Western Blackheaded Budworm
Acleris gloverana Walsingham
Primary host(s) in Alaska:
Western hemlock (Tsuga heterophylla) is the preferred host.
Sitka spruce (Picea sitchensis) & mountain hemlock (T. mertensiana)
are also hosts.
Defoliation is usually restricted to new needles.
Damage starts in the crown then moves throughout the tree.
Trees of all ages can be affected.
- Current Status
- ID, Symptoms, Biology & Impacts
- Historic Activity & Reports
- Survey Methods
- Management & Control
- Resources, News & Social Media
The western blackheaded budworm outbreak is subsiding throughout Southeast Alaska. Caterpillars were still found during ground surveys, however damage associated with their feeding was minimal. Light western blackheaded budworm activity was recorded south of Juneau near Holkham Bay and extended north to Skagway. There was active defoliation also recorded near Hoonah and Excursion Inlet. The defoliation impacts diminished north and stopped more abruptly to the south. Hemlock mortality from past defoliation was minimal in most areas, but very high in some pockets on Admiralty and Chichagof Islands and along Excursion Inlet. Previously damaged areas were dominated by top-killed trees with some trees showing needle regeneration that cast a subtle green shade to gray trees.Outbreaks typically last two to three years and then crash due to parasitism, predation, disease and ultimately starvation. Areas on the fringe of the outbreak may still experience damage as these natural control agents catch up.
Western blackheaded budworm populations began to rise in 2020, resulting in a largescale outbreak in 2021 and 2022. Defoliation extended from Haines to Ketchikan and is most notable on Admiralty, Kupreanof, Mitkof, and Wrangell Islands, as well as several drainages on the mainland. Caterpillars were commonly observed hanging from silk threads both in urban and forested settings, with high levels of frass accumulating on understory plants. A notable difference from 2021, defoliation was no longer concentrated on hemlock; instead, the caterpillars were found feeding in Sitka spruce and several ornamental conifer species planted in urban areas. A systematic ground detection survey was conducted in late July along the road system in Southeast, confirming western blackheaded budworm as the predominate defoliator.
Forest Health Protection has been working to integrate satellite-based remote sensing methods into forest health monitoring. The Landscape Change Monitoring System (LCMS) is a tool developed to map significant changes in vegetation and land use. By adding magnitude change values, we created a heat map to indicate areas most heavily impacted. These remote sensing maps simply indicate where a decline in vegetation has occurred and may represent events in addition to defoliation, such as landslides, harvest activity, or other disturbance. Anyone can access these maps here.
During early development stages (instars), larvae are creme-colored with a distinct black head. Their body color changes to green as they develop and the head capsule is brown during the last instar. Mature larvae are one-half to three-quarters inch long. Pupae are green or brown and are approximately one-third inch in length. The adult is a small moth with a three-quarter inch wingspan. Moths show great variability in wing color and pattern. The predominant wing color is grey, with mixtures of brown, black, orange and white. Eggs are yellow, flat, and are laid singly on the underside of host needles.
Western blackheaded budworms overwinter as eggs. The eggs hatch in late May or June, and young larvae begin feeding in unopened buds. Larval feeding and growth coincides with the host's bud and shoot development. Feeding is typically confined to the current year's needles. Defoliation of older needles is an indication of large populations. In their last stage of development, larvae build a pupation shelter by webbing. live and cut needles together. Pupation occurs from mid-July to mid-August. Moths emerge, mate, and female moths deposit eggs from late August through September.
Western blackheaded budworms are wasteful feeders, often clipping loose needles that are not completely consumed. By mid-summer, these needles have dried and turn red. Crowns of heavily defoliated trees appear scorched due to large concentrations of dead, dry needles. Defoliation is most severe in the upper portions of tree crowns, but entire crowns may defoliated during outbreaks. A single year of defoliation often causes reduced tree growth. Years of repeated defoliation may result in reduced cone production, top-kill, or in severe cases, death of the tree. Not all impacts of western blackheaded budworm defoliation are negative. Foliage ingestion hastens nutrient cycling and crown density reduction may increase light intensity to the ground. However, negative impacts to managed young growth forests and to urban ornamental trees often outweigh potential benefits.
During western blackheaded budworm outbreaks, both forest and ornamental trees are attacked. Western blackheaded budworm populations in Alaska have been cyclic, rising quickly over a few years, covering vast areas, and then subsiding suddenly. Recurrent infestations have been noted in Southeast Alaska since the early 1900s. An outbreak in the late-1940s to mid-1950s affected almost every forested acre in Southeast Alaska. W.F. McCambridge, the first entomologist stationed in Alaska, prepared five reports on budworm surveys and studies in the 1950s.
The most recent outbreak took place from 1992-1995 and was concentrated north of Fredrick Sound. The summary paper, Budworm in Coastal Alaska (John Hard 1974), provides a detailed account of previous outbreaks.
During outbreaks, moderate to severe western blackheaded budworm defoliation can be mapped by aerial survey. A ground survey in late July 2022 was conducted along the road system in Southeast Alaska to determine which species were active, the amount of damage visible from the ground, and whether there are any indicators that the outbreak will continue into 2023. Western blackheaded budworm was the most common defoliator observed during ground surveys in all locations. The presence of diseased larvae and pupae are positive indicators that the outbreak may have reached its peak. While we can expect defoliator activity to continue in 2023, we are predicting that populations will begin to decrease. You can read the full report here.
Large-scale control of the western blackheaded budworm in forest settings in Alaska has not been attempted and is not feasible.
Control measures to protect high value ornamentals in urban settings may however, be desirable. A simple form of control for small, lightly infested ornamentals, is to physically remove infested shoots. Use of insecticides may be desireable to protect heavily infested trees. Both biological and chemical insecticides are registered for budworm control. Spray applications (e.g., carbaryl) have proven effective in reducing budworm defoliation when applied after bud break. Biological insecticides (e.g., the bacterium, Bacillus thuringiensis (B.t)), have also been used against budworms, but proper timing of application is critical to achieve adequate control.
Natural controls of budworm populations vary by budworm life stage. Predators of larvae (and in some cases, moths) include birds, spiders and several species of insects. Some wasps parasitize budworm eggs, larvae and pupae. An unusual impact to overwintering budworm eggs is their removal by snow, as it slides from tree branches. A virus, fungi, larval starvation, and weather have also been noted as having contributed to budworm population declines
Expand to read answers to frequently asked questions about western blackheaded budworm and the current outbreak in Southeast Alaska. For more information, contact USFS Entomologist Dr. Elizabeth Graham, firstname.lastname@example.org.
- Question 1: Where is most of the activity of the western blackheaded budworm outbreak?
Areas with heavy defoliation include Admiralty, Baranof, Kuiu, Kupreanof, Mitkof, Prince of Wales, Wrangell, and Zarembo Islands, as well as several drainages on the mainland as far north as Juneau.
- Question 2: What trees do western blackheaded budworms eat?
Western hemlock is the preferred host. Sitka spruce and mountain hemlock can also serve as hosts during outbreaks. The caterpillars mine new buds and then move on to new foliage.
- Question 3. What is the life cycle of western blackheaded budworms?
Blackheaded budworms overwinter as eggs. The eggs hatch in late May or June, and young larvae begin feeding in unopened buds. Larval feeding and growth coincide with the host's bud and shoot development. Budworm feeding is typically confined to the current year's needles. Defoliation of older needles is an indication of large budworm populations. In their last stage of development, larvae build a pupation shelter by webbing live and cut needles together. Pupation occurs from mid-July to mid-August. Moths emerge, mate, and female moths deposit eggs from late August through September.
- Question 4: What happens to the trees when the western blackheaded budworm are present?
Budworms are wasteful feeders, often clipping loose needles that are not completely consumed. By mid-summer, these needles have dried and turn red. Crowns of heavily defoliated trees appear scorched due to large concentrations of dead, dry needles. Defoliation is most severe in the upper portions of tree crowns, but entire crowns may become defoliated during budworm outbreaks.
- Question 5: How are western blackheaded budworms monitored?
During outbreaks, moderate to severe western blackheaded budworm defoliation can be mapped by aerial survey. Ground-based plots are used to assess western hemlock defoliation damage severity and the presence of other defoliating insects. While little defoliator activity was recorded in 2020, in 2021 western blackheaded budworm was found throughout ground surveys in high numbers. Remote sensing tools are being explored for quantify the extent of the outbreak throughout Southeast.
- Question 6: How are western blackheaded budworms controlled?
Natural controls of budworm populations vary by budworm life stage. Predators of larvae (and in some cases, moths) include birds, spiders, and several species of insects. Certain species of wasps parasitize budworm eggs, larvae, and pupae. Viruses, fungi, larval starvation, and weather have also been noted as having contributed to population declines. Large-scale control of the western blackheaded budworm in forest settings in Alaska has not been attempted. Most outbreaks only last a few years and collapse on their own.
- Question 7: Is this being caused by climate change? Will climate change be a factor in these outbreaks?
Climate plays a role in budworm populations. Western blackheaded budworm outbreaks have been tied to warmer than average summers. Additionally, drier than normal conditions can limit the growth of fungi that are natural population controls. With a changing climate, it is possible that these outbreaks could occur with more frequency than in the past.
- Question 8: What does damage from other insects look like on hemlock?
Hemlock sawfly is the other common hemlock defoliator found in Southeast Alaska. It feeds on the older needles, whereas the budworm feeds on the newer needles. The hemlock sawfly damage is concentrated in the inner crown, which creates a more yellowish appearance in the needles versus the resultant red needle appearance of western blackheaded budworm damage. Several other caterpillars and insects can be found on hemlock. If you are interested, upload your pictures of any forest insect pests that you observe to iNaturalist for identification!
Mask, R. 1993. Blackheaded Budworm in Alaska. Alaska Region Forest Leaflet. USDA Forest Service. Available here.
Hard, J. 1974. Budworm in Coastal Alaska. Journal of Forestry January 1974. Available here.
Warmer summers fuel western blackheaded budworm infestation of Southeast hemlocks, by Tash Kimmell, KCAW - Sitka. Available here.
Find out more about the very hungry caterpillar devouring Southeast Alaska's trees, September 11, 2021, by Ben Hohenstatt, Juneau Empire. Available here.
2022 Mendenhall Glacier Visitor Center Fireside Lecture Series: Western blackheaded budworm impacts on the Tongass. Available here.
For more information, please contact Dr. Elizabeth Graham, Entomologist, email@example.com. Content adapted from Blackheaded Budworm in Alaska