Caring for the Land and
Serving PeopleThe mountain pine beetle (MPB), Dendroctonus ponderosae , is the most aggressive and destructive insect affecting pines in western North America. It belongs to a group of insects known as bark beetles that feed in phloem tissue. Lodgepole and ponderosa pines are the primary hosts, but whitebark and limber pines also are attacked in the Rocky Mountain Region. Occasional hosts may include pinyon, bristlecone, and Scots pines.
The mountain pine beetle typically completes its development in one year, although two years may be needed at high elevations. Adult beetles emerge from previously infested trees in late July or August and fly to attack green trees, where they bore through the bark to lay eggs. The beetle mixes resin with sawdust as it bores and pushes the material to the outside of the bark to form a pitch tube. The mountain pine beetle emits attractant chemicals (aggregating pheromone) that call other beetles to join in the attack on a particular tree, and enables males to locate females. This mass attack is necessary in order to overcome the natural defenses of the tree. When a tree reaches its capacity in beetles, as determined by the proximity of sounds and vibrations made by neighboring beetles boring under the bark, the beetles emit another chemical complex (anti-aggregating pheromone) that tells incoming beetles that the tree is at its maximum capacity. Flying beetles will then search for other host trees, often selecting trees adjacent to those already mass attacked.
Under the bark, the beetles lay eggs along straight, unbranched, vertical galleries. Beetles tightly pack the egg galleries with partially digested wood particles (frass) as they move upward. Eggs hatch into small, grub-like larvae within one to two weeks. These larvae feed horizontally in the phloem, effectively girdling it, which contributes to the tree's death. Additionally, the beetles transmit a blue stain fungus that grows through and clogs the conductive tissues in the tree, and makes moisture conditions more suitable for beetle development. Larvae (and sometimes adults) overwinter under the bark, and resume feeding in spring and early summer. When done feeding, larvae hollow out an oval chamber in which to pupate and then transform to an adult beetle. Adults remain under the bark for a couple weeks or so to harden, mature sexually, and feed prior to emerging. When weather conditions are favorable, beetles bore through the bark to emerge and fly to new host trees.
Although the tree is effectively dead after being mass attacked by MPB, its needles will remain green through the winter following attack. Needles on infested pine begin fading to yellow in the spring, and turn red by late summer. Whole mountainsides may appear to wear a telltale red "sash," though the insects have by then emerged to infest new areas of green pine. Patches of red pines are frequently the first sign of MPB activity to be detected by field personnel. Forest Health Management staff use the red color to estimate damage levels and identify problem areas from aerial surveys. However, inspection from the ground is necessary to determine which trees are currently infested, because their crowns are still green.
The first indication that a green pine tree has been infested by MPB is the appearance of pitch tubes on the bole, where females have entered the tree. Pitch tubes resemble wads of bubblegum, about 1/4 to 1/2 inch in diameter and usually with a central circular hole. Mass-attacked trees generally have pitch tubes scattered a couple inches apart over the entire trunk. Pitch tubes can vary in color from cream to dark red. Closer examination may reveal the presence of reddish boring dust, resembling fine sawdust, in bark crevices and at the base of the tree. Sometimes larger pitch tubes, 3/4 to 1 inch in diameter, may be found widely scattered or on one side of a tree, or pitch tubes may be runny or gooey. These conditions suggest that an attack was not successful and the tree was able to "pitch out" the beetles. Such trees may be successfully reattacked in a subsequent year.
Cutting a section of bark from the trunk of a tree with fresh (soft, not hard and brittle) pitch tubes will reveal the characteristic galleries constructed by the female for egg laying and by larvae during feeding, and the presence of eggs, larvae, pupae, or adults, depending upon the time of year. Within one to several months after the tree has been attacked, bluestain fungi begin to discolor the sapwood, giving the wood a dark blue or blackish appearance.
Woodpeckers may feed on larvae under the bark, especially during winter months. Individual holes in the bark on thick-barked trees or entire strips of bark missing from thinner-barked trees are signs of their activity. Bark flakes may accumulate around the bases of infested trees.
After beetle emergence, small round emergence holes will be present in the bark and densely scattered over the bole of the tree. By two years after attack by MPB, needles turn rusty brown and drop from the dead trees. Trees will appear grey and have hard, brittle pitch tubes, although many of the pitch tubes will likely have fallen off by then.
Millions of trees may be killed each year by MPB throughout the west, and epidemics frequently deplete merchantable pine stands, sometimes converting them to less desirable plant species. In the Rocky Mountain states, MPB may account for about 3/4 of all mortality in pines. MPB epidemics can substantially alter the ecosystem by reducing crown, thermal, and hiding cover, increasing forage production, releasing or converting to other tree species, creating large amounts of dead trees and logs, limiting access for large ungulates and recreationists, increasing fire danger, and providing a different mix of habitats for a variety of animal species.
MPB is a native insect that is always present in pine forests. At endemic levels, beetles survive in stressed or weakened trees and are frequently associated with lightening strikes, breakage of crown or bole, presence of Armillaria root rot, and previous attack by MPB and other bark beetles (Eckberg et al., unpublished). When favorable conditions exist, populations can quickly increase to epidemic proportions. At such times, apparently healthy pines are attacked and frequently killed. Food availability, in the form of thick phloem often found in large trees, is most directly associated with development of MPB epidemics. Generally, weather conditions (except at high elevations for lodgepole pine) and natural enemies play a comparatively minor role in regulating MPB populations, and have little or no association with the initiation of MPB epidemics. Drought has not been found to be associated with MPB outbreaks in this region. Unseasonably low temperatures during early fall (below 0o F), winter (below -34 o F), or mid-spring (below 0o F), can greatly retard outbreaks, but survivors (often those located below snow line, in thick-barked trees, or on slopes above the coldest areas) provide a source for the continuation of epidemics.
MPB populations can build and spread relatively quickly. Triggering mechanisms are not well understood. However, damage levels can increase within a few years from a few isolated trees, to small group kills, to large group kills. The transition from endemic to epidemic is characterized initially by selective killing of the larger diameter trees in a stand. During epidemics, contagious spread is possible, with even small trees being attacked if they are near other attacked trees. In lodgepole pine, MPB can decimate mature forests, often killing virtually all trees over extensive areas. In ponderosa pine, large group killing often occurs over a widespread area of mature and younger overstocked stands; some trees in all age classes escape attack when areas greater than 100 acres are considered. Widespread epidemics on a forest scale are really a composite of a number of patchily-distributed infestations occurring at the stand level. During epidemics, damage is generally heaviest in dense stands of mature or commercial-sized pine. Descriptors of these conditions are the basis for risk rating systems for MPB in lodgepole and ponderosa pine stands.
Stand risk indicates the degree to which the forest vegetation characteristics provide suitable habitat for development of MPB and the likely magnitude of damage should an infestation occur. The actual presence or absence of MPB is not factored into risk determination. Therefore, risk rating in itself does not predict where, when, or how frequently a given stand will be attacked by MPB. However, risk-rating is an important tool for the recognition and prevention of susceptible stand conditions that may lead to epidemics should MPB populations move into specific sites or areas.
In general, MPB prefers stands of dense, large-diameter, mature and overmature pine. Extensive areas of such forested habitat is at greater risk of large-scale epidemic, given the presence of a MPB population, than forested areas that include a mixture of age classes (either between stands or within stands) and tree species. Risk increases the closer the forest resembles an even-aged, pine monoculture of large trees with a closed canopy. Depending on host species, risk-rating systems for MPB are based upon elevation and latitude or stand structure, average tree diameter, and average age or average basal area for a particular stand. Although the rating systems are based on stand averages, dense pockets of commercial-sized trees within a stand may serve as focal points for MPB infestations to get started, even when the stand rating may indicate low risk.
Stand risk ratings should be determined for a given project or analysis area, or during planning for harvest scheduling activities. Low, medium, and high risk stands should be displayed on a map to allow examination of the spatial relationship of these stand conditions. This information can be used to prioritize harvest activity to reduce risk of MPB infestation and damage. Concentrations of high risk stands should be given priority for treatment, based upon consideration of management objectives and desired future condition. High priority should be given to areas where susceptibility to attack and damage is a primary concern in reaching or maintaining management objectives and the potential MPB risk is high or medium. Intermediate priority may be given to high or medium risk stands where susceptibility to attack and damage is a concern in attaining management objectives, but where a higher level of damage may be acceptable than for high priority areas. Low priority may be given to stands where susceptibility to attack and damage is not a concern or where MPB risk is low. Potential for damage in relation to management objectives may be determined by a biological evaluation of probable MPB population trends and possible impacts. Forest Health Management personnel should be consulted to conduct such evaluations.
The risk-rating systems have been incorporated as model subroutines and are automatically calculated by RMSTAND. Risk-rating systems are calculated differently depending upon whether the stand cover type is lodgepole pine or ponderosa pine. Stand risk, regardless of cover type, is available for inventoried stands in the RMRIS/Oracle database and is displayed in the optional field "BUG RISK" in the table R2RIS_SITE. Values of 1, 3, and 5 indicate low, medium, and high risk, respectively. A value of zero indicates no risk calculation was made.
The risk rating system that is currently used for lodgepole pine stands is based upon climatic factors, average tree age, and average tree diameter (Amman et al., 1977). Stands where epidemics typically occur tend to be located at lower elevations, have average tree diameters of 8 inches or greater, and average 80 or more years of age. Risk Classes are assigned for each of the three stand characteristics as indicated in the table below.
| 1 = low | 2 = medium | 3 = high | |
|---|---|---|---|
| Elevation - latitude | high | medium | low |
| Ave. stand DBH (in.) for trees >5 in DBH | < 7 | 7 - 8 | > 8 |
| Ave. tree age | < 60 | 60 - 80 | >80 |
High, moderate, and low elevation-latitude combinations can be determined from figure 1 (the RIS subroutine uses forest/district and elevation information). The Risk Classes are multiplied together to determine a Stand Risk Value, which can then be assigned a Stand Risk Rating from the Table below.
| Low (1) | 1 - 9 |
| Medium (3) | 12 - 18 |
| High (5) | 27 |
EXAMPLE: A low elevation-latitude stand (Risk Class = 3) with ave. DBH = 8 (Risk Class = 2) and ave. age = 90 (Risk Class = 3) would have a Stand Risk Value of 18 (3 x 2 x 3). This would give a medium Stand Risk Rating and be coded as 3 in the R2RIS_SITE table field labelled BUG RISK.
Sartwell and Stevens (1975) identified stands with basal area equal to or greater than 150 sq. ft. per acre and average tree diameters equal to or greater than eight inches to be conducive to MPB outbreaks. Lessard (1982) reported that even-aged stands with about 75% of more of the trees in the 7.1-13.0 inch diameter classes were most susceptible to attack. A risk rating system for identifying high, medium, and low risk stands was developed for the Black Hills (Stevens et al., 1980) and has been applied generally to other ponderosa pine forests in Region 2. More recently, Schmid and Mata (1992) suggested that 120 sq. ft. per acre may be the critical basal area for separating high risk stands of susceptible-sized trees in the Black Hills, rather than 150 sq. ft. per acre. This change has been incorporated into a 1993 version of RMSTAND for the Black Hills, Nebraska, and Samuel McKelvie National Forests. The original system developed by Stevens, et al. (1980) is used for all other Forests in Region 2.
a. Ponderosa Pine Risk Rating for R-2 Forests, except BLKH & NEBR
Stand Risk Rating can be calculated as follows (independently of the RMSTAND model). First, Risk Classes are assigned to stand characteristics as shown in the table below.
| 1 = low | 2 = medium | 3 = high | |
|---|---|---|---|
| Stand structure | -- | 2 - storied | single- storied |
| Ave. stand DBH (in.) for trees >5 in DBH | < 6 | 6 - 10 | > 10 |
| Stand density (sq ft/ac) for trees >5 in. DBH | < 80 | 80 - 150 | >150 |
The Risk Classes for each stand characteristic are multiplied together to get a Stand Risk Value (i.e., a stand is not high risk simply because ave. DBH is >10, for example--the 3 characteristics must be considered together).
Stand Risk Rating is assigned depending upon the Stand Risk Value as follows.
| low (1) | 2-6 |
| medium (3) | 8-12 |
| high (5) | 18-27 |
EXAMPLE: A single-storied stand (Risk Class = 3) with ave. DBH = 8 (Risk Class = 2) and a BA = 180 (Risk Class = 3) would have a Stand Risk Value of 18 (3 x 2 x 3). This would just make the high Stand Risk Rating and be coded as 5 in the R2RIS_SITE table field labelled BUGRISK.
b. Ponderosa Pine Risk Rating for Black Hills & Nebraska NFs
A risk rating for the Black Hills and Nebraska National Forests, including the Samuel McKelvie, can be calculated in the same manner as described in 2a; however, a value of 120 BA should be substituted wherever the value of 150 BA for stand density appears in the table of Risk Categories.
Several methods are available to reduce populations of MPB and the resultant tree mortality. These pest management strategies may focus on the reduction of infested material, reduction of susceptible host material, or prevention of new attacks. No method is sufficient to suppress an epidemic. The older strategy of simply trying to remove infested trees (i.e., chasing beetles) gave poor results because harvesting crews could not catch up with a wide-spread epidemic. Currently, an important strategy in mountain pine beetle management is the recognition and prevention of susceptible stand conditions that may lead to epidemics. The intent of forest management should be to reduce adverse impacts to the affected areas and minimize spread of the problem to adjacent stands. The decision to use a particular method should be predicated on considerations of stand conditions, location, management objectives, economic factors, other resource values, and other pertinent variables and issues. Often, the best strategy is a combination of salvage/sanitation harvesting, thinning green stands, and removing large diameter trees in advance of MPB attacks.
In order to destroy developing brood, fell infested trees (generally having green or fading crowns with pitch tubes present on the bole) and remove them from the site for mill processing prior to adult MPB emergence in late July and early August. This sanitation method may be effective in reducing MPB populations in localized areas or stands by removing most or all infested trees prior to beetle emergence. However, it is best used in combination with silvicultural treatments, especially where infestations are large or widespread. Salvage cutting, or removing trees after beetle emergence (crowns red, brownish, or grey and emergence holes evident on bole), can maximize utilization of infested material, but does not affect MPB population levels.
For small infestations, especially in or adjacent to developed sites, currently-infested trees can be felled and treated by burning, peeling the bark, chipping the logs, or drenching with an appropriately registered insecticide prior to beetle emergence in July.
Pheromone baits can be used to contain and concentrate beetles in an area where infested trees can be treated, and reduce MPB dispersal to other areas. Commercial baits, containing attractant chemicals that draw MPB to susceptible trees, can be stapled to trunks of uninfested trees within currently-infested areas several weeks prior to the initiation of beetle flight about mid-July. Following attack, infested trees must be treated by removal or systemic insecticide application. Trees can also be treated with insecticides prior to baiting and attack (useful where tree removal is not possible). Harvesting following baiting can be used to eliminate small spot infestations, remove residual MPB populations following tree harvest or treatment operations, limit MPB spread beyond barrier areas, or divert beetles away from leave strips. Number of baits needed and their placement varies with size and configuration of MPB infestation and the type of baiting intended. Forest Health Management staff should be consulted to help design baiting programs.
To lower susceptibility in green and infested stands, thinning and harvesting strategies are generally recommended. In general, MPB are less attracted to stands where tree crowns are not touching and light is able to penetrate to the forest floor.
a. Partial Cutting. Partial cutting is a strategy that works for ponderosa pine and some lodgepole pine stands. Care must be taken during marking to avoid leaving dense pockets in a stand, as even small areas of dense, mature trees can be highly susceptible to attack and serve as loci for continuing MPB populations. For ponderosa pine, maintaining densities at < 100 basal area will minimize tree mortality (Schmid and Mata 1992). Stands cut to basal area 60 would remain relatively unsusceptible for about 50 years, those cut to basal area 80 for about 25-30 years, and those cut to basal area 100 for about 11-15 years (Schmid and Amman, 1992). Similar figures apply to lodgepole pine. Reducing stand densities to 60-80 basal area generally provide the best compromise between maximizing growth rates, minimizing MPB risk, and allowing sufficient time between entries. The best density within this range will vary with site conditions (moisture availability, site index, regeneration potential) and importance of other management objectives. Acceptable partial cutting methods that can be used to reduce MPB risk include commercial thinning, shelterwood cutting, seed cutting (for ponderosa pine), and overstory removal.
In stands that are lightly infested, all trees that are attacked may be removed along with the most susceptible trees (generally those in the larger DBH ranges for lodgepole pine, and mature trees in dense clumps for ponderosa pine) without exceeding standard basal area prescriptions. In ponderosa pine stands that are heavily attacked by MPB, a greater partial cut can be made to remove both infested and susceptible trees. However, where infestations are heavy in lodgepole pine stands, more substantial partial cuts generally are not advisable (due to windthrow problems), and partial cutting may leave the stand susceptible to additional future losses. Partial cutting also may give poor results in understocked stands of lodgepole pine because small diameter residual trees may have thick phloem sufficient to maintain a MPB population.
b. Clearcutting. Clearcutting still remains one the best methods to create conditions conducive to regenerating lodgepole pine and converting mature stands to younger stands. Block or patch cutting within extensive areas of pure, even-aged stands of lodgepole pine can reduce the potential for MPB epidemics, by reducing the area likely to be infested at one time. Also, clearcutting is generally preferable to partial cutting in lodgepole pine stands that are understocked or heavily infested by dwarf mistletoe. Clearcutting may also be necessary to effectively treat large, concentrated pockets of MPB-infested trees, and heavy infestations in lodgepole pine stands.
c. Type Conversion. In mixed species stands, management can be designed to favor non-host species such as Douglas-fir and spruce. Although MPB attacks pines in mixed stands as readily as in pure stands, greater residual stocking will remain following an epidemic because of the presence of non-host species. Also, less acreage will be affected during epidemics if non-host stands are intermingled with pine stands. Losses from other insect pests, such as Douglas-fir beetle and spruce beetle can be expected to increase when hosts of these species are emphasized; however, these pests generally are less aggressive than MPB. Increasing the amount of Douglas-fir, especially in uneven-aged stands, can increase problems of defoliation by western spruce budworm and Douglas-fir tussock moth, though.
d. Promoting Diversity. Increasing the mix of age classes and species within and between stands reduces the amount of susceptible host type available in an area at a given time. Long-range planning and a variety of silvicultural techniques, including even-aged and uneven-aged practices, may be needed to create age and species mosaics.
A mountain pine beetle extension for lodgepole pine is available for the Forest Stand Prognosis System, Central Rockies Variant and may be especially useful for project-level planning. A similar model extension for mountain pine beetle in ponderosa pine is in the development phase.
Amman, G.D., M.D. McGregor, D.B. Cahill, and W.H. Klein. 1977. Guidelines for reducing losses of lodgepole pine to the mountain pine beetle in unmanaged stands in the Rocky Mountains. USDA For. Ser. Gen. Tech. Rep. INT-36, 19 p. Intermountain For. and Range Exp. Stn., Ogden, UT.
Eckberg, T.C., J.M. Schmid, S.A. Mata, and J.E. Lundquist. (Draft). Focus trees for the mountain pine beetle in the Black Hills. USDA For. Ser., Res. Note RM-xxx. Rocky Mountain For. and Range Exp. Stn., Ft. Collins, CO.
Lessard, G. 1982. Factors affecting ponderosa pine stand susceptibility to mountain pine beetle in the Black Hills. USDA For. Ser., Rocky Mtn. Region, Timber, Forest Pest, and Cooperative Forestry Management, Tech. Rep. R2-26, 16 p.
Sartwell, C. and R.E. Stevens. 1975. Mountain pine beetle in ponderosa pine; prospects for silvicultural control in second-growth stands. J. Forestry 73: 136-140.
Schmid, J.M. and G.D. Amman. 1992. Dendroctonus beetles and old-growth forests in the Rockies, pp. 51-59. In: Kaufmann, M.R., W.H. Moir, and R.L. Bassett (tech. coord.). Old-growth Forests in the Southwest and Rocky Mountain Regions, Proceedings of a Workshop. USDA For. Ser., Rocky Mountain For. and Range Exp. Stn. Gen. Tech. Rep. RM-213, 201 p. Ft. Collins, CO.
Schmid, J.M. and S.A. Mata. 1992. Stand density and mountain pine beetle-caused tree mortality in ponderosa pine stands. USDA For. Serv., Res. Note RM-515, 4 p. Rocky Mountain For. and Range Exp. Stn., Ft. Collins, CO.
Stevens, R.E., W.F. McCambridge and C.B. Edminster. 1980. Risk rating guide for mountain pine beetle in Black Hills ponderosa pine. USDA For. Ser., Res. Note RM-385, 2 p. Rocky Mountain For. and Range Exp. Stn., Ft. Collins, CO.
Caring for the Land and
Serving People