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Pest Status
Chapter 60
Operophtera bruceata (Hulst), Bruce
Spanworm (Lepidoptera: Geometridae)
W.G.H.IVES
The Bruce spanworm, Operophlera bruceala (Hulst), attacks a variety of deciduous
trees (Prentice 1963), and is very similar in appearance to the winter moth, Operophlera
brumala (L.). Adults of the two species can be distinguished by differences in genitalia
in males and in vestigial wing length in females (Eidt el al. 1966). Pupae can be separated
by differences in the form of the cremaster, and larvae can be distinguished by the
placement of the ocelli (Eidt & Embree 1968). In eastern Canada, sugar maples, Acer
saccharum Marsh., and beech trees, Fagus grandi/olia Ehrh., are favoured hosts, but in
western Canada, trembling aspen. Populus Iremuloides Michx., seems to be the principal
host, particularly in Alberta. Although the Bruce spanworrn is not usually considered a
major pest, a number of localized infestations, as well as more extensive outbreaks in
which damage has been primarily due to this insect, have been reported by the Forest
Insect and Disease Survey (Anon. 1952-79). The following account of infestation
history is based on these reports, except where otherwise noted.
In central and eastern Canada. the first reported damage occurred in a small stand of
sugar maple near Merrivale, Ontario, in 1951 and 1952. In Newfoundland a small
infestation was reported on white birch, Belula papyri/era Marsh., in 1956. The first
reported damage in the maritimes occurred in 1962, when a patch of sugar maple in
Nova Scotia was severely defoliated. and a larger area of defoliation was reported in
New Brunswick. Infestations in both Maritime Provinces increased in scope and intensity
in 1963, and the outbreak reached its peak in 1964, when over 200 000 ha of severe
defoliation occurred in Nova Scotia alone (Harrington 1968). Declines in populations
were noted in 1965, and by 1966 the outbreak had collapsed. A short-lived outbreak also
occurred in Quebec, where the insect reached epidemic proportions in many areas in
1963. However, populations were lower in 1964, and only small pockets of defoliation
were reported in 1965. At its peak, this outbreak covered some 39 000 km 2 (Martineau &
Monnier 1966). In Ontario, where infestations of Bruce spanworrn were first reported,
no further damage occurred until 1963, when a small pocket of aspen defoliation was
reported near Sudbury. In 1965, three widely separated areas of severe sugar maple
defoliation occurred: one was in Algonquin Park; another was on an island in Lake
Huron; and the third was on hilltops and ridges near Sault Stet Marie. The outbreak
decreased in intensity in 1966 and collapsed completely in 1967.
In western Canada, damage to trembling aspen attributable to the Bruce spanworm
was first reported from the Obed area of Alberta in 1956. Widespread areas of defoliation.
much of it severe, were reported in 1957 and again in 1958, when about 130000 kml
were moderately or heavily infested (Brown 1962). In 1959 some infestations had
subsided, and all outbreaks collapsed in 1960. Severe defoliation of trembling aspen in
northern British Columbia was reported in 1958 and 1959, but these infestations also
collapsed in 1960.
No further significant damage attributable to the Bruce spanworm was reported until
1968, when extensive areas of moderate to severe defoliation of trembling aspen were
again observed in Alberta. Both the intensity and size of this outbreak increased in 1969.
Extensive damage also occurred in 1970, but only patches of aspen were severely
defoliated in 1971. In Quebec, several maple groves were infested in 1970, and these
were completely stripped in 1971 and again in 1972. The insect was somewhat less
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