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Incidence and impact of Entomophaga aulicae (Zygomycetes: Entomophthorales) and a nucleopolyhedrovirus in an outbreak of the whitemarked tussock moth (Lepidoptera: Lymantriidae)

Published online by Cambridge University Press:  31 May 2012

K. van Frankenhuyzen ,
P. Ebling ,
G. Thurston ,
C. Lucarotti ,
T. Royama ,
R. Guscott ,
E. Georgeson  and
J. Silver
K. van Frankenhuyzen*
Affiliation:
Natural Resources Canada, Canadian Forest Service, Great Lakes Forestry Centre, 1219 Queen Street East, Sault Ste. Marie, Ontario, Canada P6A 2E5
P. Ebling
Affiliation:
Natural Resources Canada, Canadian Forest Service, Great Lakes Forestry Centre, 1219 Queen Street East, Sault Ste. Marie, Ontario, Canada P6A 2E5
G. Thurston
Affiliation:
Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, PO Box 4000, Fredericton, New Brunswick, Canada E3B 5P7
C. Lucarotti
Affiliation:
Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, PO Box 4000, Fredericton, New Brunswick, Canada E3B 5P7
T. Royama
Affiliation:
Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, PO Box 4000, Fredericton, New Brunswick, Canada E3B 5P7
R. Guscott
Affiliation:
Nova Scotia Department of Natural Resources, Integrated Pest Management Section, PO Box 130, Shubenacadie, Nova Scotia, Canada B0H 2H0
E. Georgeson
Affiliation:
Nova Scotia Department of Natural Resources, Integrated Pest Management Section, PO Box 130, Shubenacadie, Nova Scotia, Canada B0H 2H0
J. Silver
Affiliation:
University of Toronto, Division of Life Sciences, Scarborough, Ontario, Canada M1C 1A4
*
1Corresponding author (e-mail: kvanfran@nrcan.gc.ca).
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Abstract

In Nova Scotia, the whitemarked tussock moth, Orgyia leucostigma Fitch, periodically erupts in outbreaks that typically last 3–5 years. Population changes during a recent outbreak were monitored by means of aerial defoliation surveys and fall egg-mass surveys that were conducted between 1997 and 2001. Severe defoliation was first recorded on approximately 250 ha in 1996. The defoliated area increased rapidly to hundreds of thousands hectares in 1998, after which it sharply declined to about 4700 ha in 2000 and 0 ha in 2001. The total infested area [>0.01 egg masses per three branches of Abies balsamea L. (Pinaceae)] decreased from about 1.4 million ha in 1997 to about 13 500 ha in 2001. Between 1996 and 2001, the infestation involved a cumulative total of 2.4 million ha, covering most of the province. The collapse of larval populations during 1998 was associated with widespread prevalence of a singly embedded nucleopolyhedrovirus (NPV) and Entomophaga aulicae (Reichardt in Bail) Humber (Zygomycetes: Entomophthorales). Sampling of larval populations in late July and August 1998 revealed a widespread and virtually sympatric occurrence of those pathogens in areas that were under defoliation pressure, with infection levels by each pathogen exceeding 75% in many sample sites. Pathogen impacts on larval survival were studied in 1999 in a persisting pocket of severe infestation in Hants County. Larvae were collected every 3 d from balsam fir branch samples between 17 June and 21 July and reared to determine cause of death. The two pathogens together accounted for at least 50% of cohort mortality, calculated as marginal mortality rates according to Royama (2001). Although cohort mortality due to disease on balsam fir was significantly correlated with between-generation reduction in mean egg-mass density, overall pathogen-induced mortality was not high enough to drive the populations into an endemic state, and a moderate infestation persisted into 2000.

Résumé

En Nouvelle-Écosse, les infestations périodiques de chenilles à houppes blanches, Orgyia leucostigma Fitch, durent ordinairement de 3 à 5 ans. Les changements dans la population au cours d'une infestation récente ont été suivis de 1997 à 2001 par évaluation aérienne de la défoliation et par inventaire des masses d'oeufs en automne. La première défoliation importante a eu lieu en 1996 et recouvrait 250 ha. La zone défoliée a augmenté rapidement jusqu'à envahir des centaines de milliers d'hectares en 1998 pour diminuer par la suite jusqu'à environ 4700 ha en 2000 et 0 ha en 2001. La zone totale infestée [>0,01 masse d'oeufs par trois branches d'Abies balsamea L. (Pinaceae)] a diminué d'environ 1,4 million ha en 1997 à environ 13 500 ha en 2001. Entre 1996 et 2001, un total cumulatif de 2,4 millions ha ont été affectés, recouvrant presque la totalité de la province. L'effondrement des populations de chenilles en 1998 était associé à l'omniprésence d'un nucléopolyédrovirus (NPV) à enrobement simple et d'Entomophaga aulicae (Reichardt in Bail) Humber (Zygomycetes : Entomophthorales). L'échantillonnage des populations de chenilles à la fin de juillet et en août 1998 a révélé l'omniprésence de ces pathogènes pratiquement sympatriques dans les régions soumises à la pression de la défoliation et les infections par chacun de ces deux pathogènes affectaient plus de 75% des chenilles à plusieurs sites. L'impact des pathogènes sur la survie des chenilles a été étudié en 1999 dans une enclave où persistait une infestation grave et tenace, dans le comté de Hants. Des chenilles ont été prélevées tous les 3 jours sur des branches de sapin baumier entre le 17 juin et le 21 juillet et gardées en élevage pour déterminer les causes de leur mortalité. Les deux pathogènes combinés ont été responsables d'au moins 50% de la mortalité de la cohorte, calculée comme taux de mortalité marginale selon la méthode de Royama (2001). Bien que la mortalité de la cohorte sur les sapins baumiers attribuable à la maladie se soit avérée en corrélation significative avec la réduction de la densité moyenne des masses d'oeufs d'une génération à l'autre, la mortalité causée par les pathogènes n'était pas assez élevée pour rendre les populations endémiques et une infestation d'importance moyenne a persisté jusqu'en 2000.

[Traduit par la Rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 134 , Issue 6 , December 2002 , pp. 825 - 845
Copyright
Copyright © Entomological Society of Canada 2002

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