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Protozoa
E. Review of biological control opportunities 275
Protozoan parasitism of agricultural pests is commonplace and is reported to reduce
population vigour. Many species occur in forest defoliator insects, including two species
of microsporidia in spruce budworm. However, in pest control history the applied use of
protozoa has been rare, because they are costly to propagate in quantity, and exhibit low
infectiveness after artificial distribution.
Canadian research on microsporidian parasites of spruce budworm was restricted to
the Forest Pest Management Institute in the 1970s, which documented their natural
incidence and parasitic role. Spray experiments on single trees demonstrated that
parasitism rates can be artificially raised, but left the problem of mass production
unsolved. Operational application is not yet practicable.
Parasitoids
Canada has enjoyed some success in the importation of exotic parasitoids to control
introduced pest insects in both agriculture and forestry, but the record of releases to
control native forest pests has been discouraging (Munroe 1971). However, it is not
known whether the low success rate in the control of native hosts is associated with the
separate evolutions of host and parasitoid, or stems from imperfect testing techniques.
In agriculture, inundative release of a few species of natural enemies is routine practice,
but experimentation against forest pests has been fragmentary and unsustained (Pschom­
Walcher 1977).
Canadian research on manipulation of spruce budworm parasitism was scant in the
1970s, but was on a sounder base than the hit-and-miss releases of the 1940s and 1950s.
Research was on field-cage studies of host and site compatibility with exotic parasitoids
and studies of the numerical response of native parasitoids, with the inundative release
method in mind.
The case for increasing the Canadian effort with respect to spruce budworm parasitism
is not strong if the probability of establishing additional natural enemies decreases with
increasing diversity of the natural enemy complement. There is a need for better
theoretical guidelines in order to achieve maximum success. The empirical approach -
to collect homologous parasitoids abroad, then release and monitor their distribution
and persistence - is unpredictable and does not increase understanding of the processes
unless accompanied by studies of host-parasitoid compatibility.
Predators
In world agriculture, some striking successes have been achieved by inoculative release
of exotic predaceous arthropods, but in Canadian forestry few such releases have been
attempted (Munroe 1971). In general, the release of an exotic broad-spectrum predator
gives rise to more concern about ecological integrity than the introduction of hostspecific
parasitoids. In Canada, the only releases of non-native arthropod predators of
forest pests in 1969-80 were the successful introductions of two red wood ant species
from Italy and Manitoba to Quebec (Finnegan 1978); the experiments show encouraging
results in the control of spruce budworm populations in young stands. However, in
general. the technical and economic feasibilities of predator releases against spruce
budworm have not been assessed.
Genetic approaches
Pest insect genetics is an important means of biological control, because it leads to
autocidal methods for manipulating popUlations. The technique of releasing sterile