Some parasites stop here as we see in the genus Lasius. But some Formica take it to a whole new level. Formica pergandei (most likely identification) is one such species that's recently moved into my yard.
Throughout the eastern and Midwestern US, F. pergandei is the premier dulotic Formica in grasslands and open woodland habitats, utilizing virtually all other Formica species (except other F. sanguinea group species) as hosts. The host workers are obtained through regular nest-to-nest raids to steal brood of the host. Often, some stolen larvae are eaten, especially any pillaged sexual brood, but at least a portion of the pupae are reared by host workers in the F. pergandei nest. Raids start in the morning, slow down or cease in mid-day, then pick up again in the late afternoon. F. pallidefulva and F. subsericea are the prevalent host species in woodlands, and F. incerta or F. montana prevail as the host in drier and wetter grassland habitats, respectively. Any one, or often, a combination of these species may occur among the host workers, with lesser numbers of other species from the pallidefulva, fusca or even the rufa-microgyna groups also in the mixed population. In Lincoln Co., one F. pergandei nest contained a mélange of six host species, including (in order of decreasing relative abundance) F. pallidefulva, F. subsericea, F. biophilica, F. dolosa, F. incerta, F. obscuriventris, certainly the most species-rich, naturally occurring ant colony on record! Nest architecture, and more interestingly, total mixed colony population level, seem to closely approximate those of the host, such that F. pergandei parasitizing F. montana or F. subsericea typically lives in a mound with the typical mound structure and a population like those of these host species, while those parasitizing F. pallidefulva often live in a moundless soil nest or in rotten wood with the much lesser total numbers of workers, as does this latter host. F. pergandei workers in the mixed nest do not care for brood, gather sweets, or effectively contribute to nest maintenance.
Overall this seems like a fun addition to the normal array of ants found in my yard. I've lived here for 26 years and never come across this species. I can't help but feel it's to do with my fairly recent hobby of gardening with native plants. What's most exciting though is I only ever see this type of ant in the forested areas at the nature preserve I visit now and then. Hopefully I'll be able to post more on this species as they slowly devastate the colonies of Formica around the yard.