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264 G. G. Wilson, D. Tyrrell and T. J. Ennis
Recommendations
schubergi spores resulted in infection rates of 64.8% for white spruce and 96.3% for
balsam fir 19 days after spraying. There was no infection in larvae from the check area
and no carry-over of the microsporidium to the following year. All samples showed
higher levels of infection in spruce budworm taken from balsam fir.
Based on laboratory results there is a relationship between levels of infection and
mortality. As an example, spore concentrations of 10" spores/ml fed to second-instar
larvae resulted in 100% mortality in up to 24 days, with 50% mortality occurring about 14
days after infection (balsam fir buds were dipped in the spore suspension). As a
comparison, spore concentration of 100/ml resulted in 61 % mortality over a period of
6-28 days. For fifth-instar larvae Hr spores/ml gave 75% mortality over 8-33 days,
and 10" sporeslml resulted in only 5.6% mortality, which was similar to the controls.
Fungi
E. egressa does not sporulate well on artificial media and conidia are normally obtained
from infected insect cadavers. These are easily produced by injection of insect larvae
with protoplasts, which have been shown to be part of the natural life cycle of this fungus
(Tyrrell 1977). Transmission of E. egressa from laboratory-infected to natural populations
of spruce budworm under field conditions using conidia produced in this manner has
been demonstrated (Lim el al. unpublished).
Genetic manipulation
Topical application of thiotepa or 250-300 Sv of Co'" irradiation induced complete
sterility in male spruce budworm (Retnakaran 1970, 1971). Males sterilized with thiotepa
remained fully competitive and could suppress fertility of caged laboratory populations
by up to 90% (Ennis unpublished data). However, release of chemosterilized males into
caged field populations produced no detectable reduction in popUlation, mainly because
of adverse effects on adult male mating behaviour. High level irradiation of male pupae
or adults reduces fitness and mating competitiveness, militating against their effective
use. However, at 20-50 Sv, induced chromosome aberrations have a heritable effect on
fertility of subsequent generations, with depression of fertility persisting for up to three
generations in the laboratory (Ennis 19790). The potential of inherited semi-sterility has
not yet been tested in field populations. Dispersal and behaviour of irradiated males
have also been studied using pheromone re-trapping techniques (Ennis 1979b).
Few microsporidian parasites have been investigated as possible control agents of pest
insects. If we take the amount of research on Bacillus Ihuringiensis Berliner (B.I.)
before it was accepted as an insecticide as being representative in the development of a
biological control agent, then it is obvious that we have barely begun the task with
microsporidia. However, there are indications that many microsporidia do have good
potential as biological control agents that, given appropriate research and development,
may one day be realized. There is still much research needed on the development of
microsporidia in a control plan for the spruce budworm. Thorough studies are required
on the host-parasite relationships and the incidence and effects of the microsporidia in
their natural setting. Although these parasites can be introduced into a population of the
spruce budworm, their effect on insect numbers has not been quantified. The use of
these parasites in integrated control should be investigated, not only in conjunction with
chemicals, but also with other spruce budworm pathogens.