The invasive woodwasp, Sirex noctilio, has significant negative impacts on pine trees. Since the discovery of established populations of S. noctilio in New York State and Ontario in 2005, a parasitic nematode, Deladenus siricidicola, has been considered for release for biological control in the United States. North American Sirex control proves to be complicated, due to the presence of native species of Sirex which are not considered to be pests, different species and isolates of the obligate fungal symbiont of S. noctilio, Amylostereum areolatum, and the presence of different species and strains of Deladenus nematodes. Nematodes in the genus Deladenus associated with siricids have two different forms: a mycophagous form that feeds on the Sirexsymbiotic fungi and a form that parasitizes Sirex and is transferred from tree to tree during oviposition by Sirex. I evaluated the ability of D. siricidicola (Kamona strain) to reproduce when feeding on different isolates of A. areolatum and found that nematode populations persisted on all isolates tested. One of the slowest growing fungal isolates, ScyME, produced the most nematodes when the fungus was given five and ten days of growth prior to nematode inoculation, while the fastest growing fungus, Aussie, never produced the most nematodes. Although nematodes in all treatments produced eggs, D. siricidicola populations were unable to replace themselves when feeding on the fungal isolate SedDF. In another study, a culture of Deladenus nematodes established from a native Sirex nigricornis was identified as Deladenus proximus using molecular and morphometric techniques. I compared the ability of D. proximus and D. siricidicola (Kamona) to reproduce when feeding on native and invasive isolates of Amylostereum fungus. D. siricidicola were able to reproduce on all isolates of A. areolatum tested, but reproduced poorly on the A. areolatum isolate they would be most likely to encounter in northeastern North America, should the nematode be released. D. proximus were able to reproduce well on both A. chailletii and A. areolatum, despite prior evidence suggesting only A. chailletii is a suitable food source, leading to the suggestion that this native nematode should be evaluated for its ability to parasitize and sterilize S. noctilio. To study phylogenetic relationships among native Deladenus spp. in the northeastern United States and the Kamona strain of D. siricidicola, three genes (mtCO1, LSU, and ITS) from nematodes extracted from parasitized Sirex spp. collected inside and outside of the range of S. noctilio were analyzed. Results showed each Sirex species has its own associated Deladenus parasite. This study provided evidence that D. proximus can parasitize S. noctilio, and that D. siricidicola can parasitize S. nigricornis, indicating potential for non-target impacts of a biological control program using D. siricidicola against S. noctilio. In another study, I investigated a hypothesized role reversal wherein fungal hyphae invade and kill nematode eggs. D. siricidicola eggs were exposed to multiple isolates of A. areolatum to quantify the number of eggs lost to fungal invasion. A. areolatum and A. chailletii were observed via a combination of cryogenic scanning electron microscopy and fluorescence microscopy for their ability to parasitize both eggs and adults of D. siricidicola and D. proximus. This study reports the first evidence of a Basidiomycete destroying nematode eggs, as well as a novel trapping mechanism used to capture and parasitize two species of adult female Deladenus.