What voltage do electrical fly killers use?
I visited a factory making the 50/60Hz transformer type some years ago. They were several kV (3-4kV) and similar in construction to ballast transformers for neon signs, but without the magnetic shunts to increase the output impedance.
These ones create quite a profound arc which is intended to burn the insect off the grids. Here's a photo of one from Alibaba:
One which I repaired once used rectified 240V mains with a doubler, so about 680V
Here's a teardown video of a cheap plug-in type, which uses voltage doublers to product -630V and +630V across the grid:
https://www.youtube.com/watch?v=LtXmsxRdiUw