THE NATURAL IMMUNITY OF THE LARCH SAWFLY (PRISTIPHORA ERICHSONII (HTG.)) TO THE INTRODUCED PARASITE MESOLEIUS TENTHREDINIS MORLEY, IN MANITOBA AND SASKATCHEWAN

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Abstract
By 1945 it was suspected that the larch sawfly in Saskatchewan and Manitoba had developed an immunity to the introduced parasite Mesoleius tenthredinis Morley. Results of subsequent studies showed that embryonic development of M. tenthredinis in sawfly larvae from Manitoba and Saskatchewan was inhibited three to four days after oviposition. No such inhibition occurred in larvae from British Columbia where the parasite is still highly effective. Inhibition seemed to be related to the deposition of phagocytic capsules around parasite embryos, which occurred in host larvae from Manitoba and Saskatchewan but not in larvae from British Columbia. Encapsulated parasite embryos were shown to be viable, for a number of them hatched after being placed in Ringer's solution. Viability decreased with age. Some embryos were viable seven months after oviposition, though the normal incubation period is 7 to 10 days. Unencapsulated embryos developed more rapidly than encapsulated embryos in Ringer's solution. Results obtained indicate that the phagocytes of the larch sawfly from Manitoba and Saskatchewan play an important role in the immunity reaction of this insect to M. tenthredinis.

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