Increasing outbreaks of invasive insect pests pose a substantial threat to the functioning and viability of cultivated and wild woody perennial species worldwide. In the eastern U.S., the spotted lanternfly (Lycorma delicatula White; SLF), an invasive phloem-feeding planthopper, was reported to be able to negatively impact late-season plant carbon dynamics in various woody perennial species following repeated or prolonged infestation and feeding events. However, it remains unclear if SLF infestations also impact plant water relations and if SLF-mediated effects persist when populations are controlled and feeding stops. This study investigated how late-season exposure to SLF impacts whole-plant water relations by assessing diurnal sap flow and trunk radius changes in grapevines subjected to varying infestation densities of adult SLF. In two seasons, vines exposed to high infestation densities (an average of 180 SLFs per vine) for up to 31–32 days of cumulative SLF exposure had significantly lower sap flow rate than those with no SLF, resulting in up to 38 % less daily total water use. Trunk diurnal amplitudes increased under SLF infestation, but impacts were less dependent on infestation density, suggesting that grapevines may be utilizing trunk water storage during infestation to meet both grapevine water use and SLF sap ingestion. In both cases, SLF-mediated effects persisted following removal of SLFs, suggesting that exposure to high populations of SLF can alter patterns of late-season grapevine water use, at least when populations are not effectively controlled. These results indicate that SLF can modify both whole-plant water relations and carbon dynamics concurrently, further defining the implications that intensive infestations by high populations of SLF have for woody perennial whole-plant physiology.