Chronicle of an impact foretold: the fate and effect of the introduced Formica paralugubris ant

Starting in 1958, red wood ants (Formica rufa group) from the Alps were transplanted to several Apennine forests along the Italian peninsula to be employed as biological control agents for tree insect pests. In the Campigna Biogenetic Nature Reserve, central Italy, hundreds of mounds of the dominant Formica paralugubris were repeatedly introduced, creating several populations that still survive today. In this study, we analyzed the temporal dynamics and the ecological impact of five of these populations. Their present state was assessed by censusing the total number of nest mounds and their volumes, while past changes were reconstructed from literature records. We also evaluated the impact of this species on autochthonous arthropod communities by comparing impacted and non-impacted areas and performing predation experiments. The density of nests and their volume remained stable or declined for a few years after transplant, and then they began to grow steadily. Local arthropods were severely affected, since almost all collected groups were significantly less represented in impacted than in non-impacted sites. Additionally, experiments with live bait demonstrated that potential prey have a significantly greater probability of being consumed in the areas occupied by F. paralugubris. These results prompt a thorough assessment of the fate of the introduced red wood ant populations, since their role as biological control agents has to be traded against the ecological impact on native arthropod communities. This is particularly relevant for highly biodiverse areas, such as the Campigna forest, that are home of several invertebrate species with conservation interest.