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366 J. C. Cunningham and R. F. Shepherd
Evaluation of Control Attempts
Recommendations
Some of the treatments were designated operational and others experimental.
Twenty-one days after spraying, larval mortality due to the B.t. treatments averaged
34% with a high of 57% with Thuricide® and a low of 13% with Dipel®. A double
application of both materials and a higher volume of Thuricide® gave about 20% more
control than single applications or the lower volume. Increasing the dosage of Dipel®
gave no significant change, but using molasses in the tank mix instead of sorbitol gave a
significant increase in the effectiveness of Dipel® and brought it to the same level as
Thuricide® .
Neither single nor double applications of Dipel® provided adequate foliage protection-
59% defoliation was recorded on untreated check plots and 50-58% on Dipel®-treated
plots. Better foliage protection was recorded with Thuricide 81 with only 18-25% defoliation
on treated trees.
The various operational and experimental treatments with B.t. gave a wide range of
results. However, the best population reductions due to treatment were less than 60%.
This did not provide adequate foliage protection and did not prevent the Douglas-fir
tussock moth population increasing to high densities in the next generation.
Results with B.t. on Douglas-fir tussock moth in 1975 were considered unsatisfactory
and applications of B.t. on other defoliating lepidopterous forest pests during the last
decade in British Columbia have generally proved disappointing. Recent improvements
in application technology of B.t. have resulted in improved control of spruce budworm,
Choristoneurafumiferana (Oem.), and similar improvements may eventually be possible
with Douglas-fir tussock moth. On the other hand, results with NPV are already most
encouraging and it appears that this biological control agent can now provide a useful
tool for the regulation of Douglas-fir tussock moth populations.
Douglas-fir tussock moth MNPV was registered by the Environmental Protection
Agency in the United States in 1975 under the name TM Biocontrol-1. It is proposed to
apply for Canadian registration in 1982. At present this virus is not commercially
produced and this poses two major problems that need solving: a source of supply; and
production at a reasonable cost. Small amounts, sufficient to treat about 400 ha annually,
are produced at the Forest Pest Management Institute, Sault Ste. Marie, Ontario, and
larger amounts are produced for use in the United States by staff of the U.S. Forest
Service, Corvallis, Oregon. A figure of $40 US per hectare was quoted for production at
Corvallis in late 1980 (M.E. Martignoni personal communication) and this figure is
probably even higher for material from Sault Ste. Marie. This cost figure is based on a
dosage of 250 x 10' PIBlha. This dosage can probably be reduced to 125 x 10 9 PIBlha
(I1nytzky et al. 1977), and possibly even lower.
The mountainous terrain and consistently low relative humidity encountered in the
interior of British Columbia pose problems for aerial application of pest control agents
and particularly for application of aqueous spray formulations. Use of an oil-based
formulation should be investigated, for this may well enhance the deposit and facilitate
application of lower dosages.
Ideally, application of a virus initiates an epizootic in the pest insect population,
regulating the pest either in the year of application if applied on early-instar larvae, or in
the subsequent year. Some NPVs have the potential to initiate epizootics and others
have not; the status of Douglas-fir tussock moth NPV has not been established and the