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.

1. Variation in mortality due to parasitoids is expected to be affected by host sex and size. 2. We use selection analyses to examine the effects of Stylogaster neglecta (Diptera: Conopidae), a recently discovered parasitoid of nymphal Oecanthus nigricornis (Orthoptera: Gryllidae), on sex and size viability in the host crickets. 3. O. nigricornis nymphs were sampled across Ontario and New York State and dissected to determine whether they were parasitised. Logistic regression was used to examine parasitism by host body size and sex. 4. There was strong evidence for an effect of sex and body size on parasitism, with males and smaller crickets being more likely to be parasitised and containing more parasitoids in them. There was also weaker evidence of an interaction effect between sex and body size, with body size being a significant predictor of parasitism only in males. Using a selection analysis, we found evidence of positive and disruptive selection on male body size. 5. Causes of these viability differences may include sex differences in larval cricket immune function, riskier behaviour of male nymphs as they acquire resources necessary for adult trait development, as well as the effects of S. neglecta flies targeting hosts that maximise fitness.