Parasites and Insect Predators of the Larch Sawfly

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Abstract
Before the introduction of the parasite Mesoleius tenthredinis Morley into Canada in 1910, the larch sawfly had periodically caused devastating outbreaks that eventually resulted in the destruction of nearly all the mature larch east of the Rockies. The parasite at first gave control wherever it became established and, fallowing its release in Central Canada, the larch sawfly caused no serious damage there until 1938, when it again began to flare up, first in central Manitoba and then in Saskatchewan, Alberta, Ontario, Minnesota, and Wisconsin. Associated with the present resurgence is a loss of effectiveness on the part of M. tenthredinis and this appears to be related to the development of an immunity reaction on the part of the host (Muldrew, 1953a). The larch sawfly in British Columbia is, however, still highly susceptible to the parasite and, apparently, parasitism by N. tenthredinis keeps the level of abundance of the larch sawfly so low that at difficulty is experienced in obtaining host material for study purposes.