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TREE UTILIZATION AND DENSITY OF ATTACKING AND EMERGING POPULATIONS OF THE WESTERN PINE BEETLE (COLEOPTERA: SCOLYTIDAE) AND ITS NATURAL ENEMIES, BASS LAKE, CALIFORNIA, 1970–19711,2

Published online by Cambridge University Press:  31 May 2012

Clarence J. DeMars Jr. ,
Donald L. Dahlsten ,
Nancy X. Sharpnack  and
David L. Rowney
Clarence J. DeMars Jr.
Affiliation:
Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Berkeley, California, USA 94701
Donald L. Dahlsten
Affiliation:
Division of Biological Control, University of California, Berkeley, California, USA 94720
Nancy X. Sharpnack
Affiliation:
Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Berkeley, California, USA 94701
David L. Rowney
Affiliation:
Division of Biological Control, University of California, Berkeley, California, USA 94720
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Abstract

Tree size and bark surface area infested in trees killed by the western pine beetle (WPB), Dendroctonus brevicomis LeConte, and the density of attacking and emerging WPB were measured on 91 trees spanning five consecutive generations of the insect. The emergence densities of six natural enemies were also estimated.

Trees averaged 50.9 cm in diameter at breast height (DBH) with a mean height to base of infestation (HBI) of 1.1 m. Heights to top of infestation (HTI) averaged 16.0 m, but were significantly lower in trees attacked by the second [70(1)] and third [70(2)] generations, during synthetic pheromone elution, than in the first [69(3)], or fourth [70(3)], and fifth [71(1)] generations, before and after the treatment, respectively. Infested bark area averaged 2005 dm2 per tree for all generations.

WPB attack density declined steadily from 4.36 beetles/dm2 in generation 69(3) to 0.86 beetles/dm2 in 71(1). Sampling errors ranged from 6.67 to 26.27%, higher than the desired 15% in some cases. WPB emergence in field sticky traps (STIK) followed an opposite trend from 1.66 beetles/dm2 in 69(3) to 5.43 beetles/dm2 in 71(1). Sampling errors ranged from 8.39 to 28.26%. WPB emergence in laboratory rearings (REAR) averaged 4.03 beetles/dm2 with no significant differences among the generations. Sampling errors ranged from 8.20 to 28.77%. Because of inconsistent differences between these two methods of measuring WPB emergence, we have reported estimates of both.

Densities of attacking and emerging WPB were found to be uncorrelated with tree diameter, indicating that density was not a function of tree size and that these two variables may be treated as independent random variables when used in product models to estimate area-wide population totals.

Sampling errors for natural enemies were quite large, ranging up to 400%; therefore few conclusions could be drawn. Predators, in descending order of abundance, were Aulonium longum, Medetera aldrichii, Enoclerus lecontei, and Temnochila chlorodia. Parasitoids were less abundant overall than predators, with Roptrocerus xylophagorum more consistently present and at higher average densities than Dinotiscus burkei.

The suppression treatment was applied during the collapse phase of the population cycle and apparently prolonged that phase for two generations. This delay resulted in the death of an additional 120 trees. WPB productivity increased with decreasing density/square decimetre of attacking beetles. Density of attacking beetles appears regulated by exogenous processes acting immediately in an inverse density-dependent manner. The WPB population at Bass Lake would be released from endemic to epidemic status at the equilibrium point reached at a density of attacking adults of loge 1.65 beetles/dm2. Mortality of the developing within-tree life-stages appears regulated by delayed density-dependent processes, but the details of the relationships differ depending upon the assessment method (STIK or REAR) used.

Résumé

Pendant cinq générations consécutives du dendroctone occidental du pin (WPB), on a mesuré, sur 91 arbres tués par le coléoptère, la grandeur des arbres, la contenance surface infestée des coléoptères, et la densité des populations du coléoptère en attaquant et en sortant de l’arbre. Les densités des ennemis naturels en sortant furent aussi estimés.

Les arbres eurent en moyenne 50,9 cm de diamètre à hauteur de poitrine (DBH) et une hauteur moyenne de base de l’invasion (HBI) de 1,1 m. Les sommets des invasions eurent, en moyenne, une hauteur de 16,0 m, mais ils furent significativement plus bas aux arbres attaqués par les deuxième [70(1)] et troisième [70(2)] générations, pendant des élutions du phéromone synthétique, que dans les première [69(3)], quatrième [70(3)], et cinquième [71(1)] générations avant et après le traitement, respectivement. La contenance surface infestée fut 2005 dm2 par arbre pour toutes les générations.La densité de l’attaque par WPB baissa uniment de 4,36 coléoptères/dm2 dans génération 69(3) à 0,86 coléoptères/dm2 dans génération 71(1). Les erreurs d’échantillonnage s’étendirent de 6,67 à 26,27%—plus haut, dans quelques cas, que le souhaitable 15%. La sortie des WPB—comme mesurée par les "sticky traps" (STIK) —a pris le contre pied, de 1,66 coléoptères/dm2 en 69(3) et 5,43 coléoptères/dm2 en 71 (1). Les erreurs d’échantillonage s’étendirent de 8,39 à 28,26%. La sortie de WPB en laboratoire (REAR) fut en moyenne 4,03 coléoptères/dm2 avec aucune différence significative entre les générations. Les erreurs d’échantillonnage s’étendirent de 8,20 à 28.77%. À cause de ces différences inconséquentes entre ces deux méthodes de mesurer la sortie de WPB, on a rapporté des évaluations de tous les deux.

On a trouvé que les densités des WPB en attaquant et en sortant ne furent pas corrélatives aux diamètres des arbres. Cela indique que la densité ne fut pas une fonction de la grandeur des arbres et qu’on peut considérer ces deux variables comme des variables indépendentes et distribuées au hasard quand employées aux modèles pour estimer des populations régionales.

Les erreurs d’échantillonnage pour les ennemis naturels furent assez grandes, s’étendant jusqu’à 400%, ainsi on ne put formuler que peu de conclusions. Les insectes de proie furent, par rang descendant de l’abondance : Aulonium longum, Medetera aldrichii, Enoclerus lecontei, et Temnochila chlorodia. Les parasitoïdes furent moins abondants que les prédateurs, Roptrocerus xylophagorum étant plus abondant que Dinotiscus burkei par exemple.

Le traitement de suppression se fit pendant l’effondrement de la population et, évidemment, prolongea cette phase pour deux générations. Ce sursis provoqua la mort de 120 arbres de plus. La productivité de WPB s’agrandit selon que la densité des coléoptères attaquants/dm2 décrurent. La densité des coléoptères attaquants est évidemment regulé par des processus exogènes qui agissent d’une façon dépendant de la densité. La population du WPB à Bass Lake peut être changée d’un niveau endémique à un niveau épidémique au point d’appui attendu quand la densité des coléoptères attaquants est loge 1,65 coléoptères/dm2. Le taux de mortalité des stades à l’intérieur des arbres fut évidemment réglée par des processus dépendant de densité; les détails des rapports diffèrent, selon la méthode d’évaluation (STIK ou REAR)

Type
Articles
Information
The Canadian Entomologist , Volume 118 , Issue 9 , September 1986 , pp. 881 - 900
Copyright
Copyright © Entomological Society of Canada 1986

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