The first part of this dissertation details studies involving an important group of insects attacking oaks, the oak gall wasps (Cynipidae: Cynipini). The second part deals with pteromalid parasitoids involved in thousand cankers disease affecting walnuts. Oak gall wasps are parasites that mainly attack oaks and induce highly differentiated plant growths, called galls, in which they develop. Most gall wasps alternate between dimorphic asexual and sexual generations. However, most species are only known from one generation with the other remaining undescribed. The taxonomy across much of Cynipini is in need of revision. Ultraconserved elements are used to collect phylogenomic data (average 956loci per specimen) for the oak gall wasps. Numerous genera were found to be polyphyletic with separate Nearctic and Palearctic lineages. The ancestor to Cynipini is thought to have been Palearctic based on the taxa sampled. One of the genera found to be polyphyletic, Disholcaspis Dalla

Torre and Kieffer, is being thoroughly evaluated in preparation for revision. These efforts include the discovery and identification of new sexual generations, discovery of a new species, locating type specimens, and imaging of species in the genus. The molecular tools used for the identification of sexual generations exposed a great need for more loci, lending support for why phylogenomics is a valid option for this group.

Thousand cankers disease is threatening cultivated and natural walnut tree populations. The disease is caused by a phytopathenogenic fungus, Geosmithia morbida Kolařík , Freeland, Utley, and Tisserat, that is vectored by the walnut twig beetle, Pityophthorus juglandis Blackman. Studies that have reared insects from trees infected with thousand cankers disease have resulted in the discovery of two new species of parasitoid wasps in the subfamily Cerocephalinae (Pteromalidae). The first was Theocolax americanus McEwen which was found in Colorado, USA though the natural origin and host are unknown. The second was Cerocephala flavus Cooke-McEwen which was found in the Piemonte region of Italy though its natural origin is also unknown. Cerocephaline wasps are known to attack wood boring beetles so these parasitoid wasps are thought to be associated with P. juglandis.