Interspecific competition has been identified as a major structuring force in phytophagous arthropod communities. We would expect to find particularly strong competitive effects in communities with outbreaking components, where the joint food resource is depleted by a single super-abundant species. To assess how arthropod assemblages of the trembling aspen (Populus tremuloides, Salicaceae) respond to outbreaks by the forest tent caterpillar (Malacosoma disstria, Lepidoptera: Lasiocampidae), we sampled galls and leaf-mines in central Alberta, western Canada. Both the incidence and rank order of sympatric taxa varied with the abundance of M. disstria. While most species were “susceptible” to defoliation by M. disstria, declining in abundance with increasing densities of forest tent caterpillars, two species showed no response and one “resistant” species even increased. These patterns at the landscape scale appear to reflect competitive mechanisms acting at a local scale. In a laboratory choice experiment, M. disstria larvae were found to damage a higher proportion of “susceptible” galls than “resistant” galls. Patterns at the landscape scale were also indicative of large-scale movement of gallers and leaf-miners. Following a year of severe defoliation, population densities within an outbreak area declined with increasing distance from the outbreak edge. This finding identifies regional processes as important in the population dynamics of the target taxa. Our study suggests a major structuring role for M. disstria in local arthropod communities. As outbreaks regularly affect a large proportion of temperate forests, they create ample opportunity for competitive interactions among phytophagous arthropods.