Fir Engraver
Scolytus ventralis
Key Wildlife Value:
The fir engraver causes significant mortality of mature and pole-size true fir trees, creating snags ranging in size from the largest host trees down to 10 cm (4 in) dbh. Colonized trees provide good woodpecker foraging habitat. Trees killed by S. ventralis eventually contribute to levels of down wood when they break or fall over. Canopy gaps and concentrations of down wood result when groups of trees are killed, increasing structural diversity and enhancing habitat for various cavity-dependent wildlife. Topkill and dead patches of cambium caused by fir engraver facilitate stem colonization by decay fungi and contribute to the formation of unique dead wood structures in living trees.
Distribution in Oregon and Washington:
Found throughout both states. (See also Important Habitats and Outbreak Dynamics).
Hosts:
Primarily grand fir, white fir, and Shasta red fir, occasionally subalpine fir and other Abies species.
Diagnosis:
Trees may be killed singly or in groups. Dying firs fade to yellow-green and orange, and then turn a deep maroon-red. Colonized trees are difficult to detect until foliage discolors the year following successful attack because external signs are not always present. During the summer, red-brown or white boring dust may mark new attack sites on the bole. Clear pitch sometimes streams from the point of attack. Trees with less than 10 pitch streams on the main bole are likely to survive. Abundant buckshot-size emergence holes are sometimes evident on the bark of trees after broods have emerged. Small white conks of Cryptoporous volvatus, a saprot fungus that is often carried into trees by fir engraver beetles, frequently fruit on the bark surface one to two years following attack (pouch fungus is believed to have additional modes of transmission, so the presence of P. volvatus conks alone is not a diagnostic feature for fir engraver).
Larvae are white legless grubs with brown heads. Pupae are white and soft, with body forms somewhat resembling adults. Adult beetles are shiny, black, and about 4 mm long. When viewed from the side, the incurved posterior end of the adult's abdomen has a “sawed-off” appearance. During winter, mostly larvae are present under the bark of colonized trees.
Fir engraver mortality is frequently associated with root disease, drought, and defoliation. The fir engraver also colonizes recent windthrows and freshly cut logs.
Life History:
In Oregon and Washington, the fir engraver beetle most commonly has a one-year life cycle. There are four life stages: egg, larva, pupa, and adult. Fir engravers attack trees from June through August with peak activity occurring during July and August. They prefer to attack portions of stem that have diameters greater than 10 cm (4 in), and seem to especially favor the “branch collar” area at the junction of a branch and the tree bole. After mating, the female begins construction of a two-branched egg gallery extending in opposite directions from the nuptial chamber, depositing her eggs along both sides of the gallery. Fir engravers carry spores of a brown stain fungus into the sapwood as they excavate their egg galleries. The fungus discolors the sapwood surrounding the egg gallery to a yellowish brown within four to six days, blocking the flow of tree sap and water and helping the broods to survive, even in trees that do not die as a result of fir engraver attack. After hatching, larvae tunnel away at right angles to the egg gallery, feeding on the phloem. Winter is spent primarily in the larval stage. In spring, larvae feed for a short period before pupating in cells they construct at the ends of the larval mines. Pupation lasts one to two weeks, and the new adults remain in the tree for another two weeks before emerging to attack other trees.
Fir engravers frequently colonize trees without killing them, successfully colonizing only portions of the bole in what are called “strip attacks”. Healthy trees can recover from strip attacks, and some trees are attacked in this way many times. Strip attacks cause roughened patches of bark where the cambium layer beneath has died, or scattered dead branches that are girdled at the branch collar by egg galleries.
Woodborers such as the flatheaded fir borer or the roundheaded fir borer are frequently found in the thick-barked portions of a tree in association with fir engraver.
Important Habitats and Outbreak Dynamics:
Fir engraver activity throughout Oregon and Washington is strongly associated with root disease. Fir engraver activity is most prevalent east of the Cascade Mountains crest and in southwest Oregon, where drier conditions normally prevail and periodic drought is not uncommon. During droughty periods fir engraver may also cause considerable mortality of grand fir in the Willamette Valley of western Oregon. Weak, low vigor trees that are pole-size and larger and freshly windthrown material provide important habitat for fir engraver, because it is unable to colonize entire trees unless they are significantly stressed or weakened. Low-level populations survive in patches of cambium, windthrown trees, and trees infected with root disease.
During periods of drought in southern Oregon, fir engraver activity in white fir and Shasta red fir may increase in logged areas following selective harvest logging operations that provide ample breeding material in the form of slash having diameters exceeding 10 cm (4 in), and that result in “thinning shock” of the residual trees. Thinning shock is a temporary condition that is most likely to occur when dense stands of trees having low crown ratios are suddenly opened, exposing residual trees to unaccustomed levels of solar radiation and wind. Soil compaction may also contribute to tree stress in recently logged areas. Increased fir engraver activity associated with logging is uncommon elsewhere in Oregon and Washington.
Major fir engraver outbreaks are frequently associated with large-scale disturbance events that severely stress host trees over a large geographical area, such as sustained regional drought or significant defoliator events caused by true fir defoliating insects, especially western spruce budworm or Douglas-fir tussock moth. Beetle populations respond to the plentiful, easily colonized habitat available during such periods by increasing rapidly. Fir engraver activity in stands affected by defoliation typically reaches its highest level within one to three years following peak defoliation. Dry true fir sites that under natural fire regimes were occupied by pines experience the highest levels of mortality during periods of drought. Fir engraver caused spectacular levels of mortality in true fir forests growing on very dry sites near the forest-grassland interface in southcentral Oregon during a protracted drought period in the late 1980’s and early 1990’s, sometimes resulting in death of entire stands. Fir engraver populations typically subside naturally when tree vigor returns to more normal levels.
The pattern of tree mortality caused by fir engraver is quite varied, as it is influenced so strongly by individual tree vigor and stress.
Opportunities for Manipulation to Increase Wildlife Habitat:
Little opportunity exists to manipulate fir engraver to increase wildlife habitat. Unlike many other important bark beetles species, fir engraver beetles do not seem to produce aggregating pheromones. They apparently rely upon attractive chemicals produced by diseased, injured, and stressed host trees for location of suitable habitat.
Potential Adverse Effects:
In stands dominated by true fir growing east of the Cascades crest and in southwestern Oregon, intense fir engraver activity may sometimes cause significant mortality over large areas, decreasing cover and negatively affecting the quality of habitat available for some wildlife species and the quality of water resources. Associated increases in fuel accumulations may increase the risk of high severity ground fires for many decades. Trees killed in campgrounds and other developed areas can pose hazards to public safety and structures, and may negatively affect screening, shading, and aesthetic values.
How to Minimize the Risk of Adverse Effects:
The extent and severity of large-scale outbreaks associated with drought conditions may be reduced by decreasing the extent and occurrence of true fir host on dry sites, e.g., dry grand fir or dry white fir plant associations that historically were occupied by fire-climax pine. Salvage and sanitation measures (the removal of trees infested with fir engraver broods and trees in poor condition that are likely to provide breeding habitat) are ineffective at halting a drought-induced outbreak once it has begun. Likewise, the extent and severity of fir engraver outbreaks that follow defoliator outbreaks, as well as the defoliator outbreaks themselves, may be reduced by reducing the abundance of true fir on the landscape, especially on dry true fir sites. This may be accomplished by encouraging the growth of non-host species during regeneration, thinning, and selective cutting activities.
Carefully executed thinning of overstocked true fir stands during periods of normal precipitation may help to reduce the impacts of fir engraver activity by reducing competition and increasing tree vigor, however, thinning of true fir stands generally is not as beneficial as thinning in pine stands. Residual true fir trees are especially sensitive to soil compaction and bole injuries during logging operations, conditions which facilitate root pathogen infection and intensification, and have often been left to grow in stands so dense that they have failed to develop good crown ratios, making them susceptible to thinning shock. As a result, thinning such stands may cause them to become more susceptible to fir engraver than they were before. When thinning white fir and Shasta red fir stands in southern Oregon during droughty periods, local intensification of fir engraver populations may be minimized by managing operations to minimize soil compaction and tree injury, and to ensure that pieces of slash greater than 10 cm (4 in) are not created from January through July.
References
Ferrell, G.T. 1986. Fir engraver. Forest Insect and Disease Leaflet 13 (revised). USDA Forest Service, Washington, D.C. 8 p.
Furniss, R.L., and Carolin, V.M. 1977. Western forest insects. USDA Forest Service, Washington, D.C. Misc. Publ. 1339. 654 pp.
Goheen, E.M. and E.A. Willhite. 2006. Field guide to common diseases and insect pests of Oregon and Washington conifers. USDA Forest Service, Pacific Northwest Region, Portland, OR. R6-NR-FID-PR-01-06. 335 pp. http://www.biodiversitylibrary.org/bibliography/80321#/summary