Two-year cycle spruce budworm
- French common name: Tordeuse bisannuelle de l'épinette
- Other common names: 2-year cycle spruce budworm, 2-year budworm
- Scientific name: Choristoneura occidentalis biennis (Freeman)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Lepidoptera
- Family: Tortricidae
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Partial list of synonyms:
- Choristoneura biennis Freeman
General information and importance
Two-year cycle spruce budworm is the only common conifer-feeding spruce budworm that lives exclusively in Canada, occurring in high-elevation, subalpine western forests north of the 49th parallel. As its name indicates, it differs from other spruce budworms, as it requires 2 years to complete its life cycle. In the first season, small larvae emerge from hibernation, feed briefly, and then re-enter hibernation. In the second season, the larvae emerge, resume feeding and complete their maturation. This is when most damage occurs. Consequently, during outbreaks, there is an “off” and “on” year of defoliation, also revealed in alternating normal and reduced increments in the width of annual tree-rings.
The distinct life cycle and host forest type of two-year cycle spruce budworm resulted in its recognition as a separate species, C. biennis, for several years. However, genomic analyses indicate that two-year cycle spruce budworm is closely related to, and interbreeds with, the more widespread western spruce budworm, C. occidentalis. It is now classified as a subspecies of western spruce budworm. Nonetheless, the 2 year life cycle and different forest ecosystems occupied by these two budworms make them more ecologically distinct than, for example, the more genetically distant spruce budworm (C. fumiferana) and jack pine budworm (C. pinus) east of the Rocky Mountains.
Distribution and hosts
Two-year cycle spruce budworm inhabits high elevation Engelmann spruce (Picea engelmannii)-subalpine fir (Abies lasiocarpa) forests along the western cordillera of British Columbia and adjacent Alberta, from the Canadian border with the United States to southern Yukon. Subalpine fir, white spruce (Picea glauca), and Engelmann spruce are the primary hosts. Black spruce (P. mariana) is a secondary host.
Tree parts affected
Current-year buds and needles, developing pollen cones.
Symptoms and signs
Green eggs are laid in overlapping rows on needles. The dark head of the developing larva can be seen in each fertile egg just before they hatch. Early-stage larvae have dark brown to black heads and pale brown bodies, with no distinctive markings. Final-stage larvae are 25 millimetres in length, with dark brown or black heads and dark brown bodies. Their bodies are distinctly marked, with two rows of cream-coloured spots on each dorsal segment. Wingspan of the adult moths is 25 to 31 millimetres. Coloration of wings is dark grey, with white to grey markings.
During the first feeding season, there is limited damage to new growth by the small larvae. In the second year, the familiar signs of budworm feeding become apparent, as larvae form feeding shelters with silk webbing and damage many needles. As needles die, trees take on a scorched red appearance. After dead needles fall off, the branch tips appear grey or bare. Stripping of all needles off the tops of trees occurs in severe cases when populations of the insect are high.
Life cycle
Two-year cycle spruce budworm requires 2 years to complete one generation, so each life stage and associated damage is biennial. Eggs are laid in overlapping rows of 20 to 50 eggs per egg mass on needles from late July to mid-August. Eggs hatch in 1 to 2 weeks. Newly hatched budworms do not feed. They settle in protected niches on the branches and trunk where they enter diapause (physiological dormancy) and hibernate. They emerge in late May and early June of the next spring and return to the branch tips in search of fresh buds. They feed for only a few weeks, moult once, construct a second hibernation site, moult again to a 4th stage larva, and enter a second diapause period for the remainder of the summer and following winter.
In their second season, two-year cycle spruce budworm display typical budworm behaviour, constructing a feeding shelter, and developing quickly in the long summer days at high latitudes. They pupate in late July and adult moths emerge in 1 to 2 weeks. The female moth emits a pheromone that attracts males to mate. Moths become active at dusk. They may fly above the tree canopy and can be transported many kilometres on air currents.
Outbreaks occur in mature forest stands dominated by primary hosts. Dry sites and years with high evaporation rates appear to increase susceptibility to outbreaks. Analyses of tree ring data suggest outbreaks occur at 30- to 40-year intervals and last an average of 11 years, although there is high variability in the available data. The similarity of the cycle length to budworms with an annual life cycle is unexpected, as it implies two-year cycle spruce budworm completes the same population cycle in half the number of generations. The high elevation forests where the two-year cycle spruce budworm occurs have only recently been exploited commercially, so many dense stands with large trees still exist and are susceptible. The most extensive outbreaks have been in the central forests of British Columbia. This area consists of susceptible forest stands that are more contiguous than farther south and east in the range.
Most of the insect parasitoids and pathogens associated with other spruce budworms have been found attacking two-year cycle spruce budworm.
Damage
There are limited detailed studies on the impacts of two-year cycle spruce budworm. The biennial reprieve in feeding damage during outbreaks reduces impacts relative to the annual defoliation inflicted by other budworms. Mortality of dominant trees is infrequent, and mortality of intermediate and understory trees is only observed where defoliation is severe (greater than 80% removal of foliage). Decreases in annual growth rates are related to the severity of defoliation. Trees recover once the outbreak ends. Subalpine fir is more affected than spruce.
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.
There are very few silvicultural options for reducing damage caused by two-year cycle spruce budworm, given the accessibility and ecology of subalpine forests.
Despite the evidence of low impacts to affected forests, aerial applications of insecticides have targeted two-year cycle spruce budworm in the past. Priorities for treatment are set according to severity of recent damage and estimates of population density based on pheromone traps and/or egg samples. Insecticides and pheromones are defined as pest control products and are regulated in Canada. Products registered for use against two-year cycle spruce 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
Full-grown larva of two-year cycle spruce budworm.
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Full-grown larva of two-year cycle spruce budworm.
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Selected references
Brunet, B.M.T.; Blackburn, G.S.; Muirhead, K.; Lumley, L.M.; Boyle, B.; Levesque, R.C.; Cusson, M.; Sperling, F.A.H. 2017. Two’s company, three’s a crowd: new insights on spruce budworm species boundaries using genotyping-by-sequence in an integrative species assessment (Lepidoptera: Tortricidae). Systematic Entomology 42(2): 317–328. https://doi.org/10.1111/syen.12211
Nealis, V.G. 2005. Diapause and voltinism in western and 2-year-cycle spruce budworms (Lepidoptera: Tortricidae) and their hybrid progeny. The Canadian Entomologist 137(5): 584–597. https://doi.org/10.4039/n05-028
Nealis, V.G. 2016. Comparative ecology of conifer-feeding spruce budworms. The Canadian Entomologist 148(S1): S33–S57. https://doi.org/10.4039/tce.2015.15
Shepherd, R.F. 1959. Phytosociological and environmental characteristics of outbreak and non-outbreak areas of the two-year cycle spruce budworm, Choristoneura fumiferana. Ecology 40(4): 608–620. https://doi.org/10.2307/1929814
Unger, L.S. 1984. Two-year cycle spruce budworm in British Columbia, 1914-1982. Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. FIDS Report 84-01. 26 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/a09a374a-333c-4a78-a49f-0d69e0bb4cde?fromSearchPage=true
Zhang, Qi-Bin; Alfaro, R.I. 2002. Periodicity of two-year cycle spruce budworm outbreaks in Central British Columbia: a dendro-ecological analysis. Forest Science 48(4): 722–731. https://doi.org/10.1093/forestscience/48.4.722