AbstractPlants grow under reduced water availability can have divergent effects on insect herbivores, in some instances producing benefits to them. However, the forces mediating these positive impacts remain mostly unclear. We conducted a manipulative field study using a specialist herbivorePieris rapae, and its host plant,Rorippa indica, in two populations to identify how water availability impacts overall plant quality and multitrophic interactions. We observed thatR. indicagrowing under low water availability led to higher survival ofP. rapaelarvae. The increase in survival of eggs and larvae was related to the reduced abundance of other herbivores and natural enemies. Water availability had differential impacts on members of the herbivore community through changes in plant quality. Low water availability decreased the quality ofR. indicato most herbivores as indicated by reduced abundance in the field and decreased relative growth rate in feeding assays. In contrast, the performance ofP. rapaelarvae were not affected by differences in sympatricR. indicagrown under different water availability. These results indicate that localP. rapaepossess some physiological adaptation to overcome fluctuations in host quality. Our findings illustrate that reduced water availability is beneficial to a specialist herbivore, but detrimental to most other herbivores. Our work highlights the complex roles of the arthropod communities associated with plants in determining the impacts of water availability on insect herbivores.