Abstract: Ants have been suggested to protect forests against insect defoliators. Here, the association between colony size and foraging distances of two common Eurasian multinest-breeding, mound-building ant species, Formica aquilonia Yarrow, 1955 and Formica exsecta Nylander, 1846, are compared, and the results are discussed in the context of the species' potential to protect trees from insect defoliators. The distance to the most distant foraging tree was over 100 m in a large nest of F. aquilonia, whereas always less than 10 m in F. exsecta. Foraging distance increased with an increase in colony size in F. aquilonia, but not in F. exsecta. Nest mounds were clearly larger in F. aquilonia, but the foraging distance differences were not due to nest size differences. Foraging distance was larger in F. aquilonia even when the effect of colony size was controlled for statistically. The smaller foraging range of F. exsecta could be compensated by its ability to form dense nest populations, but in Fennoscandian boreal forests it lives in temporarily open habitats and disappears before canopy closure. Therefore it may have a more limited use against insect defoliators than F. aquilonia. Formica aquilonia instead, like other polydomous members of the Formica rufa Linnaeus, 1761 group, also lives inside older forests and has large foraging areas and wide-spread multi-nest colonies. Thus it may have greater potential to protect trees against defoliator insects during the lifespan of a forest.