Forest & Grassland Health

Spruce Beetle

Dendroctonus rufipennis (Kirby)

Host(s) in Alaska:

White spruce (Piceae glauca), Lutz spruce (P. glauca x sitchensis), Sitka spruce (P. sitchensis) and at times black spruce (P. mariana). Rare non-spruce hosts: Scots pines (Pinus sylvestris) and Siberian larch (Larix sibirica)

Habitat(s): phloem, inner bark and bark

Current Status in Alaska (2019 Update)

Active spruce mortality from spruce beetle was mapped across 139,500 acres statewide during aerial detection surveys in 2019, down considerably from  593,000 acres in 2018 and 405,000 acres in 2017 (see Maps). Despite this substantial decrease in acreage, the outbreak in Southcentral Alaska is still in progress. Where the outbreak has been most severe, white spruce host material is nearly exhausted and there has been increased spruce beetle activity in black spruce. The outbreak continues to expand in nearly all directions from previously affected forests. 


Extensive beetle activity was documented  along the Parks Highway to just south of Broad Pass near milepost 189, and scattered damage extending north to Summit Lake (milepost 199). In 2018, there was no spruce beetle activity along the Parks Highway north of Hurricane Gulch (milepost 174). Beetle activity was also high in the western Susitna River valley west to the Alaska Range and south to Beluga Lake. Spruce beetle activity increased notably across much of the Municipality of Anchorage this year, from the Eklutna River valley south along the Chugach Mountains to southeast Anchorage. Spruce beetle remains active on the northwestern portion of the Kenai Peninsula. Activity appears to be increasing between Soldotna and Kalifornsky and is prevalent along the eastern side of Skilak Lake road. Spruce beetle activity is also scattered along the Kenai River east to Cooper Landing. The cumulative extent of the current outbreak, now in its fourth year, has exceeded one million acres. 


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Photos

Click on the image for a larger version.

Adult Spruce Beetle

Adult spruce beetles are dark brown to black and ~5mm long.

Spruce beetle larvae and pupae with galleries shown

Spruce beetle larvae and pupae with galleries.

Pitch Tubes are evidence of spruce beetle attack

Pitch tubes are evidence of spruce beetle attack.

Exit holes created by spruce beetles with knife for scale

Exit holes created by spruce beetles, with a knife for scale.

Spruce trees killed by spruce beetle

Spruce beetle outbreak in the Mat-Su Valley seen during the 2017 aerial detection survey.

Maps of Spruce Beetle Damage

Click maps for larger versions.

Cumulative outbreak extent 2016-19. Spruce beetle activity aerially mapped in 2019.

Spruce Beetle Damage in Southcentral Alaska

Spruce beetle damage detected during aerial detection survey in Southcentral Alaska.

Spruce beetle activity mapped from 1980 to 2003 from Werner et al. 2006.

Spruce beetle damage aerially mapped in Alaska 2016-2019.

Spruce beetle damage mapped in Southcentral during aerial detection survey in 2019.

Spruce beetle damage mapped in Southcentral during aerial detection survey in 2018.

Spruce beetle damage detected during aerial detection survey in 2017 in Southcentral Alaska.

Spruce beetle outbreaks 1980-2003 in (a) Cook Inlet–Kenai Peninsula and (b) Copper River Basin  (Werner et al. 2006).

Spruce Beetles: What They Are and What To Do About Them

 
 

For information on  what you can do to protect healthy spruce trees, your options if your tree is under attack, and frequently asked questions about spruce beetle see this publication by Cooperative Extension Service available here.  

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Guidelines for Reducing Damage

Spruce beetle outbreak in the Mat-Su Valley seen during the 2017 aerial detection survey.  

Landscape view of spruce trees killed by spruce beetle.

 

Various activities which disturb the environment of spruce contribute to spruce beetle attack and epidemic outbreaks. These activities include timber harvest; land clearing related to road, seismic line, pipeline, powerline, or building construction; severe winds which cause windthrown trees, and wildfire. Spruce beetle attacks may be prevented or reduced by following these guidelines:

Proper Management of Spruce Forests

Maintain spruce stands in a healthy and vigorous condition by removing overmature, slow-growing, diseased, injured, and dying trees. Remove damaged or windthrown trees from spruce stands under management. Establish a stand rotation age (harvest age) of less than 100 years. Timber sale size and orientation of cutting areas are important in creating stands that can withstand high winds. Leave-strips between clearcut or shelterwood cutting areas should be more than 100 feet wide. Timber sales should not be located along ridgetops where shallow-rooted spruce are highly susceptible to high wind.

Timber Harvest

Overmature, slow-growing trees should be removed from forest stands as they are highly susceptible to spruce beetle attack. Windthrown trees, particularly in recently logged areas, should be removed. All logs cut after March should be removed and utilized prior to beetle flight the following May. Logs cut during the summer months should be removed shortly after cutting. All slash and cull logs four inches in diameter and larger should be disposed of by burning, burying, chipping, or peeling. Stumps should be cut as low as possible. Whole tree logging will eliminate most of the breeding material usually left in the forest and concentrate it at the logging landing where it can be destroyed.

Rights-of-Way Construction

Timber along rights-of-way for roads, seismic lines, pipelines, and power lines should be cut in the fall and the logs utilized before the next spring. Slash should be treated as described earlier. Trees next to the right-of-way should be examined for beetle attacks in late summer following cutting. If trees are infested, they should be removed. Care should be taken to avoid scarring trunks with mechanical equipment, severing roots, altering drainage patterns, or severely compacting the soil. Proper slash disposal along a powerline right-of-way Improper slash disposal along a powerline right-of-way

Home Construction

Trees removed for home construction should be properly disposed of or utilized. If stockpiled for firewood or used for construction, the bolts or logs should be peeled. Mechanical damage to standing trees should be avoided. Excess soil should not be placed on top of or removed from the area over the root zone. Trees breathe to some degree through the roots and the addition or removal of soil can cause suffocation. Avoid soil compaction around the base of trees and do not surface these areas with rock, concrete, or asphalt. Sewage drainage fields should be located away from trees because excess water can create stress conditions in adjacent trees. 

Protecting High-Value Spruce

There are several ways you can help protect your high-value spruce trees from attack by spruce beetle.  Maintaining healthy and vigorous trees is essential; watering and fertilizing trees early in the growing season will help them produce defense compounds to prevent attack.  Be careful not to damage the trees on your lot during construction or landscaping.  Thinning a stand may reduce competition for water and nutrients and provide more light, but this should not be done between May and July, when the smell of freshly cut trees could attract beetles to your property.  Pruning lower branches can also help reduce the risk of attack, but again, pruning should not be done between May and July, when beetles are flying. 

Chemical Control

There are some chemical control options available for spruce beetle management. For more information on using pesticides to protect spruce trees from attack by spruce beetle click here.

Spruce beetles produce chemicals called pheromones to communicate with other beetles. They help coordinate a mass attack on a spruce tree and also send a signal to stop attacking once the tree is colonized. One such chemical, MCH, is considered one of these “anti-aggregation” pheromones. It has been shown in some studies to discourage beetles in flight from attacking particular trees, or particular stands of trees. In the lower 48, MCH can be purchased in small packets called “bubble caps,” that land managers may attach to the trunks of high-value trees to try to discourage attack by spruce beetles.

Research on the efficacy of MCH in Alaska has had mixed results. The Alaska Division of Forestry has collaborated with researchers from the USFS Rocky Mountain Research Station to determine the efficacy of using MCH to protect high-value trees in Alaska.  As of July 2018, only one product containing MCH is registered for use in Alaska (Synergy Shield MCH).  You can read more on using MCH in Alaska available here.

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Historic Activity

The spruce beetle is the most significant mortality agent of mature spruce in Alaska. An outbreak started in the 1980s in Southcentral Alaska and continued until 2003 affecting over 1.3 million hectares of forest with >90% of the trees killed in many stands. Werner et al. 2006 provides a valuable summary of 30 years of spruce beetle research in Alaska. This massive spruce beetle outbreak has been attributed to both an aging forest with high-density stands of large-diameter, slow growing spruce, and a warming trend that enabled the spruce beetle to proliferate while leaving drought stressed trees susceptible to beetle attack. Dendrochronology studies have found evidence of major spruce beetle outbreaks in southcentral and southwest Alaska in the 1810s, 1870s, 1910s, and 1970s and were synchronous across the period of record. These outbreaks tended to occur following one or more years of warmer and drier than average summer conditions.

Spruce Beetle Damage 1976-2018

Forest area (acres) in Alaska impacted by spruce beetles per year from 1976 to 2019. Areas with damage were recorded during aerial detection surveys. 

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Symptoms, Life History & Impacts

The primary indication that beetles are attacking a tree is reddish-brown dust which accumulates on the bark, in bark crevices, and on the ground beneath the attacked tree. Globules of resin or pitch tubes at the entrance hole into the bark are another sign of beetle attack. Entrance and exit holes are usually found in the roots (both exposed and underground) and lower fifteen feet of the trunk. Early detection requires close examination of trees from early June to mid-July. To determine if spruce beetles are present, remove the bark around an entrance hole to locate the adult and larval tunnels. Beetle infested trees are often sought out by woodpeckers. Pieces of bark chipped away by the woodpecker accumulate on the ground beneath the trees. This is especially noticeable in the winter when the bark accumulates on the snow.

Boring dust from spruce beetle on white spruce in the Russian River Campground.

Spruce beetle boring dust on white spruce at Russian River Campground in 2019.

 

A change in foliage color is another indication of spruce beetle attack. Needles begin to fade from dark green to pale yellowish-green one year after attack and to reddish-brown the following summer. In some cases, needle discoloration may not be noticeable until one year after the attack and sometimes not until after the beetles have left the tree. By mid-summer, one year after initial attack, many needles have dropped and the tree turns reddish-brown. Three to five years following attack, the trees appear silvery-gray and remain that way for many years.

The spruce beetle has a one- or two-year life cycle in Alaska. Adult beetles emerge from infested trees from mid-May to mid-June, and their flight to fresh host materials lasts until mid-July. When the female beetle finds a suitable host, she bores into the bark and constructs an egg gallery in the phloem parallel to the wood grain and usually above the entrance hole. After mating occurs, the female lays whitish-yellow eggs in clusters on either side of the gallery. Eggs hatch into white grub-like larvae which feed in the phloem cross-wise to the egg gallery. Larvae do not enter the wood but may score the outer surface.

One-year life cycle beetles develop from egg to pupae the first summer. New adults spend the winter under the bark at the base of the infested tree. Two-year life cycle beetles spend the first winter as larvae beneath the bark. In spring they resume development and eventually transform into white pupae for a short time and then to adult beetles. The Adults then migrate to the base of the dead or dying tree where they overwinter.

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Spruce Beetle Attacks on Non-Spruce

When beetle populations are high, spruce beetles and other related bark beetles are known to occasionally attack non-spruce conifers. In 2017, there were a few reports of non-spruce conifers (Scots pines (Pinus sylvestris) and Siberian larch (Larix sibirica)) being attacked by bark beetles in Susitna River Valley. Taxonomist Jim LaBonte (Oregon Dept. of Agriculture) confirmed that spruce beetle was the bark beetle attacking these non-native conifer hosts. Many of the initial attacks on both non-native tree species appeared to have been unsuccessful, though gallery initiation was observed at several attack sites in Scots pine and in at least one attack site in larch. Lodgepole pine (Pinus contorta) were also present but unattacked in many of the locations where Scots pines have been attacked. It is unclear whether these beetle attacks in non-spruce hosts will be successful, both in terms of beetle reproductive success and tree mortality. 

In 2019, an effort was undertaken by the Alaska Division of Forestry to assess whether spruce beetle was the unknown bark beetle mass attacking several ornamental pines, presumed to be jack pines, in a Houston, Alaska park. Emergence traps were placed on one of the attacked trees in the spring and dozens of beetles were collected from the traps through the summer. Taxonomists at the Oregon Department of Agriculture confirmed the beetles were spruce beetle. The collection of these beetles in the emergence traps suggest that they were successful in completing their life cycle in these trees. 

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Spruce Protection Research in Alaska

Two tree protection research efforts were undertaken in 2019: a systemic pesticide trial and an anti-aggregation pheromone trial using SPLAT-MCH. Both projects are being conducted collaboratively with the USDA Forest Service Pacific Southwest Research Station, Region 10 FHP, and the Alaska Division of Forestry. The systemic insecticide trial began in 2018. The study trees were initially challenged by beetles in 2019 and the success of the treatments will be assessed in the spring of 2020. The SPLAT-MCH trial is a one-year project that was initiated in 2019 and the results will be determined in the spring of 2020. The results from a separate MCH-based tree protection research project completed in 2018 by the Alaska Division of Forestry, in partnership with the USDA Forest Service Rocky Mountain Research Station, were published in 2019 and are available here. The MCH-based treatments tested in this study were not found to effectively protect white spruce from spruce beetle attack in Alaska.  

Resources & Publications

FS-R10-FHP. 2017. Forest Health Conditions in Alaska 2017. Anchorage, Alaska. U.S. Forest Service, Alaska Region. Publication R10-PR-43. 64 P. Available here

Holsten, E.H.; Their, R.W.; Munson, A.S.; Gibson, K.E. 1999. The spruce beetle. Forest Insect and Disease leaflet 127. Washington, DC: U.S. Department of Agriculture, Forest Service. 12 p. Available here.

Werner, R.A.; Holsten, E.H.; Matsuoka, S.M.; Burnside, R.E. 2006. Spruce beetles and forest ecosystems in south-central Alaska: A review of 30 years of research. Forest Ecology and Management 227: 195-206. Available here.

In addition to this website, see www.alaskasprucebeetle.org for great information on the spruce beetle damage and management in Alaska.
 

Content prepared by Elizabeth Graham, PhD Entomologist Forest Service elizabeth.e.graham@usda.gov; Content Adapted from Richard Werner. The Spruce Beetle in Alaska Forests. PMC-10060. Available here.


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