Bruce spanworm, Operophtera bruceata / Winter moth, Operophtera brumata

Fig. 305  Adult male bruce spanworms.	Fig. 306   Adult winter moths (male above, females below).	Fig. 307  Light-coloured bruce spanworm larva.	Fig. 308  Winter moth larva.
Fig. 309  Bruce spanworm eggs.	Fig. 310  Trees defoliated by bruce spanworm. Note webbing on understory vegetation.	Fig. 311  Damage caused by bruce spanworm.

Distribution:  Bruce spanworm is found throughout B.C.  The winter moth was introduced into North America from Europe, and was first recorded in Canada in 1950, in Nova Scotia.  It was first found in B.C. in 1977 on Vancouver Island, and is now found on southern Vancouver Island and the Lower Mainland. 

TREE SPECIES ATTACKED:  Trembling aspen, paper birch, balsam poplar, and bigleaf maple are attacked. The winter moth is also an important defoliator of Garry oaks on southern Vancouver Island. Willow and several other broadleaf tree species are also attacked.

WHAT TO LOOK FOR: Hairless loopers feeding within rolled or webbed leaves. Also, bright orange eggs found either in bark crevices, under lichen, or in moss somewhere on the tree. A silken web on understory vegetation can also be observed.

INSECT DESCRIPTION & DAMAGE SYMPTOMS: Larvae of both species are stout-bodied, hairless loopers about 1.8 cm long when fully grown. Bruce spanworm larvae are typically light green with one prominent and two indistinct yellowish-white lines along the sides, with a dark brown head. Some individuals are darker in colour, having blackish heads and dark grey bodies with three whitish bands along each side. Winter moth larvae have a green body, a narrow, dark green mid-dorsal line, and three creamy yellow stripes along each side. Adult females are 0.6 to 0.8 cm long and have only vestigial wings. The vestigial wings of the female winter moth are about three times longer than those of the bruce spanworm (about 0.26 cm and 0.08 cm long, respectively). Some individuals are dull ocherous brown with white patches, and others have bluish-grey scales on their abdomens. Males are light brown, slender-bodied moths. They have bluish grey wings with brownish markings most noticeable on the veins, and a wingspan of 2.5 to 3.0 cm. Adults may be found from late October to the end of January. Females lay their eggs in the fall in bark crevices, under lichen, or in moss on any part of the tree. The eggs are light green at first, later changing to bright orange. Larvae hatch in the spring and initially feed by mining developing buds. This results in holes in the leaves, which become conspicuous as the leaves expand. Later, they feed either openly, or within rolled or webbed leaves. During severe outbreaks, trees and understory vegetation can become covered with webbing. There is one generation per year.

DAMAGE: Because feeding is completed early in the growing season, infested trees will produce new leaves. Outbreaks usually last 2 to 3 years and result in reduced radial growth. Tree mortality will result only if the outbreak lasts for several consecutive years. Garry oaks on southern Vancouver Island have been severely affected by the winter moth; however, defoliation has been greatly reduced by the introduction in 1979 to 1981 of two parasitoids, Cyzenis albicans and Agrypon flaveolatum, which were also introduced in south Vancouver in 1995.

SIMILAR DAMAGE: Damage caused by bruce spanworm can be confused with that caused by the large aspen tortrix. However, if the damage was caused by bruce spanworm, no pupal cases or egg masses will be found on the foliage, and no hibernation shelters will be present on stems and branches.

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Contact Tim Ebata if you have comments on the presentation of this information.

BC Ministry of Forests
Forest Practices Branch
P.O. Box 9513 Stn. Prov. Gov.
Victoria, BC
V8W 9C2

Section phone: (250) 387-8739
Section fax: (250) 387-2136

Last updated August 24, 2009