Discovering dark taxa: Towards a revision of the jewel wasps in the subfamily Tetrastichinae (Hymenoptera: Eulophidae)

Insects are by far the most successful group of animals. They occupy virtually every region and habitat on Earth and constitute roughly half of all described species on our planet. Although it has been estimated that there is perhaps as many as 20 million species of insects, only a fraction of them have been described to date. This leaves millions of them undiscovered and thus undescribed – a daunting yet fascinating task for science. A substantial part of these undescribed species is found within the hyperdiverse jewel wasps (Hymenoptera: Chalcidoidea) – minute insects parasitising other insects. These tiny jewels are vital to terrestrial ecosystems worldwide as key insect population regulators, and many species are used in biological control in agriculture and forestry to eradicate pest insects. Despite their ecological and economic importance, chalcidoids remain vastly understudied. This is particularly true for the subfamily Tetrastichinae (Eulophidae) – a taxonomically challenging group due to their minute size, morphological similarity and the large number of species. The aim of this thesis is therefore to shed some light on the taxa in this group, with a particular focus on the European taxa in the so called Tetrastichus group within Tetrastichinae. With this thesis I present the first detailed phylogenetic hypothesis for the subfamily, revealing a rampant para- and polyphyly at the genus level as well as many undescribed and cryptic species, particularly within the heavily sampled genera Oomyzus Rondani and Quadrastichus Girault. Further, I identify a third previously unknown main clade within the subfamily, the Apotetrastichus group, and reveal the urgent need for a major taxonomic revision of Tetrastichinae. I also demonstrate that a whole-genome sequencing approach outperforms the commonly used target-enrichment approaches by recovering substantially more data, and also by successfully resulting in draft genomes for insects as small as microhymenopterans. By using a combination of DNA barcoding, whole-genome sequencing and morphological analyses, I also initiate the taxonomic work desperately needed for Tetrastichinae. First, I revise the genus Tamarixia Mercet and describe five species new to science: T. borealis sp. nov. Kärrnäs, Röttgers & Hansson, T. hellenica sp. nov. Kärrnäs, Koutsoukos & Hansson, T. querci sp. nov. Kärrnäs, Röttgers & Hansson, T. syki sp. nov. Kärrnäs, Röttgers & Hansson and T. xanthospila sp. nov. Kärrnäs & Hansson. By doing so, I also present a robust and holistic approach to describing new species. Furthermore, I synonymise the tetrastichine genus Chaenotetrastichus Graham under Tetrastichus Haliday and transfer the species Oomyzus sempronius (Erdős) to Tetrastichus, redefine the genus Quadrastichus and describe the new genus Acaristichus gen. nov. While I already here start addressing some of the taxonomic revisionary work needed for Tetrastichinae, obtaining a natural classification of this hyperdiverse eulophid subfamily is a monumental task that will largely be left for future taxonomic studies to address.