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MORTALITY OF LARVAE IN STOCKED COHORTS OF THE DOUGLAS-FIR TUSSOCK MOTH, ORGYIA PSEUDOTSUGATA (LEPIDOPTERA: LYMANTRIIDAE)

  • R. R. Mason (a1) and T. R. Torgersen (a1)

Abstract

Cohorts of instar I larvae of the Douglas-fir tussock moth, Orgyia pseudotsugata, were stocked on branches of white fir, Abies concolor. Branches were either exposed naturally over drop-trays or protected with cages of nylon netting. Only 8.7% of the larvae in exposed cohorts survived to pupate, compared with 68.0% survival in caged cohorts where dispersal and predation were prevented. Losses from the exposed cohorts were caused mostly by arthropod predation, disappearance, and non-replaced dispersal. Disappearance of early larvae was attributed primarily to predation by spiders and insects while disappearance of late larvae was suspected to be due mostly to bird predation. In an analysis of k-values, actual or suspected predation accounted for 47.2% and dispersal 40.5% of the total loss.

Des cohortes de larves du premier stade de la chenille à houppes du Douglas, Orgyia pseudotsugata, ont été placées sur des branches de sapin concolore, Abies concolor. Les branches ont soit été exposées naturellement au-dessus de plateaux collecteurs, soit été protégées par des cages de mousseline de nylon. Seulement 8.7% des larves exposées ont survécu jusqu'à la pupaison, alors que l'incidence de survie était de 68.0% pour les cohortes dont la prédation et la dispersion ont été empêchées. Les pertes encourues par les cohortes exposées étaient dues principalement à la prédation par arthropodes, leur disparition, ou leur dispersion sans remplacement. La disparition des jeunes larves a été attribuée principalement à la prédation par les araignées et les insectes, alors que la disparition des larves âgées a été présumée être due surtout à la prédation par les oiseaux. Une analyse des facteurs K a montré que la prédation vérifiée ou présumée a compté pour 47.2%, et la dispersion pour 40.5% des pertes totales.

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MORTALITY OF LARVAE IN STOCKED COHORTS OF THE DOUGLAS-FIR TUSSOCK MOTH, ORGYIA PSEUDOTSUGATA (LEPIDOPTERA: LYMANTRIIDAE)

  • R. R. Mason (a1) and T. R. Torgersen (a1)

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