Forest and Wildlife Extension

Forest Health Notes

A Series for the Non-Industrial Private Forest Landowner

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Fir Engraver
(Scolytus ventralis)

Hosts: Grand fir, rarely Douglas-fir, Engelmann spruce, mountain hemlock, western larch

Quick ID:

• Reddish-brown or white boring dust in bark crevices or at base of tree
  
• Egg galleries straight, across the grain (horizontal)
  
• Larval galleries at 90 from the egg gallery
  
• Galleries deeply etch sapwood
  
• Galleries free of frass
  
• Foliage may thin, yellow, or redden

Field Identification

Tree: In Washington this beetle primarily attacks grand fir, although it has been observed on occasion attacking Douglas-fir, Engelmann spruce, mountain hemlock and western larch.
Reddish-brown or white boring dust may be found in bark crevices, caught in spiderwebs, or at the base of the attacked tree. No pitch tube is found as true firs are nonresinous. Under
the bark, the egg gallery is straight and perpendicular to the grain (horizontal). Midway along its length a larger nuptial chamber is found. Larval galleries are formed at right angles to the egg
gallery. The egg gallery and the larval galleries deeply etch the sapwood, leaving marks that can easily be identified well after the death of the tree. Both egg and larval galleries are free of frass (tightly packed boring dust and insect droppings). On successfully attacked trees, the foliage may thin, yellow, or redden. On unsuccessfully attacked trees, the attack may heal over leaving an area of rough bark 1/4 to 1/2 inch across.

Insect: Adult fir engravers average about 4 mm in length. They are black and the posterior of the abdomen is prominently concave on the underside. Larvae are small and white.

May be confused with: No other bark beetle has straight, across-the-grain galleries.

Life cycle: Adult insects emerge and fly in search of new hosts from June to September, with the greatest emergence in mid-July. Attacks seem to be made at random on both resistant and susceptible trees. Females attack trees first, tunnel into the inner bark, and await the males in the nuptial chamber. Other females will attack the same tree after a first attack, particularly if the first attack was successful. After mating, the female will tunnel horizontally from the nuptial chamber, depositing eggs in niches cut in the walls. Resistant reactions by the host tree may cause the female to abandon the attack at this point; in this case, the attack heals over, leaving a brown
pitch pocket in the wood that will degrade the value. In contrast to Douglas-fir and mountain pine beetles, fir engravers are not very tolerant of the resins produced by resistant trees, and will readily abandon the attack if they incur too much resistance. Furthermore, eggs laid in resistant trees will be poisoned by these resins. However, in a susceptible tree, eggs will hatch within two weeks. Larvae tunnel away from the egg gallery to feed, and pupate at the end of the larval mine. Larvae mostly overwinter before pupating and developing into adults in the spring. New adults bore through the bark in summer and seek out new hosts. Normally there is one generation per year.

Predisposing agents: Fir engravers are a secondary pest, meaning that they cannot successfully attack and kill healthy vigorous trees (in contrast to other bark beetles such as Douglas-fir, mountain pine and western pine beetles). They prey upon weakened, dying, or recently killed fir trees, and are well-adapted to finding this normally scattered food source. Certain agents of stress have been shown to predispose firs to engraver attack. These include drought; overstocking, especially if a high percentage of the stand is true fir; root disease, particularly Annosus root disease; defoliation, particularly by Douglas-fir tussock moth; and the presence of abundant dead
material such as slash and windthrow nearby.

Impact: Fir engraver is considered to be a major pest of true fir in western forests, but beetle-induced mortality can mask mortality due to predisposing factors such as drought and root disease. Nevertheless, in 1990 the USFS reported the fir engraver had killed 238,000 trees over 245,000 acres in Washington, with a loss of 12.744 million cubic feet of lumber. In 1991, it was responsible for the death of 68,674 trees over 146,542 acres with a volume loss of 3.684 million cubic feet of lumber.

Management: Direct control measures (e.g. felling, burning, spraying) for fir engravers are impractical. Therefore, minimizing stand stresses to keep trees healthy and vigorous is the only real option for control.

Fir engravers evolved here together with grand fir, and as such will never be completely eradicated. Under normal conditions, they serve to thin out weak trees and open up the stand for regeneration. The species shift in Eastern Washington in the last century away from ponderosa pine and western larch and towards grand fir and Douglas-fir, coupled with the suppression of fire, has created hundreds of thousands of acres of true firsunder stress from overstocking and served to create a habitat for fir engravers that is very favorable. Gradual correction of that situation by good stand management practices will bring down the incidence of fir engraver attack.

Many stand management techniques work well to prevent outbreaks. Attention to root disease centers and overstocking are two big steps towards a healthy, insect-resistant stand. To minimize stand stresses and maintain vigorous growing conditions, stand managers should: (adapted from Berryman: Forest Insects, 1986)

• Choose tree species that are adapted to the area on which they'll be planted. Harvest trees in a way that mimics natural processes, such as cutting small patches or making a seed tree/shelterwood cut to mimic a fire. Particularly with shade-tolerant species such as grand fir, keep in mind other stand health issues such as planting other, more resistant species on root disease centers.
• Remove diseased and unhealthy trees and logging debris; minimize soil compaction and damage to residual trees. However, salvage logging of fir engraver-killed trees should only be undertaken with caution: in Washington fir engravers generally indicate the presence of root diseases, and salvage logging on root disease centers has been shown to worsen their severity (see WSU Extension resources on root diseases).
• Practice "good housekeeping" in the forest by removing windthrown and fire-damaged material before fir engraver beetles breed in it. Large numbers of cull stems should not be left in the forest after a logging operation.
• Encourage diversity in species and age classes. A mixed-species stand is much more resistant to insect pests and diseases than is a pure stand.
• Use thinning, fertilization, prescribed fire, etc. to maintain stand diversity and vigor.

Adapted from Berryman, A.A. Forest insects and Population dynamics of forest insects.

Note: Use pesticides with care. Apply them only to plants, animals, or sites listed on label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

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Forest Health Notes were prepared by Donna Dekker-Robertson, Peter Griessmann, Dave Baumgartner, and Don Hanley, Washington State University Extension. The assistance of Robert L. Edmonds and Robert I. Gara, University of Washington College of Forest Resources, is gratefully acknowledged.

Insect or disease treatments should always be part of an overall forest management planning process. It's a good idea to have a forest management plan for your property. This tool will help you prevent problems and keep your forest healthy. For information on forest stewardship educational programs, contact your local WSU Extension office. For information on technical assistance and financial incentive programs, contact the Washington Department of Natural Resources. The Natural Resources Conservation Service will assist landowners in developing conservation plans.