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Host–instar selection in the aphid parasitoid Monoctonus paulensis (Hymenoptera: Braconidae, Aphidiinae): assessing costs and benefits

Published online by Cambridge University Press:  31 May 2012

Amanda Chau  and
Manfred Mackauer
Amanda Chau
Affiliation:
Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6
Manfred Mackauer*
Affiliation:
Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6
*
1 Author to whom all correspondence should be addressed (E-mail: mackauer@sfu.ca).
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Abstract

Females of Monoctonus paulensis (Ashmead), a solitary parasitoid of aphids, generally select the relatively smaller over equally available larger instars of the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphidoidea: Aphididae). Large hosts contain more resources for parasitoid development and hence have presumably higher quality; however, they require more time to subdue and are more likely to escape. We tested the hypothesis that a female’s choice among first (L1), second (L2), third (L3), and fourth (L4) instars of the pea aphid is based on the optimal balance between fitness costs in terms of time and fitness gains in terms of offspring number and size. Prepupal mortality did not vary with host instar, but pupal mortality was greater among parasitoids developing in L4 than in any younger instars. Offspring mortality was not influenced by clutch size in that mortality risk did not differ between parasitoids developing alone and counterparts developing in a clutch. The sex ratio, measured as proportion of daughters among offspring, was female-biased on all four host instars; the degree of bias increased from 0.70 (in L1) to 0.92 (in L4). Parasitoid body size was a function of aphid size at parasitism. Females were larger than males; the magnitude of the difference in body size was constant and independent of host and hence parasitoid size. A female’s potential fecundity as measured by the number of ovarial eggs at eclosion varied with her size and larval ontogeny. The four instars of the pea aphid were ranked in the order L1 > L2 > L3 > L4 both in terms of the number of offspring produced per encountered host and in terms of a female’s time costs; first instars are easier to handle and are more abundant in the field than older instars. The four host types were ranked in the order L2 > L3 > L1 > L4 in terms of the proportion and potential fecundity of daughters among offspring. The observed preference pattern (L1 > L2 > L3 > L4) suggests that, in choosing hosts, females of M. paulensis maximize the number of offspring per unit of search time rather than simply offspring quality.

Résumé

Les femelles de Monoctonus paulensis (Ashmead), un parasitoîde solitaire des pucerons, choisissent généralement les stades de petite taille plutôt que les stades de plus grande taille, tout aussi disponibles, du Puceron du pois, Acyrthosiphon pisum (Harris) (Hemiptera : Aphidoidea : Aphididae). Les hôtes de grande taille contiennent plus de ressources pour le développement des parasitoïdes et sont donc présumément de qualité supérieure; mais il faut plus de temps pour les maîtriser et ils sont plus susceptibles de s’échapper. Nous avons examiné si le choix d’un hôte de premier (L1), de deuxième (L2), de troisième (L3) et de quatrième (L4) stades du puceron repose sur la notion d’équilibre optimal entre les coûts reliés au fitness en fonction du temps et les bénéfices du fitness en nombre et en taille des rejetons. La mortalité des stades qui précèdent la nymphose ne varie pas en fonction du stade de l’hôte, mais la mortalité s’est avérée plus importante chez les nymphes provenant de larves élevées sur des pucerons de L4 que chez celles provenant de larves élevées sur des pucerons plus jeunes. La mortalité des rejetons n’est pas influencée par leur nombre et les risques de mortalité sont les mêmes chez les parasitoïdes solitaires au cours de leur développement que chez ceux qui se développent au sein d’une couvée. Le rapport femelles : mâles, équivalant à la proportion des femelles parmi les rejetons, favorise les femelles chez les hôtes des quatre stades; l’importance de cette tendance augmente de 0,70 (L1) à 0,92 (L4). La taille du parasitoïde varie en fonction de la taille des pucerons au moment de l’infestation. Les femelles sont de plus grande taille que les mâles; l’amplitude de la différence ente la taille des femelles et celle des mâles est constante et indépendante de la talle de l’hôte et donc de la taille des parasitoïdes. La fécondité potentielle d’une femelle, évaluée d’après le nombre d’oeufs ovariens qu’elle contient à l’éclosion de l’adulte, varie selon sa taille et son développement larvaire. Les quatre stades larvaires du Puceron du pois ont été ordonnés L1 > L2 > L3 > L4, en fonction du nombre de rejetons produits par hôte rencontré et des coûts temporels imposés aux femelles; les larves de premier stade sont plus faciles à manipuler sur le terrain et sont plus abondantes que les larves des autres stades. Les quatre types d’hôtes suivent l’ordre L2 > L3 > L1 > L4 en fonction du pourcentage de femelles dans la progéniture et de leur fécondité potentielle. Les préférences des femelles (L1 > L2 > L3 > L4) indiquent qu’au moment du choix d’un hôte, les femelles de M. paulensis maximisent le nombre de leurs rejetons par unité de temps de recherche plutôt que seulement la qualité de leur progéniture.

[Traduit par la Rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 133 , Issue 4 , August 2001 , pp. 549 - 564
Copyright
Copyright © Entomological Society of Canada 2001

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