For the past 40 months I have been spraying a solution of drano and water twice a day on a colony of fruit flies. I started with a solution of 12.5% drano to 87.5% water. I was able to spray them without killing any of them. At a solution of 12.75 drano I had a substantial amount of flies drop dead.

I am 24 generations in (about 40 months) and I am now able to do a solution of 12.8% drano without any substantial loss of life. A solution of 13% seems to be the upper limit of safely spraying the colony.