The European species of the genus Tetrastichus (Insecta, Hymenoptera, Eulophidae, Tetrastichinae) are revised with 93 species, including 50 species described as new. The revision was conducted using an integrative taxonomic approach, based on DNA barcoding in combination with morphological characters. The Tetrastichinae are a biologically diverse and species-rich group of parasitoid wasps with numerous complexes of morphologically often very similar species that attack a wide range of hosts in over 100 insect families in 10 different orders. The genus Tetrastichus is, with almost 500 described species, the third largest genus of Tetrastichinae. Although biological information is lacking for most species, current data indicate that Tetrastichus species are gregarious koinobiont endoparasitoids developing on juvenile stages of mainly holometabolous insects. Due to their host specificity, several species of Tetrastichus are used as biological control agents.
The European species of Tetrastichus Haliday (Hymenoptera: Eulophidae) are revised using a combination of externo-morphological and DNA barcoding data. This is the first integrative approach for any of the large genera of the Tetrastichinae. A total of 93 species are included, of which 50 are described as new: T. agonussp. n., T. antonjanssonisp. n., T. argeisp. n., T. argutussp. n., T. asilissp. n., T. ballotussp. n., T. blediussp. n., T. broncussp. n., T. calcariussp. n., T. calmiussp. n., T. clisiussp. n., T. cosidissp. n., T. cumulussp. n., T. cyprussp. n., T. delvareisp. n., T. doczkalisp. n., T. elanussp. n., T. elodiussp. n., T. ennissp. n., T. enodissp. n., T. erinussp. n., T. evexussp. n., T. fadussp. n., T. fenrisisp. n., T. flacciussp. n., T. grediussp. n., T. iasisp. n., T. illydrissp. n., T. incanussp. n., T. inscitussp. n., T. intruitussp. n., T. johnnoyesisp. n., T. lacustrinussp. n., T. ladrussp. n., T. laniussp. n., T. laziussp. n., T. lixaliussp. n., T. lycussp. n., T. marcusgrahamisp. n., T. miniussp. n., T. mixtussp. n., T. nataliedaleskeyaesp. n., T. nymphaesp. n., T. pixiussp. n., T. scardiaesp. n., T. splendenssp. n., T. stisp. n., T. suecussp. n., T. tacitussp. n. and T. tartussp. n. Two keys for the identification of species are presented, one for females and one for males. Based on DNA barcode sequences for 70 of the species, a Maximum Likelihood tree to assess phylogenetic relationships within the genus is presented. These 70 species are also characterised by a combination of CO1 and morphological data. The remaining 23 species, without a DNA barcode, are characterised by morphological data. Using a combination of data from the morphology and CO1 or morphological data only, the species are separated into three species groups (clito-, hylotomarum-, murcia-groups) with 41 unplaced species outside these groups. Hosts are known for 27 of the species and they are gregarious, koinobiont endoparasitoids on a wide range of immature stages of holometabolous insects and appear to be very host specific. The first host record for Lepidoptera (Tineidae) in Europe is included.