ASSIMILATION EFFICIENCY OF FREE AND PROTEIN AMINO ACIDS BY HOMALODISCA VITRIPENNIS (HEMIPTERA: CICADELLIDAE: CICADELLINAE) FEEDING ON CITRUS SINENSIS AND VITIS VINIFERA

The feeding of Homalodisca vitripennis, also known as the glassy-winged sharpshooter (GWSS), on Citrus sinensis cv. Navel and Vitis vinifera cv. Chardonnay was monitored in relation to free and protein bound amino acids in xylem fluid. Mean consumption rate of GWSS confined to feeding assemblies was significantly higher on Chardonnay (2.0 cm(3)/d) than on Navel (0.4 cm(3)/d). The concentration of free amino acids in xylem fluid was substantially higher than amino acids in protein form for both host species. For Navel, the major amino acids were asparagine/aspartic acid and proline, which represented 75% of the free amino acids. Glutamine/glutamic acid accounted for 75% of the free amino acids in Chardonnay. To test whether GWSS feeding induced changes in xylem fluid chemistry, GWSS were caged on shoots of Navel and Chardonnay for 10 d, at which time feeding rate was determined and amino acids in xylem fluid and in insect excreta were quantified. There was no significant effect of prior feeding on GWSS feeding rate. For Navel, GWSS feeding induced a significant increase in serine, arginine, alanine, methionine, isoleucine, leucine and lysine in protein form; however, the concentrations of most protein amino acids in xylem fluid of Chardonnay were not significantly increased. The assimilation efficiency (AE) of total free amino acids exceeded 99%, whereas total amino acids in protein form were assimilated with 90 to 98% efficiency. The AE was significantly increased for GWSS feeding on GWSS-caged shoots compared to that of control shoots.