Feeding by the leafhopper, Homalodisca coagulata, in relation to xylem fluid chemistry and tension

Abstract

Feeding by the adult leafhopper [ Homalodisca coagulata (Say)], a polyphagous xylem feeder, was monitored in response to changes in xylem tension, and the concentration of amino acids, organic acids and sugars in xylem fluid. Models were constructed based on diurnal feeding patterns on a preferred host, Lagerstroemia indica L. Feeding rate (measured by excretion rate) was best correlated with the concentration of the amides. Correlation coefficients of other amino acids were low. Models were tested by monitoring feeding rates diurnally on Pyrus communis L. and Vitis hybrid cv. Suwannee, and during midday on L. indica subjected to water stress. Only total amino acids, amides and glutamine were correlated with feeding on P. communis and on Vitis. When L. indica were subjected to a high level of water stress, feeding during midday was associated with xylem tension, and not the concentration of chemical variables. Feeding rate decreased exponentially as a function of xylem tension; feeding ceased above a tension of about 2.1 MPa. Models were substantially improved when compounds were expressed on a ratio basis. Feeding rate was best correlated with the ratio of amides to total organic compounds (RAM) in the diurnal experiments. In the stress experiment, amides/xylem tension provided the best correlation. Net energy gain (Xneg) of an insect feeding on xylem fluid was described by the equation: Xneg = 0.70( Xe) - Excoef( Xt − P), where 0.70( Xe) = net energy content of xylem fluid, Excoef = the extraction coefficient, Xt = xylem tension and P = the Poiseuille's law-derived pressure to account for fluid movement through stylets.