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Held, Matthias

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Held, Matthias
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https://libra.unine.ch/handle/123456789/598

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  • Publication
    Accès libre
    A comparison of naïve and conditioned responses of three generalist endoparasitoids of lepidopteran larvae to host-induced plant odours
    (2006)
    Tamò, Cristina
    ;
    Ricard, Ingrid
    ;
    Held, Matthias 
    ;
    Davison, A. C.
    ;
    Turlings, Ted 
    Many parasitic wasps that exploit herbivores as their hosts make use of herbivoreinduced plant odours to locate their victims and these wasps often exhibit an ability to learn to associate specific plant-produced odours with the presence of hosts. This associative learning is expected to allow generalist parasitoids to focus on cues that are most reliably associated with current host presence, but evidence supporting this hypothesis is ambiguous. Using a six-arm olfactometer we compared the responses of three generalist larval endoparasitoids, Cotesia marginiventris (Hymenoptera: Braconidae), Microplitis rufiventris (Hymenoptera: Braconidae) and Campoletis sonorensis (Hymenoptera: Ichneumonidae), to the induced odours of three plant species: maize (Zea mays), cowpea (Vigna unguiculata), and cotton (Gossypium hirsutum). We tested the responses of naïve females as well as of females that were first conditioned by parasitising host larvae feeding on one of the plant species. Despite similarities in biology and host range the three wasp species responded entirely differently. Naïve C. marginiventris and C. sonorensis chose equally among the induced odours of the three plants, whereas naïve M. rufiventris, which may have a somewhat more restricted host range, tended to prefer the odour of maize. After conditioning, most C. marginiventris females chose the odour of the plant species that they had experienced, but conditioned M. rufiventris showed an even stronger preference for maize odours, independently of the plant they had experienced. Cotesia sonorensis did not show any change in its preference after conditioning. We speculate that its extremely broad host range allows C. sonorensis females to use fixed responses to cues commonly associated with plants damaged by Lepidoptera. These results imply that different generalist parasitoids may employ different foraging strategies and that associative learning is not necessarily part of it.
  • Publication
    Accès libre
    Evaluating the Induced-Odour Emission of a Bt Maize and its Attractiveness to Parasitic Wasps
    (2005)
    Turlings, Ted 
    ;
    Jeanbourquin, Philippe 
    ;
    Held, Matthias 
    ;
    Degen, Thomas 
    The current discussion on the safety of transgenic crops includes their effects on beneficial insects, such as parasitoids and predators of pest insects. One important plant trait to consider in this context is the emission of volatiles in response to herbivory. Natural enemies use the odours that result from these emissions as cues to locate their herbivorous prey and any significant change in these plant-provided signals may disrupt their search efficiency. There is a need for practical and reliable methods to evaluate transgenic crops for this and other important plant traits. Moreover, it is imperative that such evaluations are done in the context of variability for these traits among conventional genotypes of a crop. For maize and the induction of volatile emissions by caterpillar feeding this variability is known and realistic comparisons can therefore be made. Here we used a six-arm olfactometer that permits the simultaneous collection of volatiles emitted by multiple plants and testing of their attractiveness to insects. With this apparatus we measured the induced odour emissions of Bt maize (Bt11, N4640Bt) and its near-isogenic line (N4640) and the attractiveness of these odours to Cotesia marginiventris and Microplitis rufiventris, two important larval parasitoids of common lepidopteran pests. Both parasitoid species were strongly attracted to induced maize odour and neither wasp distinguished between the odours of the transgenic and the isogenic line. Also wasps that had previously experienced one of the odours during a successful oviposition divided their choices equally between the two odours. However, chemical analyses of collected odours revealed significant quantitative differences. The same 11 compounds dominated the blends of both genotypes, but the isogenic line released a larger amount of most of these. These differences may be due to altered resource allocation in the transgenic line, but it had no measurable effect on the wasps’ behaviour. All compounds identified here had been previously reported for maize and the differential quantities in which they were released fall well within the range of variability observed for other maize genotypes.