Global change affects species in many ways. To understand spatio-temporal changes in population dynamics, monitoring data are needed. Social wasps (Vespidae) are ubiquitous in many environments. Using beer traps, we studied annual and seasonal population dynamics of workers of two social wasp species, Vespula vulgaris and Dolichovespula media, in south-central Finland over six years. The abundance of both species fluctuated annually. That of Vespula vulgaris displayed some cyclicity, unlike that of D. media; fluctuations were not fully synchronous among locations. Also, seasonal fluctuations were spatially and temporally variable. Vespula vulgaris was active later in the season, had longer flight period, and more abundance peaks than D. media, which generally had one short abundance peak. Our study suggests great spatio-temporal variation and species-specificity in the population dynamics of social wasps, which should be considered in designing monitoring protocols.