This study presents the first case of an ectoparasitic fungal infection of ants from southern Russia with the description of a new species Xenomorphomyces formicartus V.A. Iliushin & I.Y. Kirtsideli, sp. nov. of a new genus Xenomorphomyces gen. nov. belonging to the family Readerielliopsidaceae. The new species was found on Lasius distinguendus worker ants within a single nest in the Botanical Garden of the Southern Federal University, Rostov-on-Don, Rostov Region, Russia. The paper evaluates the phylogenetic position of the parasitic fungus within the Readerielliopsidaceae family based on the morphological observations and multilocus phylogenetic analysis. The family Readerielliopsidaceae belongs to the Capnodiales order, the class Dothideomycetes. Partial sequences of the loci encoding ribosomal polymerase II second-largest subunit (RPB2), 28S rDNA (LSU) and internal transcribed spacer rDNA region (ITS1-5.8S-ITS2) were analyzed. The asexual morph of X. formicartus was described. A comprehensive analysis of pathogenicity was performed using live ants, histological and micro-CT studies. The sequence data, as well as macro- and micromorphological characteristics, distinguish X. formicartus from all known species in the Readerielliopsidaceae.

A new scuttle fly species, Microselia rossica sp. nov. (Diptera: Phoridae), a parasitoid of the ant species Camponotus vagus (Scopoli, 1763), is described from the floodplain habitats of the Rostov Province in southern European Russia. This is the first confirmed case of parasitisation by scuttle flies of worker ants in Russia. The behaviour of the females of the new species attacking the ants is described.

New data on the distribution of six ant species in Rostov and Volgograd provinces are given. Four species, Lasius myops Forel, 1894, Cardiocondyla sahlbergi Forel, 1913, Strongylognathus rehbinderi Forel, 1904, and Temnothorax interruptus (Schenck, 1852), are recorded for the first time for Rostov Province, the latter species is also recorded for the first time for Volgograd Province.

A new species of scuttle flies, Menozziola tanaitica sp. n. that’s a parasitoid of ants Camponotus vagus Scopoli was found in the lower reaches of the Don River in the Rostov region, Russia. The new species is similar to Menozziola obscuripes (Schmitz, 1927) but differs in large number of hairs on epandrium and anal tube. This is the first confirmed case of parasitization of scuttle flies on ants in Russia.