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i'risliphora erichsonii (Hartig). 377
outbreak proportions late in the decade 1971-1980 clearly indicates that the shrews
were not able to control the insect. However, as Warren (1971) indicated, "the shrew has
been a valuable predator of larch sawfly, and, despite some evidence of reduced
parasitism, there has been an important net gain in cocoons destroyed". Ives (1976)
showed that mortality during the cocoon and adult stages was largely responsible for
determining population trends. Small mammal predation is often a major component of
this mortality, thus agreeing with Warren's assessment of the effect of the masked shrew
in Newfoundland.
Although there is some indication that larch sawfly popUlations in and around the Pine
Falls release site may now be on the increase, there has been a period of several years
when this insect was extremely difficult to find. It was probably not as rare as in Europe,
but it was much less common than it has been in recent memory. Two recent papers
illustrate just how rare the larch sawfly can be in alpine areas of Europe. Lovis (1975)
collected only six larch sawfly (out of a total of 2816 larch insects) during a 3-year study
of insects attacking native European larch in Switzerland. Auer (1971) studied the
population dynamics of the larch bud moth, Zeiraphera diniana Gn., in the Upper
Engadine Valley in Switzerland between 1949 and 1968. He presented population curves
for eight larch insects for the period 1952 to 1968. The larch sawfly was apparently too
rare to be included.
The infestation patterns of the larch sawfly have been discussed by Turnock (1972).
He did not consider the effects of O. benefactor, but with slight changes his discussion is
still valid. He recognized three types of life systems: I) the stable latent type with a
diverse environment and a rich parasitoid complex, characteristic of infestations on larch
in alpine Europe; 2) the stable permanent type with a paucity of specific parasitoids,
characteristic of many North American outbreaks before 1920; and 3) the temporary type
with an intermediate parasitoid complex which includes one (or two) very effective larval
parasitoid(s). characteristic of European plantings. infestations on western larch in
British Columbia and on tamarack in the rest of Canada following the first successful
introduction of an ichneumonid parasitoid between 1910 and 1913 (and in southeastern
Manitoba following the successful introduction of O. benefaclor there in 1961). As this
discussion of life systems illustrates, entomologists have been able to achieve at least
partial biological control ofthe larch sawfly in North America. To continue this control, it
may be necessary to seek ways in which biotic agents in the environment can be manipulated.
Turnock & Muldrew (1971b) and Turnock et al. (1976) recognized four ways in
which insect parasites or predators might be manipulated in order to help achieve
biological control: 1) colonization releases; 2) inoculative releases; 3) inundative releases;
and 4) enhancement of biotic agents by environmental manipulation. In the case of the
larch sawfly, the last three approaches offer the most promise. Because of the presence
of M. dimidiatus, any releases of O. benefactor would need to be relatively large if they
were to be effective. Releases intermediate in size between inoculative and inundative.
say 2 000-3000 mated females, should be able to control infestations in plantations in 2
or 3 years, if released when defoliation first becomes noticeable. However, this approach
would necessitate a permanent facility capable of producing parasitoids on demand.
Reliance on field populations would be too uncertain, especially as M. dimidiatus will
almost certainly reduce the numbers of O. benefactor adults obtainable. The Bavarian
strain of Mesoleius tenthredinis could probably be managed in a similar manner. Both
species might even be able to sustain themselves in the field, and thus afford more or less
permanent protection. In Manitoba O. benefactor appeared to suppress M. tenthredinis
by sheer numbers. but this seems unlikely to happen again.
Enhancement of environmental conditions might be a particularly effective method
for increasing the effectiveness of avian and small mammalian predators in plantations.
Buckner (1971) discussed this situation as it related to forest insects in general. Birds
might be encouraged to nest in plantations by providing suitable nesting sites. Bruns