Six, Diana L.Marincowitz, SeonjuDuong, Tuan A.2026-04-102026-04-102026Six, D.L., Marincowitz, S. & Duong, T.A. Ophiostoma ipsi-confusi sp. nov. Six, Marinc. & Duong, a consistent symbiotic fungus of the pinyon ips bark beetle, Ips confusus LeConte. Symbiosis (2026). https://doi.org/10.1007/s13199-026-01135-9.0334-5114 (print)1878-7665 (online)10.1007/s13199-026-01135-9http://hdl.handle.net/2263/109507DATA AVAILABILITY : Sequence data is available in GenBank.Several tree-killing bark beetle species have nutritional mutualisms with specific fungi. However, few secondary bark beetles (those that colonize weak or dying trees) have been investigated for symbiotic fungi and most are thought to have only incidental fungal associates and no dependence on fungi for nutritional or other benefits. In contrast to this supposition, we consistently isolated (> 97%) a fungus from adult Ips confusus (pinyon ips) collected from Pinus edulis (two-needle pinyon pine) from Arizona and New Mexico, USA. Using morphology and DNA sequences for three gene regions, we found the fungus is most closely related to an obligate mutualist fungus of Dendroctonus ponderosae (mountain pine beetle), Ophiostoma montium (Ascomycota: Ophiostomatales), but is morphologically and genetically distinct from it and other known species in Ophiostoma. It is also capable of growth at relatively high temperatures compared with other Ophiostoma, reflective of its southwestern USA distribution. The high frequency of its association with the beetle indicates it is symbiotic and suggests it may be a mutualist.en© The Author(s) 2026. Open Access. This article is licensed under a Creative Commons Attribution 4.0 International License.MutualismOphiostomatalesPinus edulisScolytinaeSymbiosisArticle