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DEVELOPMENT AND VALIDATION OF A FIXED-PRECISION SEQUENTIAL SAMPLING PLAN FOR ESTIMATING BROOD ADULT DENSITY OF DENDROCTONVS PSEUDOTSUGAE (COLEOPTERA: SCOLYTIDAE)

  • José F. Negrón (a1), Willis C. Schaupp (a2) and Erik Johnson (a2)

Abstract

The Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, attacks Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco (Pinaceae), throughout western North America. Periodic outbreaks cause increased mortality of its host. Land managers and forest health specialists often need to determine population trends of this insect. Bark samples were obtained from 326 trees distributed over 21 stands during a 2-year period in late winter to early spring of 1997 and 1998 in the Colorado Front Range. The variance to mean relationship of brood adults was examined using the Taylor power law, and a fixed-precision sampling plan was developed using Green’s method. Stop lines and minimum number of samples required to estimate brood adult density per 0.046 m2 with precision levels of 0.1, 0.2, and 0.3 were calculated. A resampling simulation conducted with an independent data set indicated that desired precision levels were not met. Theoretical precision levels were adjusted until desired precision levels were achieved. Average number of samples needed to estimate brood adult densities up to 25.1 adults per 0.046 m2 with precision levels of 0.09, 0.2, and 0.3 were 91, 20, and 8, respectively. For densities greater than 25.1 brood adults per 0.046 m2, conservative estimates are obtained with 72, 15, and 6 samples for precision levels of 0.09, 0.2, and 0.3, respectively. An emergence ratio can be obtained by dividing the estimated density of brood adults by twice the number of gallery starts. This system provides the user with an immediate assessment of the population trend of Douglas-fir beetle. The data collected compare favorably with data from other Douglas-fir beetle outbreaks reported in the literature. The use of this plan outside the Colorado Front Range, or by sampling at a different height, should be cautioned until additional data from other locations and sampling heights are examined.

Le Dendroctone du Douglas, Dendroctonus pseudotsugae Hopkins, s’attaque aux sapins de Douglas, Pseudotsuga menziesti (Mirb.) Franco (Pinaceae), dans tout l’ouest nord-américain. Les épidémies périodiques entraînent une hausse de la mortalité chez les hôtes. Les responsables de l’aménagement des terres et les spécialistes en foresterie ont souvent besoin d’évaluer les tendances démographiques de cet insecte. Des échantillons d’écorce ont été recueillis sur 326 arbres répartis dans 21 boisés au cours d’une période de 2 ans, à la fin de l’hiver et au début du printemps en 1997 en 1998 dans la chaîne de montagnes Colorado Front Range. Le rapport entre la variance et la moyenne a été étudié chez la progéniture émergente à l’aide de la loi de puissance de Taylor et un plan d’échantillonnage à précision pré-établie a été conçu selon la méthode de Green. Les lignes d’arrêt et le nombre minimum d’échantillons nécessaires pour estimer la densité des adultes émergés par 0,046 m2 à des niveaux de précision de 0,1, 0,2 et 0,3 ont été calculés. La simulation d’un nouvel échantillonnage avec une nouvelle matrice indépendante de données a permis de constater que les niveaux de précision n’ont pas été atteints. Les niveaux de précision théoriques ont été ajustés jusqu’à ce que les niveaux de précision désirés soient obtenus. Le nombre d’échantillons requis pour estimer la densité des adultes à l’émergence jusqu’à 25,1 adultes par 0,046 m2 était de 91 à un niveau de précision de 0,09, de 20 à un niveau de 0,2 et de 8 à un niveau de 0,03. Aux densités supérieures à 25,1 adultes par 0,046 m2, des estimations conservatrices de 72 (0,09), 15 (0,02) et 6 (0,03) échantillons ont été obtenues. Un rapport à l’émergence peut être calculé en divisant la densité estimée d’adultes par deux fois le nombre de galeries commencées. Ce système fournit à l’utilisateur une estimation immédiate de la tendance démographique du dendroctone. Ces données se comparent favorablement à celles obtenues au cours d’épidémies de dendroctones mentionnées dans la littérature. L’utilisation de ce plan en dehors de cette chaîne de montagnes, ou l’échantillonnage à d’autres hauteurs est à déconseiller si l’on n’obtient pas d’abord des données additionnelles sur d’autres localités et hauteurs d’échantillonnage.

[Traduit par la Rédaction]

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Corresponding author

1 Author to whom all corresponding should be addressed (E-mail: jnegron@lamar.colostate.edu).

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DEVELOPMENT AND VALIDATION OF A FIXED-PRECISION SEQUENTIAL SAMPLING PLAN FOR ESTIMATING BROOD ADULT DENSITY OF DENDROCTONVS PSEUDOTSUGAE (COLEOPTERA: SCOLYTIDAE)

  • José F. Negrón (a1), Willis C. Schaupp (a2) and Erik Johnson (a2)

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