Western Blackheaded Budworm

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.

Damage:

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 in Alaska (2022)

Western blackheaded budworm outbreak continues throughout Southeast Alaska in 2022, with caterpillars concentrated in Sitka spruce as well as western hemlock. Large amounts of frass and caterpillars have been reported throughout the panhandle. Defoliation was extensive from Ketchikan to Haines with over 686,000 acres of damage recorded during aerial detection surveys. Ground surveys showed a small proportion of caterpillars were infected with disease, which can help contribute to the outbreak ending. Fewer adult moths were observed in late summer also indicating the outbreak may have peaked. Western blackheaded budworm populations began to increase in 2020, developing into a large-scale outbreak in 2021 across much of Southeast Alaska, most notably 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. Defoliation by western blackheaded budworm was documented on 520,000 acres during aerial survey flights. Caterpillars feed on the buds and new growth of hemlock, which in combination with the hemlock sawfly outbreak, could prove detrimental to affected trees. Survey plots located on Mitkof Island had the greatest number of western blackheaded budworm and feeding activity. Reports of caterpillars hanging from silk threads on western hemlock were received from several areas. The last time damage was recorded in Southeast was 2009, impacting the forested area between Frederick Sound and Juneau.

Identification, Symptoms, Biology & Impacts

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.

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 Western Blackheaded Budworm Movie

Historic Activity

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 black-headed budworm survey on the Tongass National Forest, Alaska (1952, 1953, 1954)
Studies of the biology, habits and control of the black-headed budworm in Alaska (1953, 1955

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.

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Survey Methods

During outbreaks, moderate to severe western blackheaded budworm defoliation can be mapped by aerial survey. In 2019, ground-based plots were used to assess western hemlock defoliation damage severity and the relative presence of defoliating insects using beating sheets. Hemlock sawfly was the most common defoliator detected in plots; western blackheaded budworm was present at endemic population levels (see full report here).

Control Methods

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

Frequently Asked Questions (August 2021)

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, elizabeth.e.graham@usda.gov.

  • 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!

Resources, News & Social Media

Follow #AlaskaForestHealth on Twitter and the Chugach & Tongass National Forests on Facebook (@ChugachNF and @TongassNF)

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, elizabeth.e.graham@usda.gov. Content adapted from Blackheaded Budworm in Alaska