THE DYNAMICS OF LARCH SAWFLY (HYMENOPTERA: TENTHREDINIDAE) POPULATIONS IN SOUTHEASTERN MANITOBA

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Abstract
Life-table study plots were established in southeastern Manitoba to study the population dynamics of the larch sawfly for the 1956 to 1972 generations and to evaluate the impact of two recently introduced parasites.Abbreviated life tables were used to calculate k-values, expressions of mortality during the different stages due to various factors. Graphical and principal component analyses of the k-values showed that mortality during the cocoon and adult stages (k5) largely determined population trends. None of the factors affecting populations before the introduction of the parasite Olesicampe benefactor Hinz was density-dependent. However, a form of density dependence appeared to exist for O. benefactor and for its hyperparasite Mesochorus dimidiatus Holmgren. The area of discovery for both species decreased rapidly as the density of the female parasites increased.A simple model confirmed that k5 was largely responsible for determining population trends. A second model utilizing the relationships between areas of discovery and adult female densities for O. benefactor and M. dimidiatus provided reasonable approximations to observed values for numbers of sawfly eggs, and numbers of adults, rates of attack, and attacks per female for both species of parasites.Twenty sets of 50 pairs of randomly generated values of k5 were used as simulated input in the second model. None of the values for simulated egg populations of the larch sawfly reached outbreak proportions when O. benefactor and M. dimidiatus were present, but most exceeded these levels in their absence. Larch sawfly populations in southeastern Manitoba should be controlled by O. benefactor in the foreseeable future.

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