Widespread accounts of the harmful effects of invasive species have stimulated both practical and theoretical studies on how the spread of these destructive agents can be contained. In practice, a widely used method is the deployment of biological control agents, that is, the release of an additional species (which may also spread) that creates a hostile environment for the invader. Seeding colonies of these protective biological control agents can be used to build a kind of living barrier against the spread of the harmful invader, but the ecological literature documents that attempts to establish colonies of biological control agents often fail (opening gaps in the barrier). Further, the supply of the protective species is limited, and the full supply may not be available immediately. This problem has a natural temporal component: biological control is deployed as the extent of the harmful invasion grows. How can a limited supply of unreliable biological control agents best be deploy...