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Biophysique environnementale des insectes endophytes.

Abstract : Physiology and life history traits of ectothermic organisms depend on microclimate temperature. In some insect - plant relationships, the herbivore manipulates physically and /or chemically its proximate environment, i.e. plant tissues. The effects of such modifications on the phytophage's microclimate are however still poorly understood. We investigated the physical modifications of apple leaf tissues made by the leaf mining moth Phyllonorycter blancardella (Lepidoptera: Gracillariidae), and their impact on the thermal ecology of larvae. The larva lives inside a mine, a gallery it builds throughout its development.
Spectrometric measurements showed that larvae greatly modify the optical properties of their mines when feeding on leaf tissues. The mine structure absorbs more infrared radiation than intact leaf tissues do. Moreover, the fed areas in the mine integument transmit a large amount of incoming radiation. An increase in the amount of transmitted radiation induces body temperature of larvae located below the fed areas to warm, leading to an increase in their respiration rate. Measurements of mine gas exchange and a model of CO2 diffusion within leaves were developed to show that stomata in the mine integument react by closing due to both (i) the increase in larval CO2 release as incoming radiation increases, and (ii) the effect of feeding activity on CO2 resistance pathways through the mesophyll. The two effects – larval CO2 and lesion effects – have similar quantitative impacts. A biophysical model of heat transfer was built to predict the temperature within a mine from climatic parameters, physical factors (modifications of optical properties) and the physiology of stomata. Model temperature predictions and experimental measurements were confronted to test for the validity of the model. The model predicted leaf and mine temperatures with a precision of 0.9 °C in the temperature range of 12 °C – 42 °C. The model predicted a large temperature excess within a mine: mine temperature can be up to 10 °C warmer than ambient air and up to 5 °C above leaf temperature. Temperature excess was closely related to radiation level due to the impact of this abiotic factor on both radiation absorption and stomatal closure. The modifications of plant tissues – optical properties and stomatal physiology – accounted almost equally for the temperature excess. Our approach clearly demonstrates that leaf miners control the impact of their physical and biotic environments.
Our results are discussed in an evolutionary ecology point of view. The warm microclimate of endophagous insects is shown to drive the evolution of their own thermal sensitivities and also those of their parasitoids, when compared to ectophagous herbivores.
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Sylvain Pincebourde. Biophysique environnementale des insectes endophytes.. Ecologie, Environnement. Université François Rabelais - Tours, 2005. Français. ⟨tel-00108243⟩

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