The larch sawfly, Pristiphora erichronii (Htg.), is currently considered a major forest insect pest in Canada. At the present time within Canada, the sawfly reacts to parasitism by Mesoleius tenthredinis Morley in two ways. In Manitoba and Saslratchewan the sawfly encapsulates approximately 100 per cent of the parasite eggs deposited, whereas in British Columbia encapsulation rarely exceeds four per cent (Muldrew, 1953). The reasons for the difference in degree of encapsulation are apparently unknown; however, since the origin of the sawfly itself is obscure, the possibility exists that a native species, an introduced species, Or a combination of both may he present, or that geographical or ecological units may have arisen. Studies now under way by officers of the Forest Biology and Entomology divisions are attacking the problem of identity and origin following the pattern established for the European spruce sawfly, Diprion hercyniae (Htg.). In this instance, as with the larch sawfly, parasites were introduced on the assumption that the pest had been introduced from Europe. Critical investigations by Reeks (1941) and Balch, Reeks, and Smith (1941), involving morphological, cytological, and other biological characters, showed that the species occurring in North America was one of two species common in Europe, and previously referred to there as Gilpinia polytoma (Htg.). Balch et al. (1941) showed that D. hercyniae had been introduced into North America.

The study of populations and population trends of a forest insect embraces a wide field of activity. By implication, it includes a consideration of all mortality and survival factors. Therefore, in developing this portion of the symposium, it will be necessary to refer briefly to other related topics to be covered later. In addition, to obtain a proper understanding of the subject under discussion, the mode of operation of some population influences that will not be introduced by other members of the symposium will have to be described.

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.

The greatest predatory effect of small mammals is exerted upon insects that spend a portion of their life cycle on the ground or in the soil. Sawflies afford ample opportunity for mammalian predation, since they drop to the ground as mature larvae and spin cocoons in the soil. Such sawflies as Neodiprion abietis Harr., which remain within the cocoon for about three weeks, undergo only moderate risk of being preyed upon by small mammal. However, Pristiphora erichsonii (Htg.) remains within the cocoon from about mid-August until the following June, or even over more than one year, thus greatly extending the vulnerable period. Earlier investigations suggest that small mammals may comprise the largest single biological control agent acting against this insect (Graham 1928, Lejeune 1951), but the exact role of mammalian predators of forest insects has yet to be established.

Although a virus disease of the larch sawfly, Pristiphora erichsonii (Htg.), has not been discovered, polyhedrosis viruses of several other Tenthredinids are known. They have been used to control infestations of two introduced species: the European spruce sawfly, Diprion hercyniae (Htg.), (Bird, 1954) and the European pine sawfly, Neodiprion sertifer (Geoffr.), (Bird, 1950, 1952, 1953; Dowden, 1953). Unfortunately, the viruses of these, as well as the viruses of several other species tested, are not pathogenic to the larch sawfly. Some viruses, however, appear to he pathogenic to more than one species. J. M. Burk of this laboratory found, for example, that a polyhedrosis virus affecting the native jackpine sawfly, Neudiprion americanus banksianae Roh., is pathogenic to N. sertifer, N. nanulus Schedl, and D. hercyniae.

Fungi and bacteria each play a role in the natural control of many sawflies, including P. erichsonii. Parasitic fungi, for example, have been isolated from species in the following genera: Anoplonyx, Arge, Monoctenus, Neodiprion, Trichiosoma, Hemichroa, Pikonema, and Pristiphora. The isolates include species from Cephalosporium, Sorosporella, Isaria, Beauveria, Spicaria, Hirsutella, and Empusa (MacLeod, unpub.). The last five fungus genera have been isolated from larch sawfly collections made in various parts of Canada and the United States.

The larch sawfly, Pristiphora erichsonii (Htg.), like the European spruce sawfly, Diprion (Gilpinia) hercyniae (Htg.), is an obligatoty parthenogenetic ‘species’, that is, it is endowed with the capacity of reproducing without recourse to mating. Wlde this absence of cross-fertilization with the attendant uniformity of progeny it assures renders such species ideally suited for experiments requiring the use of controls, it would at first sight appear ro preclude the adoption of conventional genetic procedures in studying them, other than by circuitous means: it is the main purpose of this contribution to suggest a possible means of circumventing the difficulty.

We have had some good illusuations of different approaches to the study of forest insects. The subject has by no means been exhausted-the physiological approach, for instance, has been barely touched on—but I think the point has been brought out that there is no essential difference between the problems and methods of forest entomology and, say, agricultural entomology. There may be some differences in emphasis–the nature of the forest crop encourages the long-term ecological approach–but even this difference seems to be disappearing, as may he illustrated by Dr. Glen's recent paper on factors governing the abundance of insects.