Plant and insect characteristics in response to increasing density of Homalodisca coagulata on three host species:: a quantification of assimilate extraction

Three experiments were performed to assess the effect of the number of the leafhopper Homalodisca coagulata (Say) (Homoptera: Auchenorrhyncha: Cicadellidae) on the plant characteristics of Lagerstroemia indica L. (Lythraceae), Prunus salicina Lindl. (Rosaceae) and Glycine max (L.) Merrill (Leguminoseae) and on subsequent consumption rates and fecundity. Leafhoppers were placed in sleeve cages (L. indica or P. salicina) or on caged plants (G. max) for 2-3 weeks at densities of 0-16 per cage. Consumption rates and fecundity were not influenced by leafhopper treatments. Shoot growth and shoot xylem tension were examined on L. indica and G. max, and were not influenced by treatment. For G. max, plant height increased with leafhopper days, however plant dry weight was unaffected. The concentrations of primary organic compounds in xylem fluid were evaluated at the termination of experiments on L. indica (amino acids and organic acids) and G. max (amino acids, ureides, organic acids, and sugars). For L. indica, there was no significant density effect on the concentration of total amino acids or total organic acids. For the smaller legume G. max, concentrations of five primary organic compounds in xylem fluid were moderately reduced (between P=0.05 and 0.1) with leafhopper days. Consumption rates were inversely related (between P=0.05 and 0.1) to the concentrations of three organic compounds in xylem fluid. The total quantity of each chemical compound, organic carbon, organic nitrogen, and energy extracted by H. coagulata feeding on G. max was estimated by incorporating consumption rates, leafhopper days, diet composition, and the heat of combustion of each compound quantified. The quantity of water and nutrients (and particularly organic N) depleted by leafhopper feeding at high densities was substantial. Individual H. coagulata extracted 3.9 cm(3) water, 57 mumol organic carbon, 21 mumol organic nitrogen, and 2.7 J of chemical bond energy during a day of feeding. Moderate to large sized plants (P. salicina and L. indica) did not exhibit effects of leafhopper feeding, however high amounts of feeding on G. max increased plant height and moderately reduced the concentrations of several compounds in xylem fluid.