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RELATIONSHIPS BETWEEN ATTACK RATES AND SURVIVAL OF WESTERN SPRUCE BUDWORM, CHORISTONEURA OCCIDENTALIS FREEMAN (LEPIDOPTERA: TORTRICIDAE), AND BUD DEVELOPMENT OF DOUGLAS-FIR, PSEUDOTSUGA MENZIESII (MIRB.) FRANCO

Published online by Cambridge University Press:  31 May 2012

Roy F. Shepherd
Roy F. Shepherd
Affiliation:
Forestry Canada, Pacific Forestry Centre, 506 West Burnside Road, Victoria, British Columbia, Canada V8Z 1M5
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Abstract

Individual larvae of western spruce budworm (Choristoneura occidentalis Freeman) were observed from overwintering emergence to pupation at six locations spread over a wide range of altitudes and thus climate. A weekly census of 100 lower-crown buds per plot indicated large differences in rates of bud development and larval survival among locations.

Emerging second-instar larvae attempted to mine swelling buds of Douglas-fir. If the buds were hard and tight, larvae mined 1-year-old needles until penetrable buds were available. Larvae dispersed over the crowns with only one larva becoming established in each bud; thus, many early-emerging and surplus larvae could not find suitable feeding sites and disappeared. Within the protective bud, survival was high. After buds flushed and larvae became exposed, densities dropped, probably due to increased predation and decreased food quality. Correlations indicated a close association between larval survival for the exposed period between bud flush and pupation, and overall larval survival.

Douglas-fir trees responded to initial bud removal, but not to needle removal, by inducing latent buds in the axils of needles to grow into active vegetative buds ready to develop and flush the next spring. The number of these new vegetative buds formed was greatest when the initial buds were removed early in the season before flush, and decreased thereafter. Trees with vigorous crowns had the greatest response to defoliation by inducing the largest number of latent buds into becoming active vegetative buds; these were found mainly on the 2- and 3-year-old internodes.

Résumé

On a observé des chenilles individuelles de la tordeuse occidentale de l’épinette (Choristoneura occidentalis Freeman), depuis l’émergence post-hivernale jusqu’à la pupaison, dans six endroits choisis de manière à couvrir une gamme étendue d’altitudes et, par conséquent, de climats. Un recensement hebdomadaire de 100 bourgeons situés dans la partie inférieure de la couronne par parcelle a révélé des écarts importants en ce qui a trait au taux de développement des bourgeons et à la survie des chenilles d’un endroit à l’autre.Les jeunes chenilles du deuxième stade larvaire ont essayé de miner les bourgeons turgescents du sapin de Douglas. Lorsque les bourgeons étaient durs et bien fermés, les chenilles se sont attaquées aux vieilles aiguilles de 1 an en attendant de pouvoir pénétrer dans les bourgeons.

Les chenilles se sont dispersées dans toute la couronne et on ne comptait qu’une seule chenille par bourgeon; aussi, bon nombre de larves précoces et excédentaires n’on pu trouver d’endroits appropriés pour s’alimenter et ont disparu. Le taux de survie a été élevé chez les chenilles abritées à l’intérieur des bourgeons. Après le débourrement et la sortie des chenilles à l’air libre, la densité des populations a chuté, probablement en raison de l’accroissement de la prédation et d’une diminution de la qualité des aliments. Les corrélations établies ont indiqué que la survie générale des chenilles était étroitement liée à leur survie durant la période de vie à l’air libre, soit entre le débourrement et la pupaison.

Les sapins de Douglas ont réagi à la disparition des bourgeons initiaux, mais pas à la disparition des aiguilles, en induisant les bourgeons latents présents à l’aisselle des aiguilles à se transformer en bougeons végétatifs actifs prêts à croître et à débourrer le printemps suivant. Le nombre de ces nouveaux bourgeons végétatifs a été plus élevé lorsque les bourgeons initiaux ont été éliminés tôt avant le débourrement et il a diminué par la suite. La réaction la plus marquée à la défoliation a été observée chez les arbres dotés d’une couronne vigoureuse chez lesquels on a constaté le plus grand nombre de bourgeons latents se transformant en bourgeons végétatifs actifs; ces derniers se trouvaient principalement sur les entrenoeuds âgés de 2 à 3 ans.

Type
Articles
Information
The Canadian Entomologist , Volume 124 , Issue 2 , April 1992 , pp. 347 - 358
Copyright
Copyright © Entomological Society of Canada 1992

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