ive never heard of fly spray being used, thats both sick, and pointless.
the chemicals used in fly spray are VERY poisonous, it dosent affect humans purely because we are millions of times bigger than flies, so they only need a small dose to kill them.

sounds like an urban myth to me, why would somone put time and effort into growing nice ( even commercial grows need to be reasonably good )plants and spray them with concentrated poison?

sorry, but i dont buy that for a second, anyone selling that on a commercial scale, would either be killed by thier outraged customers, or die from fly spray inhalation, or at least develop some nasty tumours or somthing.