Technical Abstract: The leaf beetle, Diorhabda elongata Brulle, was released in several western states as a biocontrol agent to suppress Tamarix spp. L. which has invaded riparian ecosystems; however, effects of leaf beetle herbivory on Tamarix physiology are largely undocumented and may have ecosystem ramifications. Herbivory by this insect produces discoloration of leaves and premature leaf fall in these ecosystems, yet the mechanism that promotes this is still unknown. Insect herbivory may change leaf photosynthesis and respiration and may affect a plant’s ability to regulate water loss and increase water stress. Premature leaf fall may affect plant tissue chemistry and belowground carbon allocation. We conducted a greenhouse experiment to understand how Tamarix responds to herbivory by leaf beetle adults and larvae and to determine the proximate cause of premature leaf fall. We hypothesized that herbivorized plants would have greater root and leaf respiration rates, greater photosynthesis, greater concentrations of leaf nitrogen and decreased retranslocation efficiency, increased water stress, lower root biomass and lower total non-structural carbohydrates in roots. Insect herbivory reduced photosynthesis rates, minimally affected respiration rates, but significantly increased water loss during daytime and nighttime hours and this produced increased water stress. The proximate cause for premature leaf fall appears to be desiccation. Plants exposed to herbivory were inefficient in their retranslocation of nitrogen before premature leaf drop. Root biomass showed a decreasing trend in herbivorized plants. Stress-induced by herbivory may render these trees less competitive in future growing seasons.