Steam explosion is a thermomechanicochemical pretreatment which allows the breakdown of lignocellulosic structural components by the action of heating, organic acids formed during the process and shearing forces resulting to the expansion of the moisture.Two distinct stages compose the steam explosion process: vapocracking and explosive decompression which include modification of the material components: hydrolysis of hemicellulosic components (mono and oligosaccharides released), modification of the chemical structure of lignin, modification of the cellulose cristallinity index… These effects allow the opening of lignocellulosic structures and influence the enzymatic hydrolysis yield of the material.

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1. To locate mating partners and essential resources such as food, oviposition sites and shelter, insects rely to a large extent on chemical cues. While most research has focused on cues derived from plants and insects, there is mounting evidence that indicates that micro-organisms emit volatile compounds that may play an important role in insect behaviour.2. In this study, we assessed how volatile compounds emitted by phylogenetically diverse bacteria affected the olfactory response of the primary parasitoid Aphidius colemani and one of its secondary parasitoids, Dendrocerus aphidum. Olfactory responses were evaluated for volatile blends emitted by bacteria isolated from diverse sources from the parasitoid's habitat, including aphids, aphid mummies and honeydew, and from the parasitoids themselves.3. Results revealed that A. colemani showed a wide variation in response to bacterial volatiles, ranging from significant attraction over no response to significant repellence. Our results further showed that the olfactory response of A. colemani to bacterial volatile emissions was different from that of D. aphidum. Gas chromatography-mass spectrometry analysis of the volatile blends revealed that bacterial strains repellent to A. colemani produced significantly higher amounts of esters, organic acids, aromatics and cycloalkanes than attractive strains. Strains repellent to D. aphidum produced significantly higher amounts of alcohols and ketones, whereas the strains attractive to D. aphidum produced higher amounts of the monoterpenes limonene, linalool and geraniol. 4. Overall, our results indicate that bacterial volatiles can have an important impact on insect olfactory responses, and should therefore be considered as an additional, so far often overlooked factor in studying multitrophic interactions between plants and insects. K E Y W O R D SAphidius colemani, chemical communication, Dendrocerus aphidum, microbial odour, multitrophic interactions, natural enemy, semiochemical, VOCs | 509 Functional Ecology GOELEN Et aL. Islam S. Sobhy https://orcid.org/0000-0003-4984-1823 Jetske G. de Boer https://orcid.org/0000-0002-3182-8543 Kevin J. Verstrepen https://orcid.org/0000-0002-3077-6219 Hans Jacquemyn https://orcid.org/0000-0001-9600-5794 Bart Lievens https://orcid.org/0000-0002-7698-6641 S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Goelen T, Sobhy IS, Vanderaa C, et al. Volatiles of bacteria associated with parasitoid habitats elicit distinct olfactory responses in an aphid parasitoid and its hyperparasitoid. Funct Ecol. 2020;34:507-520. https ://doi.