Technical Abstract: Systemically induced resistance is a common and important phenomenon in plant-pathogen and plant-insect interactions. The mechanisms triggered by microorganisms and leaf-herbivores that lead to systemic resistance are largely understood, but, as yet, it remains unknown how root herbivory may increase resistance in the above ground part of plants. To resolve this we investigated the systemic resistance induced by the root herbivore Diabrotica virgifera virgifera against lepidopteran herbivores in the leaves of Zea mays. D. virgifera infested plants suffered less above ground herbivory in the field and showed reduced growth of Spodoptera littoralis caterpillars in the laboratory. Root herbivory did not lead to a classical systemic wound-induced response in the leaves, but specifically triggered water loss and abscisic acid (ABA) accumulation. By chemically and genetically inhibiting ABA-biosynthesis, we show that the induction of ABA by itself is partially responsible for the induction of defense genes, but not for the early resistance against S. littoralis. Instead, the herbivore is sensitive to changes in the plant’s water balance induced by root herbivory. These changes are most pronounced under low water supply and when D. virgifera is allowed to feed on the upper root system, which was found to be the feeding side that is optimal for the development of the root herbivore. We conclude that the apparent increase in leaf resistance after root feeding by D. virgifera is the result of an upset water balance, which results in systemic hydraulic changes and reduced host-suitability of the leaves for chewing herbivores.