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360 D. G. Embree. D. E. Elgee and G. F. Estabrooks
Field Trials
Evaluation of Control Attempts
No attempt has been made to control this insect through the introduction of additional
parasitoid species. The emphasis has been on establishing control through the manipulation
of the virus (Cunningham 1972, Elgee 1975). Virus suspensions can be used as
insecticides and have been tested successfully on small trees.
A major attempt at biological control was made in 1975 to initiate an epizootic in a large
population of the whitemarked tussock moth in the hope of shortening the length of an
outbreak; it has been reported by Kurstak (in press). During the previous year the
suspected start of a general outbreak of the whitemarked tussock moth was detected in an
IS-ha stand of balsam fir near Castlereagh, Colchester County, Nova Scotia. A virus
suspension was prepared from third-instar larvae reared from eggs collected at the site
and infected with the Nova Scotia strain of NPV: the virus had been saved and stored
from the last outbreak in 1965.
On the evening of 9 July 1975, 204 I of an NPV suspension containing IO.S x HY'
polyhedral inclusion bodies (PIB) per millilitre mixed with 204 I of a virus spray adjuvant
manufactured by Sandoz Inc. and a fluorescent dye were sprayed from 30 m on IS ha of
mature balsam fir with a Cessna Ag-truck aircraft equipped with a boom and nozzle
sprayer. Whitemarked tussock moth larvae were mainly first instars with some early
second instars.
There were no true controls (untreated areas) but larvae collected before the spray
application and reared were free from disease. Twenty-four sample trees were established
in a line bisecting the spray block. However, one edge of the block was not sprayed so
that trees 20-24 were outside the block. No disease was detected in larvae collected on
tree 24 and only light mortality (33%) occurred on trees 20-23. Mortality of larvae
collected 1 day after spraying from trees 1-19, and reared on foliage from the sprayed
area, averaged 84%, whereas that of survivors collected 2 weeks later was 64%. Field
populations declined by 95%, as compared to 67% on the unsprayed trees, and defoliation
was light. From the air, the outline of the sprayed area was clearly discernible in a
severe outbreak that developed around the initial Castlereagh infestation. No appreciable
population reappeared in the sprayed area. The outbreak in the surrounding areas intensified
in 1976, covering a radius of 16 km, but collapsed from the virus disease in all but
the periphery during the same year.
Early in 1976, virus suspensions were dispersed at two locations along the periphery
of the 1975 outbreak using an explosive device. Canisters containing 1.9 I of suspension
were fired into the air using a home-made mortar and exploded over the forest canopy.
Virus particles were dispersed over an area of 1000-2000 m l depending on wind velocity.
However, by this time the virus was already present in the population. Eventually the
outbreak spread throughout the province, collapsing everywhere by 1975.
The experiment showed that the virus can be used as an effective biological insecticide.
but whether it can be used to initiate a widespread epizootic is uncertain. The rapid
spread of the O. ieucosligma infestation in 1975 resulted from dispersion of newly hatched
larvae, most of which occurred before spraying took place. Moreover. low populations
of larvae existed in the area immediately surrounding the initial 18 ha of detected outbreak.
The outbreak in 1976 was widespread and extremely intense; even though it collapsed
just 2 years after it appeared, it is unlikely that the source of the epizootic was the IS-ha
spray block. Had the initial outbreak been detected in 1973 and sprayed in 1974 a clearer