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350 W. G. H. Ivcs
Background
Field Trials
abundant in 1973. and populations continued to decline in 1974 and 1975. In Ontario,
pockets of severe defoliation of sugar maple, white birch, and beech were reported along
the south boundary of Algonquin Park in 1974, and the outbreak expanded and intensified in
1975, apparently collapsing in 1976. Severe defoliation of trembling aspen also occurred
in the Lake Nipigon area in 1976. The outbreak increased in area in 1977, but populations
declined in most areas in 1978. In Alberta, severe defoliation by Bruce spanworm was
again noted in the Obed area in 1978, and this outbreak continued in 1979 and 1980.
Most of the reported outbreaks of Bruce spanworm have been of short duration.
Consequently there was little if any tree mortality, even when defoliation was complete.
One of the reasons for the short duration of the outbreaks, at least in eastern Canada,
appears to be a nuclear polyhedrosis virus (NPV) which was described by Smirnoff
(1964). The virus apparently causes appreciable mortality and thus hastens the collapse
of the outbreaks. Although there were several reports of the virus occurring in the
maritimes and in Quebec, none of the Forest Insect and Disease Survey reports from
western Canada mentioned virus, and no experimental assessments of its virulence
could be found in the literature. A small-scale field experiment, using NPV from eastern
Canada, was therefore conducted in the Obed area of Alberta in 1979.
A small supply of NPV was obtained by infecting larvae reared in a laboratory
(J.c. Cunningham 1979 personal communications). Bruce spanworm eggs were obtained
by collecting moss from around the base of infested trees near Obed in the fall of 1978.
The eggs were incubated during the winter, and the emerging larvae were reared on an
artificial diet in plastic cups. A total of 5 000 fifth-instar larvae was infected with virus.
The dead larvae were collected and the virus purified by differential centrifugation; 4 x
1011 polyhedral inclusion bodies (PIB) were obtained. The results of the field trial have
already been described (Ives & Cunningham 1980) and only a brief description of the
experiment need be given here. A strip of moderately infested trembling aspen, mostly
under 10 m in height, was selected for the virus trials. Various concentrations of virus in
an aqueous formulation containing 50 gil Shade® (Sandoz Inc.), 1 % Chevron® sticker
and 25% v/v animal feed-grade molasses were applied until liquid dripped from the
foliage (about 350 lIha) with a backpack mist blower. Virus concentrations of 10' and 10'
PIB/ml were applied on 24 May. The larvae were in the first instar, the temperature was
19°C, relative humidity 42%, and wind speed 0-8 kmlh. Leaves on the trees receiving
10' PIB/ml were 1.5-2.0 em in diameter. The remaining trees had not yet flushed. A
second application of virus, on a different group of trees, was made on 1 June. Larvae
were in the second and third instars, the temperature was 21°C, R.H. 29%, and wind
speed 3-11 kmlh. The leaves were fully flushed, and three concentrations of virus (IO',
10', and 10' PIB/ml) were tested.
Two methods of assessing impact were used: determination of percentage mortality
by rearing of individual larvae until death or pupation, and estimation of percentage
defoliation by examining large numbers of individual leaves to determine the approximate
amount eaten. Neither method was completely satisfactory. Aseptic techniques were
not used in the collection of larvae, and all larvae for each sample were placed in the same
container, so that contamination after collection probably occurred. The foliage samples
were not collected until mid-August, and there was some evidence that the petioles of
the more severely damaged leaves had fallen by then. Consequently, the larval rearings
may have tended to overestimate rates of infection, while the defoliation may have been
underestimated. A further complication was the presence of naturally occurring virus.