Population density and individual dispersal behaviour affect species’ distribution dynamics. Population densities vary over time, and some species occasionally increase to very high numbers, for example during outbreaks. In such situations, populations are expected to expand into new areas as a result of density-dependent dispersal which sometimes even results in range expansion. A local population of the northern pine processionary moth Thaumetopoea pinivora has recently reached outbreak densities at the edge of its northern range at the southern tip of Gotland Island in the Baltic Sea. We first investigated whether the outbreak had resulted in establishment of populations in suitable habitats on Gotland Island outside the outbreak area. Six small populations were found that could potentially have originated from the outbreak area. However, data from 12 microsatellite markers strongly suggest that these populations did not originate from the recent outbreak. Genetic variability was not reduced in these small, isolated populations, and there were several unique alleles, indicating instead a different population history and that there has been no recent range expansion. In addition, there was apparent genetic isolation by geographic distance, implying that despite the high density of the outbreak population, significant gene flow has not occurred.