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DISTRIBUTION OF BARBARA COLFAXIANA (KEARFOTT) (LEPIDOPTERA: TORTRICIDAE) EGGS WITHIN AND AMONG DOUGLAS-FIR CROWNS AND METHODS FOR ESTIMATING EGG DENSITIES

  • J.D. Sweeney (a1) and G.E. Miller (a1)

Abstract

The spatial and frequency distributions of Douglas-fir cone moth, Barbara colfaxiana (Kearfott), eggs in Douglas-fir trees and stands were determined by dissecting 13 262 conelets collected from 81 trees in three sites and 2 years. There were no consistent trends in egg density associated with crown level or aspect. The frequency distribution of eggs per conelet fitted the negative binomial in three of five site-years but a common k for the negative binomial could not be calculated. Green’s index of aggregation suggested that the cone moth egg distribution was significantly aggregated in each site-year.

The optimal number of conelets per tree to sample was determined to be four in forest stands and three in seed orchards. The number of sample trees required for estimating mean egg density with 10% and 20% precision and 90% confidence was calculated for a range of mean egg densities using the method of Kuno. The sample sizes required to estimate a control threshold density of 0.6 eggs per conelet with 10% precision and 90% confidence were very large and would be impractical for operational use. Therefore, a sequential sampling plan was developed for use in seed orchards that would classify cone moth egg densities as either above or below a critical density at which 10% seed loss would be expected.

Cônelets (13 262) cueillis au cours de 2 années différentes dans 81 arbres poussant dans trois stations ont été disséqués pour déterminer la distribution spatiale et de fréquence des oeufs du perce-cône du Douglas (Barbara colfaxiana [Kearfoot]) dans des arbres et des peuplements de Douglas taxifoliés. Aucune tendance régulière de la densité des oeufs associé à la hauteur de prélèvement dans le houppier ou à l’aspect de la cime n’a été relevée. La distribution de fréquence des oeufs par cônelet s’ajustait à la distribution binomiale négative dans trois des cinq combinaisons station-année, mais aucun k commun n’a pu être calculé pour la distribution binomiale négative. L’indice d’agrégation de Green laisee supposer que la distribution des oeufs du perce-cône était concentrée de façon significative dans chaque combinaison station-année.

Il a été déterminé que le nombre optimal de cônelets par arbre à échantillonner était de quatre dans les peuplements forestiers et de trois dans les vergers à graines. Nous avons calculé que le nombre d’arbres-échantillons nécessaire pour estimer la densité moyenne des oeufs à un degré de précision de 10 et 20% et à un seuil de confiance de 90% à l’aide de la méthode de Kuno pour une gamme de densités moyennes d’oeufs. La taille des échantillons nécessaires pour estimer une densité-témoin limite de 0,6 oeufs par cônelet à un degré de précision de 10% et à un seuil de confiance de 90% était trop considérable pour être utilisable de façon opérationnelle. Nous avons donc élaboré un plan d’échantillonnage séquentiel pour les vergers à graines qui permet de classer la densité des oeufs du perce-cône comme étant supérieure ou inférieure à une densité critique à laquelle il faut s’attendre à une perte de 10% des graines.

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References

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DISTRIBUTION OF BARBARA COLFAXIANA (KEARFOTT) (LEPIDOPTERA: TORTRICIDAE) EGGS WITHIN AND AMONG DOUGLAS-FIR CROWNS AND METHODS FOR ESTIMATING EGG DENSITIES

  • J.D. Sweeney (a1) and G.E. Miller (a1)

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