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EVIDENCE OF SYNCHRONIZED CYCLES IN OUTBREAK PATTERNS OF DOUGLAS-FIR TUSSOCK MOTH, ORGYIA PSEUDOTSUGATA (McDUNNOUGH) (LEPIDOPTERA: LYMANTRIIDAE)

Published online by Cambridge University Press:  31 May 2012

Roy F. Shepherd
Affiliation:
Canadian Forestry Service, Pacific Forestry Centre,506 West Burnside Rd., Victoria, British Columbia, Canada V8Z 1M5
Dayle D. Bennett
Affiliation:
USDA Forest Service, located, respectively, at 517 Gold Aveneue S.W., Albuquerque, New Mexico, 87102; 630 Sansome St., San Francisco, California, 94111; P.O. Box 7669, Missoula, Montana, 59807; P.O. Box 3623, Portland, Oregon,97208; and 1750 Front St., Boise, Idaho, 83702, U.S.A.
John W. Dale
Affiliation:
USDA Forest Service, located, respectively, at 517 Gold Aveneue S.W., Albuquerque, New Mexico, 87102; 630 Sansome St., San Francisco, California, 94111; P.O. Box 7669, Missoula, Montana, 59807; P.O. Box 3623, Portland, Oregon,97208; and 1750 Front St., Boise, Idaho, 83702, U.S.A.
Scott Tunnock
Affiliation:
USDA Forest Service, located, respectively, at 517 Gold Aveneue S.W., Albuquerque, New Mexico, 87102; 630 Sansome St., San Francisco, California, 94111; P.O. Box 7669, Missoula, Montana, 59807; P.O. Box 3623, Portland, Oregon,97208; and 1750 Front St., Boise, Idaho, 83702, U.S.A.
Robert E. Dolph
Affiliation:
USDA Forest Service, located, respectively, at 517 Gold Aveneue S.W., Albuquerque, New Mexico, 87102; 630 Sansome St., San Francisco, California, 94111; P.O. Box 7669, Missoula, Montana, 59807; P.O. Box 3623, Portland, Oregon,97208; and 1750 Front St., Boise, Idaho, 83702, U.S.A.
Ralph W. Thier
Affiliation:
USDA Forest Service, located, respectively, at 517 Gold Aveneue S.W., Albuquerque, New Mexico, 87102; 630 Sansome St., San Francisco, California, 94111; P.O. Box 7669, Missoula, Montana, 59807; P.O. Box 3623, Portland, Oregon,97208; and 1750 Front St., Boise, Idaho, 83702, U.S.A.
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Abstract

Outbreak patterns of Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), over western North America historically appear to be synchronous, particularly in British Columbia, Washington, Oregon, and northern Idaho. Populations of the insect increase to outbreak and collapse in a variable cycle, averaging 9 years between peaks. A review of all outbreaks suggests repeated, widespread, nucleopolyhedrosis viral epizootics are responsible for the collapse of the population and, hence, the cycle. The virus appears to survive in the soil between outbreaks and to be carried incidentally to foliage where it is occasionally consumed by larvae. Ingestion of a single particle is probably sufficient to cause infection. Populations of the moth increase until density reaches the point where larvae to larvae infection is established. The viral inoculum builds rapidly following that point and spreads widely so that distant populations at all densities become infected, and collapse in the same year. The epizootic continues for another year. Then foliage contamination disappears, and populations reach their lowest densities before starting the cycle again.

Résumé

Les infestations de la chenille à houppes du douglas Orgyia pseudotsugata (McDunnough), dans l'ouest de l'Amérique du Nord suivaient, historiquement, un cycle de 9 ans. L'effondrement des infestations était synchrone sur une grande étendue, plus particulièrement en Colombie-Britannique et dans le Washington, l'Oregon et le nord de l'Idaho. On émet l'hypothèse qu'un virus provoquant une polyédrose nucléaire soit responsable : le virus, qui survit dans le sol entre chaque infestation, se retrouve parfois sur le feuillage où il est consommé par des larves. L'ingestion d'une seule particule de virus suffit probablement à causer l'infection. Les faibles populations endémiques augmentent pendant quelques générations sans qu'il y ait régulation par le virus. Lorsque leur densité atteint un certain point, il se produit une réinfection de larves à larves; l'inoculum viral augmente rapidement et est dispersé sur une grande distance, de sorte que des populations éloignées de toutes densités sont infectées et s'effondrent la même année. L'épizootie se poursuit pendant une autre année avant que la contamination ne disparaisse du feuillage, et les populations atteignent alors leurs densités les plus faibles, puis le cycle recommence.

Type
Research Article
Copyright
Copyright © Entomological Society of Canada 1988

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