Forest & Grassland Health

Western Blackheaded Budworm

Blackheaded budworm damage, Turnagain Pass.

Western blackheaded budworm damage near Turnagain Arm.

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 throughout the tree crown. All tree age classes can be affected.

Photos

Click on the image for a larger version.

Western blackheaded budworm feeding damage

Western blackheaded budworm defoliation of western hemlock

Western blackheaded budworm TN

Western blackheaded budworm caterpillar

Blackheaded budworm feeding damage Turnagain Pass.

Western blackheaded budworm defoliation damage along Turnagain Arm.

Current Status in Alaska (2020)

Western blackheaded budworm activity increased from 2019 based on ground survey data. The increased activity does not yet indicate an outbreak but will continue to be monitored. 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. In some locations around Petersburg, reports of caterpillars hanging from silk threads on western hemlock were received. Reports of western blackheaded budworm in Sitka were submitted through our iNaturalist Alaska Forest Health Observations project.

No damage from western blackheaded budworm was observed during the aerial detection survey in 2019. The last time damage was recorded in Southeast was 2009. During ground surveys, western blackheaded budworm larvae were found in 50% of the plots (n=76) however within those plots the proportion of trees with larvae were low (<20%) (see full report here). Western blackheaded budworm larvae feed in the buds and on the new foliage of western hemlock. An outbreak in combination with the hemlock sawfly outbreak could result in tree mortality.

Identification, Symptoms, Biology & Impacts

During early development stages (instars), budworm 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.

Black-headed 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. 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.

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 budworm 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 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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Historic Activity

During budworm outbreaks, both forest and ornamental trees are attacked. Black-headed 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.

Survey Methods

During outbreaks, moderate to severe 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 black-headed budworm in forest settings in Alaska has not been attempted. Control measures to protect high value ornamentals in urban settings may however, be desirable. Control measures for black-headed budworm vary I based on degree of infestation and the size and setting of impacted trees.

No control may be necessary when budworm populations are low and their feeding damage is insignificant. 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. A potential control method that is currently being studied, is the use of insecticide implants (e.g., acephate). These implants are effective in protecting individual trees from a similar defoliator, the western spruce budworm. 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

Links & Resources

Mask, R. 1993. Blackheaded Budworm in Alaska. Alaska Region Forest Leaflet. USDA Forest Service. Available here.

 

For more information, please contact Dr. Elizabeth Graham, Entomologist, elizabeth.e.graham@usda.gov. Content adapted from Blackheaded Budworm in Alaska by Robin Mulvey, robin.mulvey@usda.gov. 

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