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We investigated theBotanophila(Diptera: Anthomyiidae)– Epichloë (Ascomycetes: Clavicipitaceae) interaction in cultivatedFestucaspp. (fine fescue) in Oregon in western USA.Epichloë spp. are endophytic fungi of grasses in the subfamily Pooideae. They develop a felt-like stroma on the surface of grass culms and a dense mycelium within the culms that typically prevents seed head emergence. As a result, seed yields are suppressed, and hence the disease is known as choke. Studies ofEpichloë spp. on wild grasses indicate that the fly–fungus interaction is an obligatory mutualism. During oviposition,BotanophilatransfersEpichloë spermatia between stromata of opposite mating types, and the perithecia that develop after fungal fertilization serve as food forBotanophilalarvae. In the current study, we surveyed 19 cultivated fields ofFestucaspp. in Oregon, and observed choke caused byEpichloë festucaeLeuchtmann, Schardl and Siegl in 10 of these. However, perithecia were observed in only four fields, and on only 1.0–2.6% of stromata. Perithecial development was also low, and rarely covered 50% of the stroma surface. Despite the absence or low frequency of fertilized stromata,Botanophila lobataCollin larvae were present in all choke-infested fields. Infestation levels ranged from 2.5 to 70.7%, based on an examination of 175–200 stromata from each field. Only eight (= 2%) of the 450 stromata withB. lobatahad perithecia, and the greater majority ofB. lobatalarvae completed development and exited from unfertilized stromata. This is the first report of theB. lobata–E. festucaeassociation in the USA, and ofB. lobatalarvae developing successfully on unfertilizedEpichloë stromata. The average pupal weight (0.0032 g) did not differ significantly from pupae (0.0030 g) originating from larvae that had developed on fertilized stromata ofE. typhinaonDactylis glomeratain a neighboring field. This result indicates that in cultivated fine fescue fields in Oregon, B. lobataforages onE. festucae, but fly development is not dependent on the fertilized stromata ofEpichloë. [ABSTRACT FROM AUTHOR]