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Salivary secretions play critical roles in aphid-host plant interactions and are responsible for damage associated with aphid feeding. The objectives of this study were to evaluate aspects of salivation and the salivary constituents of Diuraphis noxia (Hemiptera: Aphididae). Salivary proteins were isolated and compared from three aphid probed diets: pure water, 15% sucrose, or amino acids (100 mM serine, 100 mM methionine, 100 mM aspartic acid, and 15% sucrose). After 6 h, more aphids settled on sucrose diet compared with other diets, but there were no significant differences in the number of stylet sheaths produced per aphid after 24 h. There were differences in the amount of soluble salivary protein (watery saliva), with the greatest amount secreted in sucrose diet, followed by amino acid diet and pure water, respectively. Protein constituents secreted into sucrose and amino acid diets were compared using gel electrophoresis using standardized amounts of protein. More protein bands and bands of greater intensity were visualized from probed sucrose diet compared with probed amino acid diet, indicating qualitative differences. Phosphatase was putatively identified from D. noxia saliva from a major protein band using gel electrophoresis and mass spectrophotometry. Alkaline phosphatase activity was confirmed in sucrose diet using enzymatic assays but was not detected in aphid probed water or amino acid diets. Other peptides in sucrose diet weakly but significantly showed similarities to putative dehydrogenase and RNA helicase expressed sequence tags identified from other aphids. The implications of these findings in aphid salivation and plant-insect interactions are discussed.

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Aphid (Hemiptera: Aphididae) saliva, when injected into host plants during feeding, causes physiological changes in hosts that facilitate aphid feeding and cause injury to plants. Comparing salivary constituents among aphid species could help identify which salivary products are universally important for general aphid feeding processes, which products are involved with specific host associations, or which products elicit visible injury to hosts. We compared the salivary proteins from five aphid species, namely, Diuraphis noxia (Kurdjumov), D. tritici (Gillette), D. mexicana (Baker), Schizaphis graminum (Rondani), and Acyrthosiphon pisum (Harris). A 132-kDa protein band was detected from the saliva of all five species using sodium dodecyl sulfate polyacrylamide gel electrophoresis. Alkaline phosphatase activity was detected from the saliva of all five species and may have a universal role in the feeding process of aphids. The Diuraphis species cause similar visible injury to grass hosts, and nine electrophoretic bands were unique to the saliva of these three species. S. graminum shares mutual hosts with the Diuraphis species, but visible injury to hosts caused by S. graminum feeding differs from that of Diuraphis feeding. Only two mutual electrophoretic bands were visualized in the saliva of Diuraphis and S. graminum. Ten unique products were detected from the saliva of A. pisum, which feeds on dicotyledonous hosts. Our comparisons of aphid salivary proteins revealed similarities among species which cause similar injury on mutual hosts, fewer similarities among species that cause different injury on mutual hosts, and little similarity among species which feed on unrelated hosts.

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