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SPATIAL DISTRIBUTION OF ADULTS OF OOENCYRTUS KUVANAE (HYMENOPTERA: ENCYRTIDAE), AN EGG PARASITE OF LYMANTRIA DISPAR (LEPIDOPTERA: LYMANTRIIDAE)1

Published online by Cambridge University Press:  31 May 2012

M. W. Brown  and
E. Alan Cameron
M. W. Brown
Affiliation:
Department of Entomology, The Pennsylvania State University, University Park 16802
E. Alan Cameron
Affiliation:
Department of Entomology, The Pennsylvania State University, University Park 16802
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Abstract

The spatial distribution of adult Ooencyrtus kuvanae (Howard) (Hymenoptera: Encyrtidae) was examined in central Pennsylvania during 1978–1980. Data were gathered over a wide range of gypsy moth (Lymantria dispar (L.) (Lepidoptera: Lymantriidae)) densities and from all phases of population growth. There was a linear relationship between the log variance and log mean, fitting Taylor's Power Law with a power of 1.27. The degree of aggregation in O. kuvanae populations, as measured by the variance-to-mean ratio, was correlated positively with the abundance of the parasite, and was independent of host density. Aggregation was believed to result from the habit of the female parasite ovipositing repeatedly on the same egg mass, and the resultant progeny emerging synchronously. Behavior and density of O. kuvanae populations were found to depend upon the gypsy moth population condition, i.e., low, rising, high stable, outbreak, or collapsed. The frequency distribution of O. kuvanae populations fit the Poisson at densities less than 0.2 parasite adult per egg mass, a condition which occurred early and late (July, November) in the season, and the negative binomial above this density; a few distributions fit the log normal. The data were also analyzed using analyses of variance (multivariate and univariate), correlation, and regression techniques. All main effects, i.e., study area, host egg mass volume, egg mass height from the ground (within the 0–2 m sampling universe), aspect of the egg mass on the tree, and tree species, were important in explaining the variance in parasite abundance; study area differences were the most important. The effects of aspect and tree species were explained on the basis of small sample sizes, and not studied further. There was a positive correlation between host egg mass volume (= size) and parasite abundance. There was also a positive correlation between the height of the egg mass and parasite abundance during the summer; however, this relationship became negative by late fall. This seasonal change in vertical distribution was attributed to the tendency of O. kuvanae to be closer to the ground where they overwinter. Variation in adult abundance was generally greater among gypsy moth egg masses on different trees than among egg masses on the same tree, except at low parasite densities. This was attributed to the parasites searching for egg masses on one tree before dispersing to another.

Résumé

La distribution spatiale des adultes d'Ooencyrtus kuvanae (Howard) (Hymenoptera : Encyrtidae) a été examinée en Pensylvanie centrale au cours des années 1978–1980. Des données furent prélevées sur une étendue considérable de densités de la spongieuse (Lymantria dispar (L.) Lepidoptera : Lymantriidae)) et à toutes les phases de croissance de population. Une relation linéaire fut observée entre le log de la variance, et le log de la moyenne, correspondant à la loi des facteurs (Power Law) de Taylor avec un facteur de 1.27. Le degré d'aggregation des populations d'O. kuvanae, mesuré par le rapport variance/moyenne, était positivement corrélé avec l'abondance du parasite, et indépendante de la densité de l'hôte. L'aggrégation a semblé résulter de l'habitude qu'a la femelle du parasite de pondre répétitivement sur la même masse d'oeufs, et de l'émergence synchronisée de la progéniture résultante. Le comportement et la densité des populations d'O. kuvanae se sont montrés dépendants de l'état de la population de spongieuse, c'est à dire basse, croissante, élevée stable, épidémique ou effondrée. La distribution de fréquence des populations d'O. kuvanae correspond a une Poisson aux densités inférieures à 0,2 adultes du parasite par masse d'oeuf, une situation s'étant manifestée tôt et tard (Juillet, Novembre) dans la saison, et à une binomiale négative au dessus de cette densité; quelques distributions correspondent à une log normale. Les données ont aussi été soumises à l'analyse de variance (multivariée et univariée), de corrélation et de régression. Tous les facteurs principaux c'est à dire l'aire d'étude, le volume de la masse d'oeufs de l'hôte, la hauteur de la masse d'oeufs à partir du sol (à l'intérieur des 0–2 m échantillonnés), et l'espèce d'arbre, se sont avérés importants pour expliquer la variance de l'abondance du parasite; les différences entre aires d'étude étaient les plus importantes. Les effets de l'aspect et de l'espèce d'arbre ont pu être expliqués par la faible taille des échantillons, et n'ont pas été étudiés plus à fond. Une corrélation positive a été observée entre le volume de la masse d'oeufs de l'hôte (= taille) et l'abondance du parasite. Il y avait également une corrélation positive entre la hauteur de la masse d'oeufs et l'abondance du parasite durant l'été; cependant, cette relation était négative à l'automne. Ce changement saisonnier dans la distribution verticale a été attribué à la tendence d'O. kuvanae à se situer plus près du sol aux endroits où il hiverne. La variation de l'abondance des adultes était généralement plus grande entre les masses d'oeufs sur des arbres différents qu'entre les masses d'oeufs sur la même arbre, sauf aux basses densités du parasite. Ceci est attribuable au fiat que les parasites cherchent des masses d'oeufs sur la même arbre avant de chercher sur un autre arbre.

Type
Articles
Information
The Canadian Entomologist , Volume 114 , Issue 12 , December 1982 , pp. 1109 - 1120
Copyright
Copyright © Entomological Society of Canada 1982

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