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

Published online by Cambridge University Press:  31 May 2012

W. G. H. Ives
W. G. H. Ives
Affiliation:
Northern Forest Research Centre, Canadian Forestry Service, Edmonton, Alberta
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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.

Résumé

L’auteur établit dans le sud-est du Manitoba des parcelles-échantillons de mortalité pour étudier le dynamisme des populations de la Tenthrède du Mélèze pour les générations de 1956 à 1972, et évaluer l’impact de deux parasites introduit récemment.

Les valeurs k furent calculées avec des tables de mortalité abrégées; les valeurs k représentent la mortalité des Tenthrèdes pendant différents stades, attribuée à divers facteurs. L’analyse graphique et factorielle des valeurs k démontra que c’est la mortalité pendant les étapes de pupe et adultes (k5) qui détermine largement les tendances des populations. Aucun des facteurs affectant les populations avant l’introduction du parasite Olesicampe benefactor Hinz n’a influencé la densité. Cependant il semblait que l’hyper-parasite Mesochorus dimidiatus Holmgren influençait de quelque manière la densité de O. benefactor. Le nombre de présences signalé de ces deux espèces parasitaires a décru rapidement à mesure que la densité des femelles parasitaires augmentait.

Un simple tableau-modèle de mortalité appuie l’hypothèse que la valeur de k5 était largement responsable des tendances des populations de la Tenthrède. Un deuxième tableau-modèle, basé sur les rapports entre le nombre d’aires de découvertes et les densités des femelles adultes de parasites O. benefactor et M. dimidiatus a donné des corrélations raisonablement équivalentes aux valeurs observées du nombre d’oeufs de la Tenthrède, du nombre d’adultes, des taux d’infestations et du nombre d’attaques par femelle pour chaque espèce parasitaire.

Ont été utilisées comme données d’entrées simulées du deuxième tableau-modèle, 20 séries de 50 paires de valeurs de k5 choisies au hasard. Selon le modèle aucune des valeurs simulées des populations d’oeufs de la Tenthrède du Mélèze n’ont atteint des proportions épidémiques lors de la présence de O. benefactor et M. dimidiatus, mais la plupart les ont dépassées lors de leur absence. L’auteur conclut que le parasite O. benefactor devrait dorénavant être utilisé dans la lutte contre les infestations des Tenthrèdes du Mélèze dans le sud-est du Manitoba.

Type
Articles
Information
The Canadian Entomologist , Volume 108 , Issue 7 , July 1976 , pp. 701 - 730
Copyright
Copyright © Entomological Society of Canada 1976

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