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226 M. A. Hulme and G. W. Green
production of biological control agents is seen as a less attractive commercial proposition,
and it is thus important that government organizations maintain an interest in providing
opportunities for large-scale production of biological control agents.
Economic aspects of various biological control strategies have been briefly mentioned
in this chapter. These aspects need addressing in more detail to supplement the few cost
estimates developed here by biologists. Analysis by economists could provide a more
comprehensive picture of costs and benefits: cost figures should separate production
from application; and benefit figures should include carry-over effects following the
year of application.
Target pests for control
Judging by progress reported in this and other reviews of biological control there seems
little reason to rule out any forest pest on a priori grounds as a suitable candidate for
biological control. Each decision to attempt control must, of course, take into account
the specific situation of pest and host tree(s) and the range of control options that might
be applied. However, in considering biological control there seems good reason to give
more attention to research opportunities with the key economic pests in Canada's
forests, which would include C. fumiferana in eastern Canada and D. ponderosae and D.
ru/ipennis (Kby.) in western Canada
In the case of C. fumiferana, a good short-term option using biological control is now
available commercially. Longer term control is desirable but biological control attempts
have met with limited success. There is, of course, scope for further work and, although
all attempts will be difficult, we suggest that searches for more virulent viruses and for
exotic parasitoids be given special emphasis. Candidate viruses to date exhibit marginal
virulence against C. fumiferana but some control has nevertheless been obtained and
further work is warranted. Parasitoid introductions as a biological control option have
been given little more than a cursory examination, perhaps because C. fumiferana is a
native pest. Many of the releases were conducted in unsuitable conditions and a more
systematic study of this control option would allow a more factual comparison of its
potential with other control options. The most common candidate hosts in Europe that
have been suggested for parasitoid collections, based on the similarity of their life cycles
to C. fumiferana, are C. murinana Hb. and Epinolia nigricana L. on Abies spp., Dichelia
histrionana Froel. on Picea spp., and Archips oporana L. on Larix spp., Pinus spp., and
Picea spp. Several Zeiraphera spp. offer more remote possibilities. Collection activities
should not, however, be confined to Europe. China, for example, has a Choristoneura sp.
that attacks Picea spp. and appears to be well controlled by natural factors (Macdonald &
Pollard 1976).
With Dendroctonus spp. of bark beetles, particularly D. ponderosae and D. ru/ipennis
that are major pests in western Canada perhaps the whole range of biological control
agents needs closer examination to see which, if any, have practical potential for control.
Certainly a wealth of information exists on related scolytids such as the southern pine
bark beetle, D. frontalis Zimmerman (Thatcher et al. 1978), some of which could provide
a useful starting point in considering options with other Dendroctonus spp. More field
examinations of the natural enemy complex of Dendroctonus spp. in Canada is obviously
required. Multiphagous parasitoids of bark beetles that attack hardwoods should not be
overlooked, as some also parasitize bark beetles that attack softwoods.
Other particularly destructive pests that are not included in this review are the
hemlock looper, L. [lScellaria[lScellaria, and the western hemlock looper, L. [lScellaria
lugubrosa (Hulst); populations are now at endemic levels and irruptions are likely within
the next few years. For the present, however, the most logical approach is to give priority
to biological control research, mainly in accordance with the current importance of the
pest, as the scope of research opportunities is limited not by lack of suitable problems but
by the resources we are able to devote to biological control. There seems little doubt that