The western pine beetle creates ponderosa pine snags. Mortality typically occurs in large, scattered, individual trees, but D. brevicomis also commonly kills groups of trees and smaller trees down to pole-size. Woodpeckers forage on western pine beetle larvae throughout the year following initial attack, as well as on other wood boring insects that colonize and remain in trees for several years after the trees die. Western pine beetle and woodborer associates promote the development of quality cavity nesting habitat in the thick sapwood of large ponderosa pines because they vector the common sap rot fungus Cryptoporus volvatus and create entry courts utilized by other decay fungi when feeding and tunneling. Canopy gaps form where groups of trees or very large trees are killed, increasing the structural and sometimes the compositional diversity of stands. Trees killed by the western pine beetle contribute to levels of down wood when they eventually break or fall over.
Found throughout Oregon associated with the range of its host. Occurs in Washington east of the Cascades crest. (See also Important Habitats and Outbreak Dynamics).
Western pine beetle causes small globules of pitch, called pitch tubes, to form on the tree bole at points of entry. Pitch tubes caused by the western pine beetle are white to reddish brown, inconspicuous, and about 6 to 13 mm (1/4 to 1/2 in) in diameter. They are usually present on trees colonized by the western pine beetle, however, severely stressed trees may not produce enough pitch for normal tube formation. Orange-red boring dust on the bark and around the bases of trees provides an external clue to inspect more closely for the presence of western pine beetle. Positive identification is best achieved by removing a portion of the bark so that egg gallery patterns may be inspected. Crowns of infested trees fade from yellow to straw to sorrel within a year. Fading may begin during the summer of colonization or the following spring. Flakes of bark caused by woodpeckers foraging for larvae are frequently found at the bases of trees. As woodpeckers forage for western pine beetle larvae, they flake off the outer bark, exposing smoother areas of bright-orange inner bark and giving affected areas of the trunk a “shaved” appearance. This behavior contrasts with the pattern of woodpecker foraging for mountain pine beetles (which finish their development near the sapwood rather than in the middle and outer bark), where the bark is flaked off in spots all the way to the sapwood.
Egg galleries are randomly sinuous and long, crossing and recrossing in a spaghetti-like pattern. Fresh galleries are filled with frass. Larval galleries radiate away from the egg gallery for a short distance (up to 13 mm, or 1/2 in), and then appear to stop abruptly at the point where larvae tunnel outward into the corky outer bark. The gallery patterns often are more evident on the bark underside than the sapwood surface.
Larvae are white legless grubs with brown heads. Pupae are soft and white, with body forms somewhat resembling adults. Adult beetles are dark brown to black and about 6 mm (0.2 in) long, with clubbed antennae and rounded posteriors.
The western pine beetle has two generations per year in Oregon and Washington, with overlapping generations and two main flight periods beginning in early June and again in late August. There are four life stages: egg, larva, pupa, and adult. A relatively few individual female beetles initiate colonization. After successfully entering a tree, pioneering females release pheromones that attract males and other females. Attacks begin on the midbole, and subsequently fill in above and below as the time progresses. Mating, egg gallery construction, and egg deposition follows. Some adults may emerge to reattack and establish another brood elsewhere in the same tree or in other trees. Following egg hatch, larvae mine away from the egg gallery for a short distance, then turn and mine towards the outer bark, where they complete their development. The winter is spent mostly in the larval and adult stages. Adult beetles carry spores of a blue-staining fungus in special structures on their heads called mycangia. The stain fungi are introduced into the sapwood as the beetles excavate egg galleries, where they aid the beetles in overcoming the tree by interrupting the flow of pitch and the flow of water to the crown.
Western pine beetle colonization sometimes occurs in association with other insects such as pine engraver beetles, mountain pine beetle, or California flatheaded borer.
Western pine beetle is most active in locations east of the Cascades crest and in southwestern Oregon. Activity is rare in other regions west of the Cascades crest. It is especially prevalent in very dry areas, and may cause more ponderosa pine mortality in these areas than mountain pine beetle. Low-vigor trees weakened by overcrowding, suppression, drought, injury, or diseases provide important habitat for western pine beetle populations when beetles populations are low, and act as “focal” trees for group attacks when beetle populations are high. Western pine beetles normally do not reproduce in freshly cut trees.
When population levels are low, western pine beetles breed in older, low-vigor trees and trees damaged by lightning, wind, fire, other insects, or disease. Mortality tends to occur as scattered, individual, low vigor trees, especially those with diameters greater than 51 cm (20 in), causing a beneficial thinning effect by removing low vigor individuals from stands.
During droughty periods, western pine beetle populations behave more aggressively, attacking, in addition to low-vigor individuals, apparently healthy trees and sometimes causing extensive mortality in overcrowded mature and second growth stands. Behavior during outbreaks is similar to that of the mountain pine beetle. The first beetles starting an attack are nearly always attracted to a particular tree. This tree usually has very low vigor, and belongs to one of the larger size classes present in the stand. The aggregating effects of pheromones produced by the pioneering beetles draw in large numbers of colonizing beetles, which “spill over” onto surrounding trees. Under highly epidemic conditions, as many as 200 trees may be killed in a group, but groups of 3 to 15 trees are more common. The larger trees in a stand usually are attacked first, and proportionally more large trees are killed than smaller trees.
Like the mountain pine beetle, certain stand characteristics are particularly associated with western pine beetle activity. The most important of these are tree diameter and spacing among trees. Stands having close spacing and trees with large diameters are more attractive to beetles than stands having wide spacing or stands having trees with small diameters (averaging less than 25.4 cm). Large host trees usually have ample quantities of phloem with sufficient thickness to support successful brood production. However, their natural defenses render them resistant to attack when they are growing well and not under moisture stress. Dense stands containing large host trees provide very favorable conditions for western pine beetle because the large trees, under stress due to the effects of competition, are easily colonized and provide abundant suitable habitat that facilitates brood survival.
Table 1 displays “rules of thumb” threshold values for stand susceptibility to western pine beetle outbreaks in Oregon and Washington. Although many aspects of western pine beetle relationships old growth pine are well documented, little similar documentation is available for western pine beetle in second growth stands. Considerable work has been done for mountain pine beetle in second growth stands, however, and these relationships, with minor adjustments, are assumed to represent western pine beetle susceptibility as well. Units are given in English units because of common use. Ponderosa pine basal area thresholds are known to vary with site quality, with poorer sites having lower thresholds and more productive sites having higher thresholds. Stands meeting or exceeding these thresholds have a high likelihood of experiencing significant structural changes during the next 10 years.
Table 1. Threshold values for western pine beetle risk in Oregon and Washington.
|Host Susceptibility Class||Proportion of host1 (tpa)||Average DBH of host (in)||Average Stand Basal Area1 (sq ft/acre)|
|Ponderosa pine||75-100||101||150 (80-150)2|
|Medium-size to giant legacy ponderosa pines||N/A||N/A||150 (80-150)2|
1As computed for trees greater than or equal to 5 inches dbh
2Varies with site quality. As a general rule, aim for a maximum of 80 sq ft/acre of basal area on dry sites, 100 to 120 sq ft/acre of basal area on moderate sites, and 150 sq ft/acre of basal area on wetter sites. Refer to Cochran (1992, 1994) for site-specific guidelines.
Intense western pine beetle activity in dense second-growth ponderosa pine stands retards the formation of large tree structure because the beetles tend to remove the largest trees present. When low intensity beetle activity persists from year to year in a stand, it favors the development of increased structural and age diversity because the beetles preferentially attack a portion of the largest trees present each year, creating patches of mortality and associated new growth of various stages throughout the stand.
When scattered medium-size to giant legacy ponderosa pines growing in stands of smaller trees are killed by western pine beetle, small canopy gaps are created. This type of activity in mixed conifer stands tends to accelerate the natural succession of mixed conifer stands to more shade tolerant species.
Large ponderosa pine trees that are highly susceptible to attack by western pine beetle may be identified and selected for retention as wildlife trees by using Keen’s tree classification system for tree age and crown vigor (Figure 1). Older trees (age classes 3 and 4) with lower vigor crowns (crown-vigor classes C and D) have a high likelihood of being killed by western pine beetle. This strategy offers a lower-cost alternative for snag creation than topping or girdling, but less certainty that the selected trees will die within a prescribed period of time.
Snag creation by pine beetles could be encouraged in managed stands by selecting for retention patches of densely stocked areas meeting risk criteria for western or mountain pine beetle during selective cutting activities. Although a more passive and less certain approach to snag creation than topping or girdling, such patches could provide attractive habitat for future bark beetle colonization, and also could be used in combination with active methods of snag creation.
It may be possible to create snags using western pine beetle attractant pheromone, but methodology has not been developed for this specific use. Pheromones present some advantages for over mechanical methods for creating snags, such as topping or girdling, in that they are less expensive, safer to administer, and mimic a natural process. Snags created using beetle attractant pheromones would provide good foraging habitat for woodpeckers due to the large numbers of beetle larvae that would occupy the snags during the year following initial colonization.
During severe outbreaks, the largest trees in the stand usually are killed first, and stocking levels may be reduced below desired levels. Abundant new snags and down wood are created in these areas, but the accompanying losses of existing cover and large stand structure also may degrade the quality of habitat available for some wildlife species, limit the quality and availability of future large dead wood structure, negatively affect water quality in watersheds, impede movement through stands, or increase risk of high-severity ground fire for many decades.
In mixed conifer stands, structural and compositional diversity may be decreased when western pine beetle selectively removes legacy and other ponderosa pine trees from the stand, leaving stands of predominantly shade tolerant species. Legacy pines representing important and relatively rare structure that would take centuries to replace, may be removed from dense stands in a relatively short period by western pine beetle activity.
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.
Non-outbreak levels of western pine beetle populations are most effectively promoted by providing vigorous growing conditions for host stands, and by maintaining a diverse mosaic of age distributions and species compositions across the landscape. In general, thinning appears to reduce attacks by western pine beetle in comparison to unthinned areas. The effects of western pine beetle can be minimized at the stand level by regulating stocking levels in accordance with site-specific stocking guidelines, such as those described by Cochran (1992) and Cochran et al (1994). Thinning around individual legacy ponderosa pine trees to promote tree vigor may retain them on the landscape when it is not possible or desired to thin surrounding stands. These spot treatments typically involve removal of all shrubs and trees beneath the canopy of the tree to a prescribed distance (often 7.6 m) beyond the dripline.
In certain circumstances it may be desirable to protect high-value trees and areas, such as campgrounds, using chemical control methods. Several commercially available formulations of insecticides, both bole-applied and injectable, are effective in preventing western pine beetle attack. Proper timing of injectable insecticide application, to allow material transport up the tree bole before beetle flight occurs, is critical to success.
Currently there are no commercially-available options for utilizing semiochemical repellents of western pine beetle for tree protection. Verbenone-only products have proved ineffective. However, ongoing research of semiochemical combinations of verbenone and non-host angiosperm volatiles may someday provide effective semiochemical tools for western pine beetle management.
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