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: