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A COMPARISON OF DISCRIMINATION AND OF DENSITY RESPONSES DURING OVIPOSITION BY EXENTERUS AMICTORIUS AND E. DIPRIONIS (HYMENOPTERA: ICHNEUMONIDAE), PARASITES OF NEODIPRION SWAINEI (HYMENOPTERA: DIPRIONIDAE)

Published online by Cambridge University Press:  31 May 2012

J. M. McLeod
J. M. McLeod
Affiliation:
Laurentian Forest Research Centre, Canadian Forestry Service, Quebec, Quebec
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Abstract

The oviposition behavior of Exenterus amictorius Panzer, an introduced parasite of pre-spinning eonymphs of the Swaine jack pine sawfly, Neodiprion swainei Middleton, was studied in a series of field experiments over a 5-year period in the Province of Quebec. The ability of this parasite to discriminate against hosts with previously deposited progeny varied significantly: discrimination was lacking at the beginning of the host’s spinning period, but was rapidly acquired and persisted to the end of the spinning period. The relationships were described by negative power functions of the form Iδ = aX−b, where Iδ = Morisita’s Index of Dispersion, and X = the number of days from the beginning of the host’s spinning period. The response of E. amictorius to changing host densities, however, was positive at all host densities and described by functions of the form Y = aXb, where Y = number of parasite progeny per host and X = number of hosts dropping into funnel traps at 2-day intervals during the spinning period. Over 67% of the variation in the number of progeny deposited by E. amictorius during a field experiment in 1969 was explained by changes in adult parasite density, and an additional 19% by changes in host numbers. This indicates strongly that the observed change in discrimination against previously deposited progeny bears no direct relationship to either host or parasite density. It was suggested that the change might be influenced either by a reduction in the parasite’s egg complement in time, or by an associative learning process.

The density response of an indigenous parasite, Exenterus diprionis Rohwer, was comparatively much weaker and it seemed to lack the adaptive changes in discrimination against previously deposited progeny possessed by E. amictorius. Although attacks of the one species occurred independently of the other, in the event of multiparasitism E. amictorius always survived because of its faster rate of development in the host. The role of discrimination and of density response in the dominance of E. amictorius among the Exenterus spp. attacking N. swainei, and its successful establishment on a variety of North American diprionids, is discussed.

Résumé

Étude en une série d’expériences en forêt échelonnées sur cinq années au Québec du comportement au cours de la pondaison d’Exenterus amictorius Panzer, un parasite exotique de la Tenthrède de Swaine (Neodiprion swainei Middleton) au premier stade nymphal précédant la formation du cocon. Variation considérable de l’aptitude de ce parasite à sélectionner des hôtes déjà parasités, laquelle propriété fait défaut au seuil de la période de formation du cocon de l’hôte mais s’acquiert rapidement et dure jusqu’au terme de cette période. Description de ces relations par des fonctions à exposant négatif de la forme Iδ = aX−b, où Iδ représente le coefficient de dispersion de Morisita et X, le nombre de jours depuis le commencement de la période de formation du cocon de l’hôte. Réaction positive d’E. amictorius à tous les changements de densité chez l’hôte, décrite par des fonctions de la forme Y = aXb dans lesquelles Y = nombre de couvains du parasite par hôte et X = nombre d’hôtes tombant dans les pièges à entonnoir par intervalles de deux jours durant la formation du cocon. Plus de 67% de la variation numérique des couvains d’E. amictorius lors d’une expérience en forêt en 1969 s’expliquait par des changements numériques intervenus chez l’hôte, ce qui constitue un puissant indice que le changement observé relativement à la discrimination des couvains précédemment déposés ne comporte aucune relation directe avec la densité de l’hôte ou du parasite. On a suggéré qu’un tel changement pourrait être influencé soit par une réduction de la ponte avec le temps, soit par un processus associatif d’apprentissage.

La réaction d’un parasite indigène, Exenterus diprionis Rohwer, à la densité s’avérait comparativement plus faible et paraissait dépourvue des modifications adaptatives que possède E. amictorius dans la discrimination des couvains en place. Quoique les attaques de l’une et l’autre espèce eussent lieu indépendamment, en cas de multiparasitisme E. amictorius survivait toujours à cause de son rythme de développement plus rapide chez l’hôte. Discussion du rôle joué par la réaction à la discrimination et à la densité dans la dominance d’E. amictorius parmi les espèces d’Exenterus attaquant N. swainei, et de son succès à coloniser toute une variété de diprionidés d’Amérique du Nord.

Type
Articles
Information
The Canadian Entomologist , Volume 104 , Issue 9 , September 1972 , pp. 1313 - 1330
Copyright
Copyright © Entomological Society of Canada 1972

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