Abstract1. Migration into local populations may increase the likelihood of persistence but emigration may decrease the persistence of small and isolated populations. The dispersal behaviour of a day-flying moth Zygaena filipendulae was examined to determine whether emigration is correlated positively or negatively with population size and host plant density. 2. A mark–release–recapture study showed that most moths moved small distances (< 40 m on average) and only 6% of movements were > 100 m. 3. Twenty-five individuals moved between populations, a measured exchange rate of 8%. Moths were more likely to move between patches that were close together and they moved to relatively large patches. 4. The fraction of residents increased with increasing population size in the patch and increasing host plant cover. Relatively high proportions of individuals left small patches with small moth populations. 5. Moths released in grassland lacking Lotus corniculatus (the host plant) tended to leave the area and biased their movement towards host plant areas, whereas those released within an area containing L. corniculatus tended to stay in that area. 6. Biased movement away from small populations and areas of low host plant density (normally with low population density) was found. This migration-mediated Allee effect is likely to decrease patch occupancy in metapopulations, the opposite of the rescue effect. The effects on metapopulation persistence are not known.