The conopid fly Stylogaster neglecta Williston (Diptera: Conopidae) is a parasitoid with no known host. We report this species as the first recorded dipteran parasitoid of Oecanthus nigricornis Walker (Orthoptera: Gryllidae) (black-horned tree crickets). We reared field-collected O. nigricornis juveniles over several months in 2017 and found that larval S. neglecta emerged from them during late July into August. We estimated the incubation period for S. neglecta larvae to be around 30 days based on the length of time it took for them to emerge from the host and pupate (subsequently all hosts died). We documented several cases of multiple parasitism. In 2018, we dissected O. nigricornis sampled from four sites across southern Ontario, Canada and upstate New York, United States of America and found that the percentage of juvenile O. nigricornis parasitised ranged 2–39%. Further sampling will be necessary to determine whether this variation represents consistent population differences or between-year variation in parasitism.

In 2017, we sampled O. nigricornis nymphs approximately every two weeks from 28 June until 13 September from old fields at the University of Toronto, Mississauga Campus (43.55°N, 79.66°W) (total = 131 O. nigricornis). The start date was chosen as the first day in which we were successfully able to sample O. nigricornis from the site. The end date was chosen as the first sample in which we no longer found any S. neglecta larva emerging from the sampled O. nigricornis. We also collected 21 O. nigricornis eggs in May of 2017 by cutting short segments rasberry stems containing eggs and then placing them in petri dishes lined with filter paper. The filter paper was sprayed with water daily and kept at 25 °C and 70% humidity. Nymphs that hatched were removed and reared individually. All crickets collected were given a letter code based on the day they were collected (A-H). We reared caught and hatched O. nigricornis individually in open-ended plastic cylinders, 8 cm in diameter and 7.5 cm long, with metal mesh covering the top end, at 25 °C and 70% humidity. Oecanthus nigricornis were fed pollen and dry dog food, and containers were sprayed with water every second day. We recorded the date on which O. nigricornis were caught, when they died, cause of death, and whether a parasitoid emerged. Stylogaster neglecta pupae found in the containers were removed and placed in a petri dish on filter paper and sprayed with water daily.