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BGPI : équipe 6; In phytophagous piercing-sucking insects, salivary sheath-feeding species are often described as xylem- or phloem-sap feeding specialists. Because these two food sources have very different characteristics, two feeding tactics are often associated with this supposed specialization. Studying the feeding behavior of insects provides substantial information on their biology, ecology, and evolution. Furthermore, study of feeding behavior is of primary importance to elucidate the transmission ability of insects that act as vectors of plant pathogens. In this study, we compared the durations of ingestion performed in xylem versus phloem by a leafhopper species, Scaphoideus titanus Ball, 1932. This was done by characterizing and statistically analyzing electrical signals recorded using the electropenetrography technique, derived from the feeding behaviors of males and females. We identified three groups of S. titanus based on their feeding behavior: 1) a group that reached the phloem quickly and probed for a longer time in phloem tissue than the other groups, 2) a group that reached the xylem quickly and probed for a longer time in xylem tissue than the other groups, and 3) a group where individuals did not ingest much sap. In addition, the numbers and durations of waveforms representing ingestion of xylem and phloem saps differed significantly depending on the sex of the leafhopper, indicating that the two sexes exhibit different feeding behaviors. Males had longer phloem ingestion events than did females, which indicates that males are greater phloem feeders than females. These differences are discussed, specifically in relation to hypotheses about evolution of sap feeding and phytoplasma transmission from plant to plant.

International audience; Concanavalin A (lectin from Canavalia ensiformis L., ConA) has previously been shown to act as a feeding inhibitor for Acyrthosiphon pisum, the pea aphid. In the present study a range of histochemical and biochemical techniques were used to elucidate the target tissues and binding sites of the lectin in the aphid. Diet uptake was evaluated using a radioactive tracer (14C-methylated inulin) and demonstrated that adults were capable of ingesting high quantities of the toxin (approx. 1 microg over a 48 h period). Electophoretic analysis and enzyme-linked immuno-sorbent assay of honeydew samples confirmed these results and further demonstrated that only small levels of ConA were excreted. Histofluorescence and immunolocalisation studies on nymphs revealed that the stomach was the primary target for ConA. At concentrations up to 400 microg ml(-1), lectin binding only occurred in the stomach region, however, at high concentrations (800 microg ml(-1)) the whole digestive tract was stained, although there was no evidence of binding in either the oesophagus or rectum. In addition to binding, there was evidence to suggest that ConA was also causing systemic effects in that the lectin appeared to cross the intestinal epithelial barrier. Immunohistochemical and electron microscopy studies revealed that ConA induced severe cellular swelling of the epithelial cells, accompanied by hypersecretion and a progressive detachment of the apical membrane; however, the striated border itself did not appear to be directly affected. Furthermore, there was no lysis of the epithelium, nor loss of integrity of the epithelial cells themselves. Our results suggest that ConA interacts with glycosylated receptors at the surface of the stomach epithelial cells, interfering with normal metabolism and cell function, resulting in a rapid feedback response on feeding behaviour. Whilst our results provide a much greater understanding regarding the modes of action of ConA in insects, they suggest that different lectins, including other mannose binding lectins, have different modes of action at the cellular levels, and thus generalizations should be treated with caution.