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• Biological control of Sirex noctilio in North America with Deladenus is explored. • The woodwasp's symbiotic fungus can impact nematode parasitism. • Amylostereum fungus species and strain affected nematode reproduction. • Mycophagous Deladenus juveniles never avoided non-host fungi. • Deladenus proximus could produce some eggs on non-host fungi. Dimorphic nematodes in the genus Deladenus have been used or are being considered for use in biological control of the invasive Eurasian woodwasp, Sirex noctilio , which threatens pine (Pinus spp.) trees. Deladenus species that are parasitic on Sirex can kill woodwasp eggs and occupy these same eggs for their own dispersal. These nematodes also have mycophagous phases that feed on the white rot fungal symbionts of Sirex , Amylostereum species. The mycophagous stage of the Hungarian strain of Deladenus siricidicola developed for control of S. noctilio in Australia feeds exclusively on A. areolatum. The mycophagous stage of a North American Deladenus species being evaluated for biological control, D. proximus , feeds on either A. chailletii, or A. areolatum. Amylostereum species and strains associated with Sirex have differential impacts on survival and growth of these nematodes. We investigated whether differences in species and strains of Amylostereum influence the numbers of Deladenus juveniles adjacent to cultures, as this would impact potential parasitism of Sirex. Fungal species or strain did not influence persistence of juveniles in the fungal vicinity although retention could be influenced by the fungal strain consumed by parents. Investigating D. proximus , we tested whether the most common invasive strain of A. areolatum associated with S. noctilio in North America (IGS D) impacted nematode growth, compared with the common native Amylostereum chailletii. Deladenus proximus increased very slowly when feeding on A. areolatum IGS D, compared with A. chailletii ; when provided A. areolatum IGS D, 55 eggs were produced after 4 weeks compared with 8.1 × 104 eggs after 2 weeks when A. chailletii was provided. In summary, behavior of Deladenus juveniles resulted in no or low avoidance of Amylostereum species and strains, regardless of poor growth when mycophagous forms fed on fungi on which growth was poor.. [ABSTRACT FROM AUTHOR]