The consequences of delayed development resulting from competitive interactions between insect herbivores are rarely considered in the context of altered susceptibility to natural enemies. We measured the influence of delayed development in leaf beetles (Leptinotarsa decemlineata), resulting from plant-mediated competition with leafhoppers (Empoasca fabae), on predation by the stinkbug, Podisus maculiventris. To achieve a range of beetle development times, we exposed potato plants to 4 levels of prior feeding by leafhoppers (no, low, medium, or high). These 4 levels of leafhopper-damaged plants were crossed with the presence/absence of predators in a factorial design experiment that measured beetle survival through larval development. We found a highly significant interaction between predator presence and prior leafhopper feeding such that predator effects were relatively weak on undamaged plants where beetle development was rapid compared with damaged plants where development was delayed. Thus, predation and plant-mediated competition acted synergistically to suppress beetle densities. Altered host quality and not modified plant architecture (leaf curling) was the underlying mechanism of this effect by extending the “window of vulnerability” to consumption by beetle enemies. Functional response experiments revealed that stinkbugs were more effective predators of early-instar larvae compared with beetles in later larval stages. These results support the slow-growth-high-mortality hypothesis in explaining enhanced predation on leafhopper-damaged plants. Therefore, to accurately assess the degree of competitive interactions between beetles and leafhoppers in this system, both plant-mediated effects on herbivore development and its consequences for enhanced predation risk must be considered.