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Optimal Selection and Exploitation of Hosts in the Parasitic Wasp Colpoclypeus Florus (Hym., Eulophidae)

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

This study deals with the question how an insect parasitoid can maximize its fitness through adaptation of its reproductive behaviour. It concentrates on the behaviour of a parasitoid after it has encountered a host. Optimal exploitation of individual hosts is emphasized rather than a maximization of the number of parasitized hosts. In Chapter I the topic of optimization of behaviour is introduced in relation to the study of insect parasitoids. The choice of the experimental animals is explained and behavioural alternatives of the parasitoid are discussed. In this study the number of granddaughters is taken as a measure of fitness. The chalcidoid wasp Colpoclypeus florus (Hym., Eulophidae) is a gregarious ectoparasitoid of larvae of at least 32 species of leafrollers (Lep., Tortricidae; Table I). Host plants are predominantly trees and shrubs. The parasitoid has a west palearctic distribution (fig. 1) and is rare in natural or semi-natural habitats. However, C. florus can be found in abundance in intensively cultivated habitats. In the Netherlands they are found especially in apple orchards, during outbreaks of the summer fruit tortrix moth, Adoxofihyes orana. Efforts to control A. orana with mass releases of the parasitoid had not been successful. However, the parasitoid is considered as promising by those working on integrated control and more biological information was required. In Chapter 2 the parasitization behaviour, development and phenology of the parasitoid is described. The experimental host (A. orana), general techniques and conditions are also described. Field experiments were carried out in an experimental apple orchard. Unlike many internal and external parasitoids, C. florus has the unusual habit of ovipositing beside instead of on or in the host. This offers the opportunity to manipulate the eggs and hosts separately. In addition, the number of hosts parasitized by an individual in the field is low, about 2-3 hosts per female, and the time taken to parasitize one host is long (average 13-28 h in the laboratory at 21 °C and about twice as long in the field, in summer). Thus, C. florus is particularly suitable for studies on how it optimizes exploitation of individual hosts. Three stages in the parasitization process were analysed in detail: a) The first problem concerned the host size selection for oviposition (Chapter 3 and 4). It was hypothesized that only the most profitable hosts are selected for oviposition. Only the first of five larval instars of A. orana is rejected for oviposition by the parasitoid. In the laboratory, proportion of hosts accepted, clutch size, survival of pre-adults, proportion females and parasitization time increase with host weight (tables VI, VIII). As a result the profitability of hosts (defined as the fitness gained per unit of time or per egg) is correlated with host acceptance, but the profitability threshold of host acceptance is low (fig. 7). It was shown that this threshold is not influenced

Affiliations: 1: (Agricultural University, Department of Animal Ecology, Wageningen, The Netherlands


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