NITROGEN-FERTILIZATION AFFECTS INTERACTIONS BETWEEN THE COMPONENTS OFAN INSECT-FUNGUS-PLANT TRIPARTITE SYSTEM

1. The chrysomelid beetle, Gastrophysa viridula and the rust fungus, U romyces rumicis both occur on leaves of Rumex obtusifolius growing in a wide range of soil nutrient conditions. We investigated the effect o f fertilizing plants with eight nitrate and four ammonium concentratio ns on the components of this tripartite interaction in a controlled en vironment. 2. Leaf weight, area and total plant weight increased as bo th nitrate and ammonium concentrations increased up to 15 mmol l(-1). Between 15 and 50 mmol l(-1) added nitrate, leaf and total plant weigh t decreased. Total plant weight was unaffected by increasing ammonium fertilization from 15 to 25 mmol l(-1). 3. The density and percentage of U. rumicis pustules sporulating 8 days after infection decreased wi th increasing nitrate but were unaffected by increasing the concentrat ion of ammonium fertilization. 4. Leaf area consumed and number of egg s laid by G. viridula decreased as the concentration of nitrate fertil ization increased. Increasing the concentration of ammonium decreased leaf area eaten. 5. First instar mortality of G. viridula was increase d and,gregariousness and adult fecundity was decreased additively by t he combination of U. rumicis infection and decreasing the nitrate conc entration fed to plants from 10 to 1 mmol l(-1).