge of the drip line. Do not use sprinklers because moisture sprayed on the needles increases the risk for some needle diseases.
    2. Maintain a 3-4 inch thick layer of mulch over the root zone of the tree using wood chips, shredded bark or other organic material. This minimizes competition, moderates soil temperature and maintains soil moisture.
    3. Avoid wounding the tree stem. This weakens the tree and releases volatile resins from the wounds that may attract beetles.
    4. Avoid injuring tree roots and compacting soil, particularly during construction, telephone line installation, etc.
    5. Protect trees from defoliation by other insects. Insects such as spruce budworm or sawflies that feed on needles may reduce tree vigor making a tree more susceptible to attack by spruce beetle. An insecticide treatment to prevent defoliation may be necessary if populations of defoliating insects are present on spruce.

  3. Sanitation and removal of breeding material helps prevent the buildup of large populations of spruce beetles. Spruce beetles need to attack trees in large numbers in order to successfully enter, colonize, and kill a healthy tree.

    1. White spruce trees that have tipped over or are damaged during storms should be removed before May 1st. Trees that are damaged during the summer should be removed promptly, or at least before August 31st. If these trees are not removed promptly (within several weeks of falling or being damaged) spruce beetles may colonize and reproduce beneath the bark resulting in the development of large numbers of beetles that emerge and may attack nearby healthy white spruces. These trees should be burned, debarked, or removed from the site.
    2. In northern Minnesota, spruce beetles appear to readily colonize the stump remaining from trees that have broken or fallen during winter and spring storms. Cut off white spruce stumps as close to the ground as possible before May 1st. Debarking the remainder of the stump including the portion down to 6 inches below ground, is suggested for complete removal of breeding material.
    3. Do not store infested or recently cut (fresh) white spruce firewood in your yard unless it is either debarked or at least 3 mile from the nearest susceptible spruce. Spruce firewood that is dry (dead more than 1 year), and not infested with spruce beetle can be safely stored in your yard.
    4. Standing, or fallen white spruce trees or tree parts that have been dead for 3 or more seasons do not have to be removed because they will no longer attract or produce spruce beetles. However, standing dead trees near houses are hazardous and should be removed before they decay and potentially harm people or property.

  5. Management and control of an existing spruce beetle population is necessary if a large population has already developed on or near your property and is already attacking or killing trees.
    1. Examine the white spruce trees on your property in August, looking for evidence of recent spruce beetle attack. A tree heavily attacked during the summer should be removed before August 31st, if possible. (Heavily attacked trees will have reddish brown boring dust on more than 50 percent of the tree?s circumference at ground level.) The tree should be cut down and burned, debarked, or removed from the site. The stump should be treated as described above. Burning or debarking kills all beetle life stages under the bark and prevents them from emerging and attacking nearby trees. All outer bark must be burned or removed from the tree to ensure that all life stages of the insect are killed. Beetles may emerge from dead trees for a period of two years following attacks. Spruce beetle is generally not found in trees that have been dead 3 or more years.
    2. The two suggestions listed below have not been tested or applied in Minnesota for spruce beetle control, but have proven effective elsewhere.

    3. Trap logs are green trees that are felled before the beginning of the initial beetle flight (usually around May 1st). They are very attractive to spruce beetles. The logs must be removed or destroyed before the beetles complete their life cycle and emerge through the bark of the logs. Trap logs are often used in western North America to trap beetles, and thereby reduce or prevent attacks on living trees growing near an active spruce beetle infestation. Spruce beetles prefer downed trees to standing live trees, and according to the Canadian Forest Service, trap logs attract 10 times the number of beetles that are attracted to standing trees. Large diameter trees (> 16"diameter) make the best trap logs. Branches should be left on the trap logs because spruce beetles prefer to attack shaded portions of the tree stem or bole. Improper use of trap logs may exacerbate a spruce beetle problem. Trap logs should be at least 100 feet from live white spruce trees. If a trap log is adjacent to a live white spruce tree, the standing tree may also be attacked. All trap logs and attacked standing trees must be removed before August 31st to ensure the effectiveness of this strategy. If trap logs and infested standing trees are not removed or destroyed, beetle populations will increase significantly, attacking nearby healthy spruce during the following spring or summer.
    4. Registered insecticides for spruce beetle (such as Carbaryl 4L, Sevin SL or Astro) can be applied to the stem of uninfested trees to kill beetles as they arrive on the bark surface. These insecticides will not soak through the bark and kill beetles already in the tree. If possible, all of the main stem should be treated. On larger trees, (>16") the stem must be treated to a height of at least 45-50 feet. All bark surfaces must be thoroughly treated up to this height including the base or root collar on the tree. Applications where only a portion of the bark surface on the main stem is treated or the base of the tree is missed are often ineffective. The material should be applied so that the bark surface is dripping during the application. In the western United States, the insecticides mentioned above are effective for one to two years, if the application is properly applied. Fall pruning the lower limbs on the main stem up to a height of 15-20 feet will improve application success. Spring pruning can increase the risk of attack. A licensed commercial applicator with the proper equipment (hydraulic sprayer with 250-300 psi equipped with a #5 or #7 nozzle orifice) will be required to meet the spray height objectives. The insecticide must be handled and applied properly to avoid injury to humans, animals and aquatic habitats. The insecticides are not selective for the spruce beetles; other insects (including beneficial species) will also be killed if the insecticide is ingested or is contacted. Remember, if applying pesticides, the Label is the Law.