Saliva is a critical biochemical interface between aphids and their host plants; however, the biochemical nature and physiological functions of aphid saliva proteins are not fully elucidated. In this study we used a multidisciplinary proteomics approach combining liquid chromatography-electrospray ionization tandem mass spectrometry and two-dimensional differential in-gel electrophoresis/matrix-assisted laser desorption/ionization time-of-flight/mass spectrometry to compare the salivary proteins from three aphid species including Acyrthosiphon pisum, Megoura viciae and Myzus persicae. Comparative analyses revealed variability among aphid salivary proteomes. Among the proteins that varied, 22% were related to DNA-binding, 19% were related to GTP-binding, and 19% had oxidoreductase activity. In addition, we identified a peroxiredoxin enzyme and an ATP-binding protein that may be involved in the modulation of plant defences. Knowledge of salivary components and how they vary among aphid species may reveal how aphids target plant processes and how the aphid and host plant interact.

Herbivore insects use a broad range of chemical cues to locate their host to feed or to oviposit. Whether several plant volatiles are effective allelochemicals for insects, the latter also emit molecules which have infochemical role. The (E)-b-farnesene (EBF) is a well-known aphid alarm pheromone commonly found in all previously tested species. Analysis of the released molecules from 23 aphid species, mainly collected on their natural host plant from May to July, was performed by gas chromatography-mass spectrometry. While EBF was identified as the main volatile substance in 16 species, alone or associated with other molecules, the alarm pheromone was only a minor component of the volatile molecule pattern of five other species. Moreover, two species, Euceraphis punctipennis and Drepanosiphum platanoides, did not release EBF at all but other terpenes were identified. This original observation raised the question on the utility and the source of the non-EBF volatiles. Are these potential infochemical substances produced by the aphid or only absorbed from the host plant? Here we determined that terpenes released by insects were not only provided by the host plants. Indeed, Megoura viciae emitted additional molecules than the ones from several aphid species reared on the same host plant. Moreover, no systematic relation between the feeding behaviour of the aphid species and the volatile releases was observed. Aphid terpene composition and proportion would provide reliable cues to identify the emitting organism, plant or insect. The next step of this work will be to determine the infochemical role of terpenes found in the range of tested aphid samples to better understand the relations between the different tritrophic levels.

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