Presumably ovipositing, otherwise I don't understand the curious dance this colourful fly was doing. Quite the display given its wing and body colours and patterns.
This is a wild headline from NPR: "A fungus is turning cicadas into horny zombies — but don’t panic."
"[The fungus] Massospora cicadina has been observed in cicadas in more than half a dozen states — both those belonging to Brood XIII, which emerges every 17 years and is concentrated in Illinois, and of Brood XIX, a 13-year group that is distributed across much of the southeast."