A highly diverse microcosm in a hostile world: a review on the associates of red wood ants (Formica rufa group)

The importance of Eurasian red wood ants (RWAs) (Formica rufa group) in forest and heath ecosystems has long been recognized. One key function of RWAs is the role of their nests in supporting an intriguing ecosystem of a highly diverse group of obligate myrmecophiles and facultative guests. In this review we list 125 obligate arthropod myrmecophiles that occur in RWA mounds or in the near vicinity of the mounds. More than 40 % of them are Coleoptera, but also Hemiptera, Diptera, Hymenoptera, Acari and Araneae are well represented. RWAs are estimated to be the primary hosts for 49 species. 24 species were hitherto only recorded to be associated with RWAs, 12 with both RWAs and other mound-building Formica species and 9 were found to be associated with both mound-building and non-mound-building Formica species. The remaining associates are less specific and can be found with other ant genera or ant subfamilies. Other mound-building Formica ants (Coptoformica, F. uralensis and F. truncorum) support fewer species, most of which are known to also occur with RWAs. We discuss the biology of the different obligate myrmecophilous groups and give general notes on the facultative guests found in RWA mounds. We stress the importance of the conservation of RWAs as hosts of one of the richest and diverse associations known to date in insects.