Western blackheaded budworm

Written by V.G. Nealis Revision 2025

• French common name: Tordeuse à tête noire de l'Ouest
• Other common names: Blackheaded budworm, western black-headed budworm, hemlock budworm
• Scientific name: Acleris gloverana (Walsingham)
• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Lepidoptera
• Family: Tortricidae
• Partial list of synonyms:
  • Acleris variana Fernald
  • Peronea variana (Fernald)

General information and importance

Western blackheaded budworm is a native defoliator of conifers in North America from the Rocky Mountains west to the Pacific maritime forests. It was considered a western variant of eastern blackheaded budworm, Acleris variana, until the 1970s. Both species have nearly identical life stages and habits: adult moths are highly variable in colour, and males respond to the same sex pheromone. Few studies have occurred in the western prairies or Rocky Mountains where the species’ transition would occur. There are no genetic studies to confirm its status as a distinct species; therefore, this may be a single species with a transcontinental gradient in preferred tree hosts.

Outbreaks of western blackheaded budworm occur at about 10- to 15-year intervals and last two to four years. Outbreaks are most intense in moist western hemlock (Tsuga heterophylla) forests of the Pacific coast and the interior wet belt of southeastern British Columbia. Historically, extensive outbreaks occurred as far south as the Pacific Northwest of the United States and Vancouver Island in Canada, but the most recent outbreaks have been centred farther north in the coastal forests of Haida Gwaii and the Alaskan panhandle. Outbreaks are often concurrent with those of other irruptive insects such as hemlock sawfly (Neodiprion tsugae). 

Distribution and hosts

This insect is found from the Rocky Mountains westward, from northern California and Colorado in the south to Alaska in the north, including all of British Columbia and southwestern Yukon. In British Columbia, it is most abundant in coastal and interior hemlock forests where it feeds primarily on western hemlock and mountain hemlock (T. mertensiana), and occasionally spruces (Picea) and Douglas-fir (Pseudotsuga menziesii). True firs (Abies) are more common hosts in the United States than in Canada.

Tree parts affected

Foliage

Symptoms and signs

Pale yellow, oval eggs are laid singly on the underside of needles. Newly hatched larvae are 1 millimetre long and pale green with dark brown or black heads. Fully grown larvae are 11 to 15 millimetres long, and uniformly green. Colour of head capsule varies from solid reddish-brown to dark brown and black. Adults are medium-sized moths with wingspans from 8.0 to 9.8 millimetres. Colour patterns of the wings are variable in the extreme, ranging from a simple grey colour with a light-colour median band to ochreous colour with bands and various mottled markings.

Defoliation appears suddenly. The first evidence of damage is a red tinge in the upper crowns of host trees from needles that were detached and used to construct feeding shelters. Defoliation is most severe in younger, regenerating trees, with the entire crown turning red and complete top-stripping of all the needles of the dominant leader.

Life cycle

Western blackheaded budworm has one generation per year. Eggs are laid singly on the underside of needles near branch tips in the upper tree crowns of host trees in late summer to early fall. The insect overwinters in the egg stage. Eggs hatch in May to June and larvae bore into fresh buds. As shoots flush, budworms chew needles at the base and web them together to form individual feeding shelters. Where densities are high, larvae can feed on previous years’ needles as well, particularly on hemlocks and firs. Pupae are formed in the final feeding shelter in late August and adults emerge throughout September. Females emit a sex pheromone to attract males.

Moths are active flyers and may disperse long distances. Dispersal of moths away from damaged forests contributes to the short duration of outbreaks.

Little is known about the causes of outbreaks or their decline. It is likely that the same factors that influence outbreaks of eastern blackheaded budworm, such as natural enemies and variable weather patterns, are also important with western blackheaded budworm in western forests.

Damage

Tree mortality is not common unless severe defoliation lasts more than two years or is exacerbated by co-occurrence of other defoliators such as hemlock sawfly. The most significant damage occurs in juvenile stands where some tree mortality and top-kill occurs. The tallest trees in juvenile stands are most susceptible. Western hemlock recovers from top-kill rapidly because of its indeterminate, or continuous, growth throughout the season. Growth rates decrease when defoliated but surviving trees recover within a few years.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches; and
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

Juvenile stands less than 20 years of age are at greatest risk, especially if such stands have been thinned to increase spacing between trees. Pheromone traps and egg samples can track increases in annual population densities of western blackheaded budworm and forecast outbreaks a few years in advance. During this period, thinning juvenile stands should be curtailed.

Limited aerial applications of insecticides were carried out until the 1980s. The brief period of defoliation makes large-scale treatments difficult to target. Low rates of mortality in mature stands also make them unnecessary. Pheromones and insecticides are defined as pest control products and are regulated in Canada. Products registered for use against western blackheaded budworm under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered products and product information for use against this budworm. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Photos

Dorsal view of mature larva, on Engelmann spruce

Lateral view of mature larva, on Engelmann spruce

Dorso-lateral view of mature larva (in feeding shelter), on western hemlock

Selected references

Lejeune, R.R. 1975. Western black-headed budworm, Acleris gloverana (Wals.). Pages 159–166in M.L. Prebble, editor. Aerial control of forest insects in Canada. Department of the Environment, Canadian forestry Service. Ottawa Ontario.

Nealis, V.G.; Silk, P.; Turnquist, R.; Wu, J. 2010. Baited pheromone traps track changes in populations of western blackheaded budworm (Lepidoptera: Tortricidae). The Canadian Entomologist 142(5): 458–465. https://doi.org/10.4039/n10-033

Nealis, V.G.; Turnquist, R. 2010. Impact and recovery of western hemlock following disturbances by forestry and insect defoliation. Forest Ecology and Management 260(5): 699–706. https://doi.org/10.1016/j.foreco.2010.05.025

Nealis, V.G.; Turnquist, R.; Garbutt, R. 2004. Defoliation of juvenile western hemlock by western blackheaded budworm in Pacific coastal forests. Forest Ecology and Management 198(1–3): 291–301. https://doi.org/10.1016/j.foreco.2004.04.013

Otvos, I.S.; Fajrajsl, A. 1997. An annotated bibliography of Acleris variana and Acleris gloverana. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. Information Report BC-X-371. 81 p.

Shepherd, R.F.; Gray, T.G. 2001. Comparative rates of density change in declining populations of the blackheaded budworm Acleris gloverana (Lepidoptera: Tortricidae) among different sites on Vancouver Island. Environmental Entomology 30(5): 883–891. https://doi.org/10.1603/0046-225X-30.5.883

van Sickle, G.A. 1995. Forest insect pests in the Pacific and Yukon region. Pages 73–89 in J.A. Armstrong, and W.G.H. Ives, editors. Forest insect pests in Canada. Natural Resources Canada, Canadian Forest Service, Science and Sustainable Development Directorate. Ottawa, Ontario. https://publications.gc.ca/site/eng/9.889231/publication.html

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