Spruce Budworm

Eastern spruce budworm (Choristoneura fumiferana (Clemens))
Spruce budworm (Choristoneura orae Fallen)

Host(s) in Alaska:

Lutz (Picea x lutzii), Sitka (P. sitchensis), and White (P. glauca) spruce. They are rarely found on black spruce (P. mariana), larch (Larix sp.), and hemlock (Tsuga spp.). Balsam fir (Abies balsamea) is a primary host for this species outside of Alaska.

Habitat(s): Spruce tree foliage

Current Status in Alaska (2022 Update)

No discernable spruce budworm damage was recorded in 2022.  Spruce budworm populations are monitored using pheromone traps. Two green bucket traps were placed at each site and baited with either a Choristoneura fumiferana lure or a C. orae lure. Overall, damage from spruce budworm was low for the areas surveyed in 2022. 

Historic Activity

Spruce budworm populations were monitored in 2020 using pheromone traps at 4 different sites in Southcentral Alaska. The number of locations decreased from 9 in 2019 due to logistical restrictions caused by the COVID-19 pandemic. Between the four trapping locations, the C. orae traps collected 238 specimens. These numbers are consistent with 2019 trap catches for the same locations. Spruce budworm damage was low for the areas surveyed in 2020. Spruce budworm is native to Alaska, and can be found throughout the range of its hosts. To date, spruce budworm is the most common defoliator of spruce recorded during aerial detection surveys. In the 1990s, over a million acres of spruce, primarily in the Interior, were defoliated by spruce budworm. The most widespread defoliation events were recorded in 1994 at the confluence of the Tanana and Yukon Rivers (150,000 acres); in 1996 along the Yukon River from the village of Tanana to Ruby (145,000 acres); and in 1998 along the Yukon River between Ruby and Weir Island (87,000 acres). The outbreaks appeared to collapse by 1999, when only 700 acres of budworm defoliation were recorded between Ruby and Weir Island along the Yukon River.

From 2002-2007, a relatively small budworm outbreak appeared along Nenana, Parks, and Chena Ridges, defoliating roughly 123,000 acres of spruce. At that time, experts feared the outbreak would spread, but populations collapsed before growing to high levels observed in the previous decade. The collapse of both the 1990s and 2000s outbreaks is presumed to have been caused by predators, parasites, and parasitoids.    

Since 2007, spruce budworm activity has remained at relatively low levels with occasional spikes in activity. In 2012, 13,000 acres of defoliation was detected 10 miles upriver from Ruby on the Yukon River. The following year, 6,000 scattered acres were recorded along the Kobuk and Koyukuk Rivers. Since 2013, spruce budworm defoliation mapped during aerial surveys has not exceeded 800 acres.

Roughly 5,000 budworm were collected in 2018. Traps baited with C. orae lures captured moths from Coldfoot to Seward. Traps baited with C. fumiferana lures only captured moths north of the Alaska Range, but at higher numbers compared to C. orae. The highest trap captures for both species were in Delta Junction, where over 900 C. fumiferana and 400 C. orae were collected (see Maps). 

No spruce budworm damage was mapped during aerial detection surveys in 2018 or 2019. Spruce budworm population monitoring was conducted in 2019 using pheromone traps targeting both species of spruce budworm at 46 sites along major roadways in Southcentral and Interior Alaska, an increase of 22 sites from previous years. On average, 93 budworm moths were collected per site in 2019, compared to 208 per site in 2018. As of 2019, the spruce budworm outbreak previously recorded in Eagle, Alaska in 2016 and 2017 appears to have collapsed. 

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Biology, Symptoms, Impacts & Vulnerable stands

Biology: Adult budworms emerge in mid-June to mid-July, females attract males with pheromones, and commence mating. Females oviposit roughly 170 greenish eggs in multiple clusters over a 1-2 week period. A single female can create 5-10 egg clusters each containing 5-50 eggs. The majority of egg clusters will be found in upper spruce crowns, but can be also found among lower branches. Newly emerged females are not active fliers, but take wing once roughly half their eggs have been oviposited. Female moths are capable of using wind directed dispersal to travel (and oviposit eggs) across distances of 150 miles. Eggs hatch in 10-14 days and larvae undergo six instars (developmental stages). First instar larvae do not feed, but spread throughout forested stands, construct silken shelters (hibernacula), and molt. Second instar larvae overwinter in hibernacula on host and non-host trees. In mid-May of the second year, larvae emerge from hibernacula and move towards feeding sites. Dispersal via ballooning may also occur at this time. Ballooning caterpillars produce silken threads which are taken up by wind currents, transporting caterpillars to new locations. Second instar larvae mine old spruce needles and unopened vegetative buds, or feed on staminate flowers of host trees. As spruce buds open, larvae switch to feeding on new foliage. Older larvae feed within web-enclosed shelters, constructed by tying branch tips together. Mature larvae are 20-30 mm (1 to 1 ½ in) long, brown, green, or black, with white spots running along their flanks. Following completion of larval development caterpillars build loose cocoons and pupate among damaged foliage.            

Symptoms & Impacts: Spruce budworms feed primarily on current year needles, but will “back feed” on older needles when populations are at high levels and new growth has been consumed. Needles damaged by spruce budworm feeding turn reddish-brown. Damage occurs primarily in upper crowns, but can affect entire trees. As older larvae feed, they construct shelters (described above) which can easily be identified among the tips of branches. Heavy feeding for consecutive years can result in complete defoliation, top-kill, and occasionally mortality. Tree mortality due to budworm defoliation has the potential to alter stand composition, but rarely occurs in Alaska. Top-kill, or death of a tree’s apical leader (primary central stem) at the top half to top third of affected trees, may cause forks to form as multiple lower branches compete for dominance. Competition for apical dominance can result in more “rounded” tops of damaged trees as several branches strive to become the new leader. Feeding can cause substantial reductions in growth and wood production in merchantable spruce stands, and top-kill can set back stand growth by five to ten years. Consecutive years of heavy budworm defoliation has the potential to reduce tree vitality making spruce more vulnerable to bark beetle attack, notably the spruce beetle (Dendroctonus rufipennis) and northern spruce engraver (Ips perturbatus).

Vulnerable stands: Factors affecting spruce budworm survival and stand vulnerability specific to Alaska have not been calculated, but key stand factors affecting budworm populations in other locations may be applicable to Alaskan spruce forests. Stands with the following conditions are considered vulnerable:

1. ≥ 50% of a stand’s composition is comprised of preferred hosts.
2. A stand is > 50 years of age, possessing overmature, slow-growing trees producing flowers.
3. Stands with a large number of host tree tops protruding from the forest canopy, and/or having a large number of host trees in direct sunlight.
4. Stands are under water stress, either due to overly dry or overly wet conditions.

Survey Method

Trapping, ground, and aerial survey.

Maps of Spruce Budworm Trap Captures

Click maps for larger versions.

Left map: Spruce budworm (C. orae) trap captures from summer 2019 (2018 map available here). 
Right map: Spruce budworm (C. fumiferana) trap captures from summer 2019 (2018 map available here).

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. 

Spruce Budworm. Written by Ed Holsten, Research Entomologist; Revised by James Kruse, Forest Entomologist and Nicholas Lisuzzo Biological Science Technician, USDA Forest Service, Alaska Region, State and Private Forestry. Available here.

Coulson, Robert N., and John A. Witter. Forest entomology: ecology and management. John Wiley & Sons, 1984.

Content prepared by Dr. Stephen J. Burr, Forest Entomologist, USDA Forest Service, stephen.burr@usda.gov and Dr. Sydney Brannoch, Entomologist, USDA Forest Service, sydney.brannoch@usda.gov.

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