Bruce spanworm complex
- French common name: Arpenteuse de Bruce
- Other common names: Hunter’s moth
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Scientific names:
- Operophtera bruceata (Hulst)
- Operophtera danbyi (Hulst)
- Operophtera occidentalis (Hulst)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Lepidoptera
- Family: Geometridae
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Partial list of synonyms:
- Operophtera groenlandica Lesse (synonym of O. bruceata)
- Operophtera hyperborea (Hulst) (synonym of O. occidentalis)
- Operophtera latipennis (Hulst) (synonym of O. occidentalis)
General information and importance
Bruce spanworm is a complex of closely related native defoliators of many hardwood species throughout Canadian forests. They are also closely related to the winter moth (Operophtera brumata), an invasive alien species that has been introduced to Canada’s eastern and western coastal forests. Genomic evidence suggests the species are hybridizing where their populations overlap. Their close similarity obscured the invasion of winter moth for several years because it was assumed the damage was caused by the native Bruce spanworm complex.
The name “Bruce” is a colloquial version of the species name “bruceata”. The species was named after Mrs. Rachel Bruce, the wife of an early lepidopterist. The name “spanworm” (also called “looper,” or “inchworm”) refers to the characteristic crawling habit of this caterpillar and its relatives in the family Geometridae or “earth measurers”. Caterpillars first stretch the front half of their bodies forward, then pull the back half of the body forward to meet the front legs, making a loop. As the front of the caterpillar stretches forward again, the body appears to span or “measure” a distance the length of their bodies. Another common name, “hunter’s moth”, comes from the adult male moth’s flight on cold autumn nights, during hunting season.
Distribution and hosts
Of the three species in the complex, O. bruceata is by far the most widespread, occurring from the Atlantic Provinces to the Rocky Mountains in Canada, as well as in adjacent northern United States. Populations west of the Rocky Mountains are sufficiently distinct genetically that they have recently been recognized as two separate species: O. occidentalis on Vancouver Island and the coastal mainland and O. danbyi on Vancouver Island. Where more than one Operophtera species occurs (Atlantic and Pacific coasts), they cannot be separated reliably except via DNA analysis.
The Bruce spanworm complex feeds on most hardwood species in Canada including oak (Quercus), maple (Acer), poplar (Populus), willow (Salix), birch (Betula), and alder (Alnus), as well as wild and cultivated fruit trees. Apparent preferences reflect regional abundance or commercial value of available hosts. Poplar (aspen) and willow are the most common hosts in the Prairie provinces and northern Ontario, sugar maple (A. saccharum) and beech (Fagus grandifolia) in southern and central Ontario and Quebec, and oak and fruit trees including apple (Malus), pear (Pyrus), and cherry (Prunus) in the Maritime provinces.
Tree parts affected
Symptoms and signs
Pale green eggs are laid singly in crevices on the tree and are less than 1 millimetre long. They become orange within a few days. First-stage larvae are pale and about 2 millimetres long. As larvae grow, the colour of their body and head is highly variable from green to dark brown with distinct lateral and dorsal lines on the body. Length of mature larvae is about 18 millimetres. Female moths have vestigial wings and are covered in large brown scales with irregular patches of white. Males are medium-sized moths with semi-transparent forewings banded with brown or grey markings. Male moths fly at night from late autumn to early winter and are attracted to lights.
Small larvae may be seen in early spring hanging from trees on silk threads. Early emerging larvae bore into developing buds where they are well hidden. Much of the damage occurs during this early season feeding and becomes more evident as leaves unfold to reveal a characteristic pattern of holes in the expanding leaves. Large larvae web leaves together and tend to avoid feeding on leaf veins, which “skeletonize” the foliage.
Life cycle
All species within the Bruce spanworm complex have one generation per year. Eggs are laid singly in late fall to early winter in bark crevices, on lichens, or any other protected locations on the tree. They remain dormant until the following May. Egg hatch coincides with the springtime flush of buds. Small larvae disperse surprising distances on air currents by ballooning on silk threads. Once settled, they first bore into buds and later feed openly or in protected shelters that are loosely formed by webbing leaves together until mid- to late-June. When larval development is complete, the caterpillars drop from trees on silk threads and form pupal cells in the soil or duff beneath the tree. They remain in the soil as pupae until late fall when adults emerge. Female moths do not fly. They climb from the soil to the lower stem of trees and other plants and emit a pheromone to attract mating males. The male moths are active flyers, and they are one of the few moths seen on cold nights late in the season.
There have been occasional outbreaks covering extensive areas of aspen in Alberta and sugar maple in Quebec. In general, however, extensive outbreaks of Bruce spanworm are uncommon. A richly developed group of insect natural enemies and birds attack Bruce spanworm but do not appear to have much of an impact. A virus has been observed where populations have reached epidemic levels.
Damage
Outbreaks are not common and only extensive in areas such as aspen parklands where the preferred host is dominant. If severe defoliation occurs early in the season, most hardwood tree species can produce a second flush of foliage. Outbreaks seldom last more than a few years so tree mortality is not significant. Local defoliation of valued trees such as sugar maple, fruit trees, and urban ornamental trees may be an annoyance.
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.
Species within the Bruce spanworm complex are common insects, but damage is infrequent and generally not significant. Because they feed on a wide variety of trees, there are few options for avoiding occasional damage. Sticky bands placed around tree trunks in autumn can trap female moths in the fall and impede larvae from crawling up the tree to the foliage in the spring.
Pesticides registered for use against Bruce spanworm 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.
Photos
Adult female (wingless) laying green eggs in bark crevices (length: 6-7 mm)
Thérèse Arcand
Adult male on a maple trunk (wingspan: 25-30 mm)
Thérèse Arcand
Pupa in its open cocoon (length: 6-7 mm)
Jean-Paul Laplante
Lateral view of a green larva (lenght: 18 mm)
Thérèse Arcand
Dorsal view of a dark feeding larva
Jean-Paul Laplante
Severely defoliated maple grove
Thérèse Arcand
Selected references
Brown, C.E. 1962. The life history and dispersal of the Bruce spanworm, Operophtera bruceata (Hulst), (Lepidoptera: Geometridae). The Canadian Entomologist 94(10): 1103–1107. https://doi.org/10.4039/Ent941103-10
Eidt, D.C.; Embree, D.G. 1968. Distinguishing larvae and pupae of the winter moth, Operophtera brumata, and the Bruce spanworm, O. bruceata (Lepidoptera: Geometridae). The Canadian Entomologist 100(5): 536–539. https://doi.org/10.4039/Ent100536-5
Eidt, D.C.; Embree, D.G.; Smith, C.C. 1966. Distinguishing adults of the winter moth, Operophtera brumata, and the Bruce spanworm, O. bruceata (Lepidoptera: Geometridae). The Canadian Entomologist 98(3): 258–261. https://doi.org/10.4039/Ent98258-3
Elkinton, J.; Boettner, G.; Liebhold, A.; Gwiazdowski, R. 2014. Biology, spread, and biological control of winter moth in the eastern United States. United States Department of Agriculture, Forest Service, Publication FHTET-2014-07. https://www.fs.usda.gov/nrs/pubs/jrnl/2015/fhtet-2014-07_elkinton_2015_001.pdf [Accessed September 2024]
Humble, L.M. 1985. Final-instar larvae of native pupal parasites and hyperparasites of Operophtera spp. (Lepidoptera: Geometridae) on southern Vancouver Island. The Canadian Entomologist 117(5): 525–534. https://doi.org/10.4039/Ent117525-5
Volney, W.J.A. 1995. Bruce spanworm. Natural Resources Canada, Canadian Forest Service, Northern Forestry Centre. Edmonton, Alberta. Forest Leaflet 34.