Little is known of the population dynamics of long-range insect migrants, and it has been suggested that the annual journeys of billions of nonhardy insects to exploit temperate zones during summer represent a sink from which future generations seldom return (the "Pied Piper" effect). We combine data from entomological radars and ground-based light traps to show that annual migrations are highly adaptive in the noctuid moth Autographa gamma (silver Y), a major agricultural pest. We estimate that 10-240 million immigrants reach the United Kingdom each spring, but that summer breeding results in a fourfold increase in the abundance of the subsequent generation of adults, all of which emigrate southward in the fall. Trajectory simulations show that 80% of emigrants will reach regions suitable for winter breeding in the Mediterranean Basin, for which our population dynamics model predicts a winter carrying capacity only 20% of that of northern Europe during the summer. We conclude not only that poleward insect migrations in spring result in major population increases, but also that the persistence of such species is dependent on summer breeding in high-latitude regions, which requires a fundamental change in our understanding of insect migration.