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258 J. C. Cunningham and G. M. Howse
Recommendations
use will probably be restricted to high value stands and environmentally sensitive areas. If it
can be demonstrated conclusively that treatment with a virus gives foliage protection for
several years following the year of application, then the high initial cost of the treatment
becomes more acceptable.
The application of contaminated virus preparations in 1971 was accidental. The
presence of a CPV contaminant in the NPV used in 1979 and 1980 was known. but the
material was still applied on the basis of the 1971 results. The minute amounts of CPV
gave dramatic infection levels in the spruce budworm population that were often higher
than the levels of NPV. This was surprising because the ratios of NPV:CPV were 400: 1.
300: 1. and 178: 1. It appears that there is a synergistic effect between NPV and CPV. but
this would require detailed laboratory investigations to prove conclusively.
Foliage protection in the year of application was demonstrated in 1979 following treatment
with NPV plus CPV on second-instar larvae as they emerged from hibernacula. Treatment
of second-instar larvae has many benefits. although timing is extremely critical and
it would only be practical to treat small. carefully monitored areas. Second-instar larvae
are much more susceptible to infection with NPV than later instars and the LD50 in terms
of numbers of PIB has been calculated for different instars as follows: second. 25; third.
204; fourth. 462; fifth, 2514; and sixth, 3 643 (F.T. Bird 1978, unpublished data). At the
time of emergence from hibernacula. leaves on the hardwood overstorey. frequently
found in spruce-fir stands, have not flushed and a good aerial spray deposit can be
obtained on the target species.
To date, the tank mix considered most suitable for spruce budworm virus applications
was an aqueous suspension containing 25% molasses, 6% Shade® and 1 % Chevron®
sticker. It has been widely used with the NPVs of both Douglas-fir tussock moth. Orgyia
pseudotsugata (McDunnough), and gypsy moth, Lyman/ria dispar (L.). Since Shade® is
no longer commercially available, it will be necessary to find a new screening agent
against ultra-violet light or a totally new formulation.
Safety tests on spruce budworm NPV conducted at Ontario Veterinary College
showed that this virus presents no hazard to birds and mammals (Valli et al. 1976). Fish
tests, although indicating no hazard. were inconclusive and should be repeated (Savan
et al. 1979). There are only limited safety testing data available for EPV (Buckner &
Cunningham 1972). and no CPV and GV safety testing data are available. These would
be required before any of these three .. iruses could be registered by Canadian authorities.
The use of any of the four viruses with inclusion bodies that infect spruce budworm is still at
the experimental stage. In the near future only treatment of small areas can be considered.
Applications of NPV or NPV plus CPV on second-instar larvae as they emerge from
hibemacula should be retested in an effort to find if. under closely monitored conditions.
this is a feasible and effective strategy.
An improved tank mix that will keep the virus in a viable state on the foliage for more than
a week would greatly increase the effectiveness of treatments and possibly allow lower
dosages. Research on formulations is being conducted by both commercial and government
agencies in the United States and formulations developed for other pathogens. such as
bacteria and fungi. will probably have the characteristics suitable for use with viruses.
The search for new viruses or strains of viruses that infect spruce budworm should be
continued. There are several isolates of NPV • GV, and EPV from different Choristoneura
species, but none has been found that is more virulent than the strains used in field trials.
The search for larger. easily reared host larva for spruce budworm virus production has also
proved unsuccessful. but should be continued.
A long-term goal in the use of viruses for the regulation of spruce budworm population is
to study the nucleic acid of spruce budworm NPV with a view to constructing a genetic