Red wood ants are ecologically dominant species that affect the composition of the invertebrate community. In the past century, one of these species, Formica paralugubris (Seifert 1996), was introduced outside its native area (the Alps), in the Apennines (Italy). Here we investigated the effect of the presence of an introduced population of F. paralugubris on the frequency of nests of other ant species in Abetone, Central Apennines. Ant nests were found only in the area unoccupied by F. paralugubris. In this area, we also found one pleometrotic association between queens of Lasius flavus (Fabricius, 1782) and two plesiobiotic associations, one between Formica fusca (Linnaeus, 1758) and L. flavus and the other one between Myrmica ruginodis (Nylander, 1846) and L. flavus. This latter is the first plesiobiotic relationship reported between these two species, the second reported for M. ruginodis and the 10th for L. flavus. Additionally, this is the first plesiobiotic association reported for the Mediterranean region and Southern Europe.