Forest & Grassland Health
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 (2021 Update)
Spruce beetle activity was observed on approximately 193,500 acres statewide during aerial detection surveys in 2021. More than 98% of the activity mapped in 2021 was within Southcentral Alaska, where an ongoing spruce beetle outbreak is now estimated to be in its sixth year. The outbreak has affected at least 1.6 million cumulative acres of mixed spruce and birch forests since 2016, when it was first documented. In 2021, the outbreak was most active in the northern Matanuska-Susitna Borough, the lower Denali Borough, and around Cooper Landing, Kenai, and Soldotna on the Kenai Peninsula. Activity has declined in the areas that were impacted most severely early in the outbreak.
In areas that were initially impacted by the spruce beetle outbreak, such as the central and lower Susitna River Valley, a near exhaustion of susceptible white spruce is visible in many areas. Most spruce beetle activity in these areas is confined to black spruce and widely scattered residual small white spruce. In 2021, nearly 30,000 acres of black spruce forests were impacted within the outbreak area. Not all black spruce mortality observed in 2021 could be confirmed as solely caused by spruce beetle. During ground assessments, a small subset of recently killed black spruce showed signs of both spruce beetle and northern spruce engraver (Ips perturbatus), with neither being more likely the cause of the mortality than the other. Additional investigation is warranted in these areas.
Various activities that 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.
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.
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
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.
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.
Forest area (acres) affected by spruce beetle in Alaska and observed during aerial
detection surveys 1976-2021. No aerial surveys were conducted in 2020.
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.
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.
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.
Region 10 FHP and DNR DOF staff were involved in two tree protection studies in 2020, both led by Dr. Christopher Fettig, USFS Pacific Southwest Research Station. A study initiated in 2019 evaluated SPLAT-MCH (ISCA Technologies, Inc.) in combination with other semiochemicals (a form of chemical communication between organisms) as a method of repelling spruce beetles from susceptible trees. This study was completed in 2020. The second study, which began in 2018, evaluated systemic pesticide efficacy in killing spruce beetles as they attack trees and limiting the introduction of blue-stain fungi. The study trees were initially challenged by beetles in 2019 and the success of the treatments was assessed in 2020. The surviving trees will be reassessed after the 2021 spruce beetle flight period. Final results and data analysis from both studies will be forthcoming. 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.
Click maps for larger view.
Spruce beetle damage detected in Southcentral Alaska in 2021 (left) and from 2016-2021 (right).
Spruce beetle outbreaks 1980-2003 in (a) Cook Inlet–Kenai Peninsula and (b) Copper River Basin (Werner et al. 2006).
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
Annual Forest Health Conditions Reports are available here.
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.