Large aspen tortrix
- French common name: Tordeuse du tremble
- Other common names: Tortrix
- Scientific name: Choristoneura conflictana (Walker)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Lepidoptera
- Family: Tortricidae
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Partial list of synonyms:
- Archips conflictana (Walker)
- Tortrix conflictana Walker
General information and importance
Large aspen tortrix is a native defoliator of poplars (Populus) and other northern hardwoods in North America. Its extensive distribution and severe outbreaks make it a familiar insect in northern forest communities. The common name, tortrix, is derived from the Latin word for twist. It refers to how large aspen tortrix (and its relatives) twists or rolls the leaves of its host plant to form feeding shelters. Insect relatives of large aspen tortrix that feed on conifers, such as spruce budworm (Choristoneura fumiferana), have a similar habit. A notable difference is that they cannot twist rigid needles, so they attach them together with silk to form their feeding shelters.
Distribution and hosts
The distribution of large aspen tortrix coincides with that of its primary host, trembling aspen (Populus tremuloides), the most widespread tree species in North America. The insect and its host have a transcontinental distribution in Canada. This encompasses all major forest regions, including the northern plains and aspen parkland, where aspen and related balsam poplar (P. balsamifera) are often the only tree species present. Large aspen tortrix is also found extensively throughout the central and northeastern forests of the United States, Alaska, and at higher elevations in the western states as far south as aspen occurs.
Primary hosts are trembling aspen and related species, such as largetooth aspen (P. grandidentata) and balsam poplar. The ability of large aspen tortrix to utilize other hardwoods as food results in an extension of its distribution beyond the range of trembling aspen to include riparian zones where secondary hosts, such as cottonwood (P. deltoides), willow (Salix), or alder (Alnus), dominate. During outbreaks, primary hosts may be completely defoliated and large aspen tortrix will feed on different nearby deciduous trees and shrubs, including cherry (Prunus).
Tree parts affected
Symptoms and signs
Eggs may be visible in midsummer in flat clusters of 60 to 450 eggs, covering 15 to 85 square millimetres on leaves. Newly hatched larvae are 1.8 to 2.0 millimetres in length. They are a pale green colour, with a light brown head and thoracic shield. These small larvae remain together in mid to late summer, feeding on leaves they tie together with silk. They avoid feeding on the leaf veins, resulting in ‘skeletonized’ leaves. The following spring, small larvae emerge and feed on new buds. As leaves expand, the larvae twist or roll the leaves, fastening the edges with silk webbing to form characteristic and obvious feeding shelters. Mature larvae are 15 to 21 millimetres in length, with dark bodies and black heads. Pupae are formed in these feeding shelters. Trees may be completely defoliated except for the rolled leaves that shelter pupae. Masses of silk and frass will be visible on shrubs beneath trees. Large aspen tortrix are medium-sized moths, with a body length of 10 to 15 millimetres and wingspan of 25 to 35 millimetres. Forewings are light grey, with darker markings on inner and outer patches divided by a middle band. Millions of moths often congregate at lights on summer nights during outbreaks. Aspen often regenerate from suckering, resulting in clusters of defoliated trees and large aspen tortrix outbreaks that are somewhat patchy, especially when viewed from an aircraft.
Life cycle
Large aspen tortrix has one generation a year. Eggs are laid midsummer. Newly hatched larvae feed as a group on leaf surfaces they web together. By late August, larvae move to the tree trunk and establish hibernation sites in protected bark crevices. They emerge in early spring, return to the tree crown, and tunnel into fresh buds. As leaves expand, the larvae twist the leaves or join two leaves together by tying the edges with silk to form a feeding shelter. Pupation occurs in this feeding shelter. Adults emerge and mate. Moths are strong fliers and may disperse many kilometres.
Outbreaks are most extensive in the transitional ecological region between the boreal forest and prairies, also known as the aspen parkland. However, poplars are abundant in most Canadian forest regions, often growing from root suckers in clusters following disturbance. These outbreaks are not clearly periodic, and the cause of irruptions is poorly understood. Outbreaks usually last 2, or at most, 3 years. Many natural enemies exploit outbreaks of large aspen tortrix. Starvation of larvae during intense outbreaks is considered the main reason for the abrupt collapse of populations.
Damage
Defoliation by large aspen tortrix rarely kills otherwise healthy aspen trees. Shoots may be lost, and growth rates slowed. Defoliated trees usually re-foliate with smaller leaves later in the summer. Older trees (greater than 80 years) and trees growing on poor sites are most vulnerable. There is no clear evidence that defoliation by large aspen tortrix contributes significantly to aspen crown dieback and decline, although changes in environmental conditions, such as drought, could affect both the health of aspen and fitness of large aspen tortrix. Occasionally outbreaks of large aspen tortrix precede or follow outbreaks of forest tent caterpillar (Malacosoma disstria). The combined outbreaks can result in more severe impacts and tree mortality.
Prevention and management
Despite the strong association between people and poplars and the commercial value of aspen in modern forestry products, direct management of large aspen tortrix populations is not undertaken often. Foresters engaged in intensive management of poplars are encouraged to maintain vigorous stands through appropriate silvicultural practices. Unsightly outbreaks in recreational areas often result in calls for pesticide applications. However, the short-term nature of outbreaks, relative tolerance of aspen to damage, and demonstrable absence of significant long-term effects usually means that no such management intervention during outbreaks is required.
Pesticides registered for use against large aspen tortrix under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides 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.
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.
Photos
Large aspen tortrix egg mass on a trembling aspen leaf. A few eggs have hatched.
Jean-Paul Laplante
Large aspen tortrix egg mass on a trembling aspen leaf. Several eggs have hatched and larvae are emerging or have already emerged.
Thérèse Arcand
Rolled trembling aspen leaves with an empty pupal case on the right from which a large aspen tortrix moth emerged.
Jean-Paul Laplante
Adult moth of large aspen tortrix at rest on a trembling aspen leaf.
Thérèse Arcand
Newly emerged large aspen tortrix moth next to its empty pupal case on trembling aspen.
Jean-Paul Laplante
Mature large aspen tortrix larva (dorsal view) on trembling aspen leaves.
Thérèse Arcand
An unrolled trembling aspen leaf exposing the mature large aspen tortrix larva feeding within it.
Thérèse Arcand
Large aspen tortrix pupa tied with silk to a trembling aspen leaf. Note also the cast skin from the last moult adjacent to the pupa.
Thérèse Arcand
An unrolled trembling aspen leaf exposing the large aspen tortrix pupa within it.
Thérèse Arcand
Large aspen tortrix moth, pinned with wings spread.
Thérèse Arcand
Rolled leaf of trembling aspen tied with silk. A single large aspen tortrix larva feeds within this rolled leaf.
Thérèse Arcand
Aerial view of a patch of trembling aspen severely defoliated by large aspen tortrix.
Jean-Claude Boutin
Crowns of several trembling aspens that have been defoliated by large aspen tortrix. In many cases, rolled leaves remain on the defoliated trees.
Roger Roy
Crowns of several trembling aspens that have been defoliated by large aspen tortrix. In many cases, rolled leaves remain on the defoliated trees.
Roger Roy
Selected references
Brandt, J.P.; Cerezke, H.F.; Mallett, K.I.; Volney, W.J.A.; Weber, J.D. 2003. Factors affecting trembling aspen (Populus tremuloides Michx.) health in the boreal forest of Alberta, Saskatchewan, and Manitoba, Canada. Forest Ecology and Management 178(3): 287– 300. https://doi.org/10.1016/S0378-1127(02)00479-6
Cerezke, H.F. 1992. Large aspen tortrix. Forestry Canada, Northern Forestry Centre. Edmonton, Alberta. Forestry Leaflet 21. https://ostrnrcan-dostrncan.canada.ca/entities/publication/ee5d974c-a052-43ef-89aa-a7be3ad9e884?fromSearchPage=true
Ciesla, W.M.; Kruse, J.J. 2009. Large aspen tortrix. United States Department of Agriculture, Forest Service. Portland, Oregon. Forest Insect & Disease Leaflet 139.
Parry, D.; Volney, W.J.A.; Currie, C.R. 1997. The relationship between trembling aspen phenology and larval development of the large aspen tortrix. Natural Resources Canada, Canadian Forest Service, Northern Forestry Centre. Edmonton, Alberta. Information Report NOR-X-350. 12 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/1f2469a3-79ea-46ff-9884-f027c8b68da6?fromSearchPage=true
Prentice, R.M. 1955. The life history and some aspects of the ecology of the large aspen tortrix, Choristoneura conflictana (Wlkr.) (N. Comb.) (Lepidoptera: Tortricidae). The Canadian Entomologist 87(11): 461–473. https://doi.org/10.4039/Ent87461-11