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224 M. A. Hulme and G. W. Green
ability, and good synchronization of reproduction with the host (Pschorn-Walcher
1977). Inbreeding that restricts genetic variability should be limited. It is always advisable to
try to ensure genetic variation between released individuals by collecting breeding stock
from several locations (Bennett 1974, Mackauer 1976).
Selection of collecting locations for exotic parasitoids and predators has rightly been
focused on the native habitat of the pest when introduced insects are the subject of
control attempts and this has inevitably led to a concentration of collection efforts in
Europe. Case histories, however. show that successful control agents have also been
found on different hosts and in different parts of the world. and to allow for this
possibility. and thus increase the probability of success, a proportion of effort should be
devoted to collecting from areas other than the native habitat of the pest (Pschorn­
Walcher 1977). This approach is, of course. essential where native pests are the subject
of control attempts by inoculative releases.
Although the above remarks are aimed principally at so-called classical biological
control. they are not intended to preclude attention to other release strategies. Augmentative
or inundative releases of parasitoids or predators should perhaps be considered
in some cases. These techniques remain essentially untested in Canadian forestry
applications despite successes claimed in Russia and China (Tropin et 01. 1980, McFadden
et al. 1981). For example, in Russia release of Trichogromma evanescens Westw. against
eggs of R. buoliana and Petrona resine/la (L.) reduced the number of damaged pine buds
by over 50% (Beglyarov & Smetnik 1977); and in China Trichogrommo dendrolimi Mats.
reduced infestation by Dendrolimus sibericus Tschetw. from 63% of trees in 1956 to 1 %
of trees in 1970 (Hussey 1977). Attention could also be given to the use of behavioural
chemicals such as kairomones (e.g. Gross Jr. 1981, Weseloh 1981, Vinson 1977); these
semiochemicals should improve control by augmenting the ability of natural enemies to
locate hosts.
Finally, two fundamental research needs in predator and parasitoid introductions
have been raised repeatedly in this overview. The first need covers rearing techniques to
ensure that adequate numbers of biological control organisms (mated females) are available
for whatever release strategy is employed. The second fundamental need is for better
knowledge of the population dynamics of the pest insect and of the candidate predators
or parasitoids. Many feel that neglecting this aspect in favour of an ad hoc approach
inevitably reduces chances of obtaining successful control.It is of course recognized. as
Pschorn-Walcher (1977) states, that "predictions derived from multivariate analysis or
population models are no guarantee that the biological control agents selected will be
effective." Judgement must be exercised in deciding how much background information
is desirable before releases are attempted. As Simmonds (1972) points out. "The introduction
... is the crucial experiment. Promising biological control agents have failed to
live up to expectations whereas apparently unlikely species have been very successful".
Entomopathogens as control agents
Bacteria and viruses have dominated control successes with entomopathogens and will
probably continue to do so in the immediate future as improved methods are found for
producing and applying these organisms. With B.t .• for example, efficacy against C.
fumi/erona has been widely demonstrated and future research emphasis will be on
reducing costs. This can be approached in a number of ways. Development of more
concentrated formulations to reduce the volume of carrier liquid will reduce both shipping
and application costs. Development of even more potent strains of B.t. will lead to similar
reductions in costs. Finally, development of better application methods will increase the
probability that larvae will ingest a lethal dose from a given quantity of applied B.t. by
optimising the distribution of B.t. on the leaf surface. These methods entail all aspects of
application technology: generating optimum-sized droplets that contain the optimum