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RESOURCE USE AND CLONAL DIFFERENCES IN ATTACK RATE BY THE DOUGLAS-FIR SEED CHALCID, MEGASTIGMUS SPERMOTROPHUS WACHTL (HYMENOPTERA: TORYMIDAE), IN FRANCE

Published online by Cambridge University Press:  31 May 2012

Nancy Rappaport  and
Alain Roques
Nancy Rappaport
Affiliation:
USDA Forest Service, Pacific Southwest Forest and Range Experiment Station, PO Box 245, Berkeley, California, USA 94701
Alain Roques
Affiliation:
Institut National de la Recherche Agronomique, Centre de Recherches d'Orléans, Zoologie Forestère, Ardon 45160 Olivet, France
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Abstract

The within-cone distribution of Megastigmus spermotrophus Wachtl (Hymenoptera: Torymidae), the Douglas-fir seed chalcid, infesting Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] cones from north-central France was compared with that in samples from California. Results indicate that the mid-region of cones was more intensively utilized by seed chalcids in France than in California, whereas in the northwestern United States the mid-region is characteristically occupied by dipteran and lepidopteran species. This difference in distribution may explain the large discrepancy in infestation rates on the two continents. The potential impact of this finding on pest management strategies is discussed.

Cones were measured and dissected or X-rayed to determine seed chalcid infestation levels. Analysis of covariance performed on data from cones that were collected in 1986 and dissected showed cone diameter at maturity to be a highly significant factor in attack rate, although slopes were different among clones. Between-tree variation was also significant, but clonal source was not. Cone diameter, clonal source, and between-tree variation constituted 87.1% of the variation in rate of attack by the seed chalcid. Although clone was not a significant factor in data from dissected cones, X-rayed cones showed significant differences in attack rates for some clones.

Résumé

La répartition des larves de Megastigmus spermotrophus Wachtl (Hymenoptère : Torymidae) aux cônes du sapin de Douglas [Pseudotsuga menziesii (Mirb.) Franco] du nord-central de la France fut comparée à celle des échantillons de la Californie. Les données montrent que la région centrale des cônes était utilisée plus intensivement par les chalcidiens en France qu’en Californie, tandis qu’au nord-ouest des Etats-Unis cette région des cônes est, en règle générale, occupée par les lépidoptères et les diptères. Il se peut que ces différences en ce qui concerne la répartition puissent expliquer la grande différence entre les taux d’attaque par le chalcidien sur les deux continents. On discute l’impact éventuel de ces renseignements sur la lutte contre cette espèce.

Des cônes furent mesurés et disséqués ou radiographiés afin de constater les taux d’attaque par le chalcidien. L’analyse de covariance conduite aux données des cônes cueillis et disséqués en 1986 a montré un rapport linéaire très significatif entre le taux d’attaque et le diamètre des cônes, bien que les pentes soient hétérogènes entre clones. La variation entre-arbres fut aussi significative, mais la souche clonale ne fut pas significative. Diamètre des cônes, souche clonale, et variation entre-arbre ont constitué 87,1% de la variation de taux d’attaque par le chalcidien. Bien que la souche clonale ne soit pas une variable significative pour les cônes disséqués, les données des cônes soumis aux rayons-X ont montré les différences significatives entre taux d’attaque pour quelques clones.

Type
Articles
Information
The Canadian Entomologist , Volume 123 , Issue 6 , December 1991 , pp. 1219 - 1228
Copyright
Copyright © Entomological Society of Canada 1991

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