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1. The population structure of a monophagous noctuid moth, Abrostola asclepiadis, living on a patchily distributed perennial herb, Vincetoxicum hirundinaria is described. The study took place over 5 years at a landscape scale (about 12 km2).

2. Patch occupancy rates and population densities were studied in relation to patch size, degree of patch isolation, level of sun exposure and distance from the coast. In addition, flight tests in the laboratory were performed to estimate the potential dispersal capacity of the moth.

3. Occupancy rates were high and the likelihood of extinction depended on patch size. Small patches were less likely to be occupied than were large patches (> 10 m2). Sun-exposed patches were occupied for a lower proportion of years than were shaded patches. No distance effects could be discerned at the spatial scale of study, presumably because the insect is a strong flier.

4. Population densities in occupied patches decreased with increasing patch size. Furthermore, insect densities tended to increase with distance from the coast. Density changes in patches were synchronized.

5. The studied insect population can be described as a ‘patchy population’sensu Harrison (1991) with spatially correlated population dynamics. These dynamics are superimposed on a landscape gradient.