Riparian corridors maintain regional biodiversity where they occur. Within the last century the floristic composition of riparian communities in the Southwest has drastically changed following introduction of the exotic tree Tamarix spp. In an attempt to control Tamarix spp. populations the Tamarisk leaf beetle (Diorhabda spp.) has been utilized as a biological control agent. Multiple years of data collection at our study sites along Fountain Creek (Fountain, CO) allowed us to characterize the response of Tamarix spp. to invasion by the biological control agent during the summer of 2013. In analyzing data collected before, during, and after the beetle invasion we saw a significant effect of foliar herbivory on Tamarix spp. physiology and survival strategy. During the beetle invasion Tamarix spp. experienced increased stomatal conductance (low abscisic acid sensitivity), and diminished foliar chlorophyll content and flower production. Phenotypic selection analysis on functional traits changed before, during, and after the beetle invasion. Before the invasion successful individuals exhibited high stomatal conductance and wasted water. During and after the invasion, selection was detected for increased chlorophyll content. These results suggest that Tamarix spp. experienced decreased photosynthetic potential during the invasion, and required high stomatal conductance to facilitate gas exchange for photosynthesis. It appears that Tamarix spp. responds to defoliation by increasing water use in an attempt to sustain photosynthate allocation to reproductive structures and below ground biomass. These results have implications for management strategies. As a facultative phreatophyte Tamarix spp. has garnered attention due to its prolific water use in areas of low water accessibility. The leaf beetle may increase the tendency of Tamarix to waste water during the multiple growing seasons needed for defoliation to be an effective management technique, which could have drastic effects on both shallow and deep groundwater reserves and water availability for native species.