Western pine budworm
- French common name: Choristoneura lambertiana
- Other common names: Sugar pine tortrix
- Scientific name: Choristoneura lambertiana (Busck)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Lepidoptera
- Family: Tortricidae
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Partial list of synonyms:
- Tortrix lambertiana Busck
General information and importance
Western pine budworm is a pine-feeding forest insect closely related to spruce budworm. It is native to North America and is the only pine-feeding member of the budworm species group west of the Rocky Mountains. A high degree of colour variation in moths of western pine budworm resulted in early confusion with the naming of various subspecies based on local characteristics. Recent genetic analyses suggest that western pine budworm is sufficiently distinct to be considered a separate species, but there is ample evidence of interbreeding and hybridization among other budworm species within the western region.
Populations of western pine budworm may become locally abundant enough to cause noticeable defoliation of trees, but extensive outbreaks characteristic of related budworm species have not been reported. The absence of recurring, significant damage to trees has resulted in much less attention to the biology and ecology of western pine budworm than other budworms in the group.
Distribution and hosts
Recent genetic analysis reveals a more extensive range for western pine budworm and distinguishes two forms associated with different pine hosts and their ranges. The northern form occurs from western Saskatchewan through the northern and central Rocky Mountains, east to central Oregon. The northern form feeds primarily on lodgepole pine (Pinus contorta) at higher elevations and to a lesser extent, jack pine (P. banksiana) at the eastern extremities of the latter pine’s range. The more common southern form is found from south coastal British Columbia eastward through the Cascades and Great Basin in the United States to the Rocky Mountains. It reaches as far south as New Mexico, with fragmented populations in California. The various populations of the southern form feed on a greater variety of pines, including sugar pine (P. lambertiana), ponderosa pine (P. ponderosa), lodgepole pine, limber pine (P. flexilis), Washoe pine (P. washoensis), and Jeffrey pine (P. jeffreyi).
Tree parts affected
Current year buds and needles, and especially immature seed and pollen cones.
Symptoms and signs
Eggs are pale green and laid in overlapping clusters on needles. Single eggs of western pine budworm are the largest of the spruce budworm group (greater than 0.2 milligrams). Final stage larvae are up to 18 millimetres long. Head and thoracic shield patterns of larvae are distinctly lighter in colour than spruce-fir feeding budworm species, with brown patterns on a reddish tan to yellowish background. Head colour is similar in both sexes, but the thoracic shield of female larvae is notably lighter in colour than those of male larvae. The body is reddish brown to pale rust. The back of each abdominal segment is marked with two pairs of pale spots. Pupae of both sexes are yellowish at first and darken as their cuticle hardens. Forewings of adult moths are 9 to 10 millimetres in length and 2.5 times longer than their width. The base colour of forewings is reddish-brown to brick-red with whitish bands common to both sexes.
Small, early emerging western pine budworm larvae damage early-season buds and pollen cones. However, this damage is only visible on close inspection. Later, large larvae construct protective feeding shelters by tying needles together with silk. The combination of webbing and damaged needles is noticeable throughout the tree crown. As damaged needles dry in mid-summer, the crown of the tree turns red. This change in appearance can occur quickly in dry, interior forests. If defoliation persists for several years, the tree crown becomes thinned, and branches may die.
Life cycle
Western pine budworm is one of the least known insects of the spruce budworm group of species. The following description of its life cycle is based largely on the well-known life cycle that is common to other budworm species in the group.
There is one generation per year, with the potential of a 2-year life cycle in northern and high-elevation locations. Moth adults lay eggs on the needles of host trees during July and August. Each cluster may contain up to several dozen eggs, with a mean of 22 eggs per egg-mass. Eggs hatch in 9 to 13 days. First-stage larvae do not feed. They settle in sheltered locations on the host tree where they moult and overwinter as second-stage larvae. These larvae emerge in the spring before current-year buds become available and exploit pollen cones before constructing feeding shelters around expanding shoots. Larvae feed for 4 to 6 weeks. Pupation occurs on the host tree, usually in the final feeding site. The pupal period lasts about 2 weeks. Freshly emerged female moths emit a pheromone that attracts male moths to mate. Both sexes are strong flyers.
Damage
Extensive outbreaks of western pine budworm are uncommon. In fact, outbreaks are so infrequent that general surveys have a difficult time finding many specimens. Nonetheless, local populations may increase to very high densities on just a few trees, causing significant damage. These defoliation episodes are short-lived, and trees recover. Western pine budworm also feeds on immature pollen and seed cones, conceivably causing significant damage in seed orchards. However, this has not been reported.
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.
The absence of significant, prolonged outbreaks across large, commercial forested areas has resulted in little need for a management strategy for western pine budworm. Well-developed methods to monitor populations of other budworm species could presumably be applied directly to western pine budworm populations, should that become necessary. Such methods include setting pheromone traps, and sampling egg masses, overwintering larvae, or feeding larvae. Calibrated methods to assess damage levels from other budworms could also likely be applied.
Pheromones and pesticides are defined as pest control products and are regulated in Canada. Pheromones and pesticides registered for use against western pine 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 insect. 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
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Selected references
Duncan, R.W. 2006. Conifer defoliators of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 359 p.
Dupuis, J.R.; Brunet, B.M.T.; Bird, H.M.; Lumley, L.M.; Fagua, G.; Boyle, B.; Levesque, R.; Cusson, M.; Powell, J.A.; Sperling, F.A.H. 2017. Genome-wide SNPs resolve phylogenetic relationships in the North American spruce budworm (Choristoneura fumiferana) species complex. Molecular Phylogenetics and Evolution 111: 158–168. https://doi.org/10.1016/j.ympev.2017.04.001
Furniss, R.L.; Carolin, V.M. 1977. Western forest insects. United States Department of Agriculture Forest Service. Washington, DC. Miscellaneous Publication 1339. 654 p.
Harvey, G.T.; Stehr, G. 1967. On coniferophagous species of Choristoneura (Lepidoptera: Tortricidae) in North America. III. Some characters of immature forms helpful in the identification of species. The Canadian Entomologist 99(5): 464–481. https://doi.org/10.4039/Ent99464-5
Lumley, L.M.; Sperling, F.A.H. 2010. Integrating morphology and mitochondrial DNA for species delimitation within the spruce budworm (Choristoneura fumiferana) cryptic species complex (Lepidoptera: Tortricidae). Systematic Entomology 35(3): 416–428. https://doi.org/10.1111/j.1365-3113.2009.00514.x
Lumley, L.M.; Sperling, F.A.H. 2011. Utility of microsatellites and mitochondrial DNA for species delimitation in the spruce budworm (Choristoneura fumiferana) species complex (Lepidoptera: Tortricidae). Molecular Phylogenetics and Evolution 58(2): 232–243. https://doi.org/10.1016/j.ympev.2010.11.023
Volney, W.J.A. 1989. Biology and dynamics of North American coniferophagous Choristoneura populations. Agricultural Zoology Reviews 3: 133–156.
Volney, W.J.A.; Fleming, R.A. 2007. Spruce budworm (Choristoneura spp.) biotype reactions to forest and climatic characteristics. Global Change Biology 13(8): 1630–1643. https://doi.org/10.1111/j.1365-2486.2007.01402.x