The flies are thrips, and they can be taken care of by some strategicly placed flystrips. By adjusting humidity levels all you are doing is possibly slowing their reproductive cycle down...not good enough. If you plan on using that duster can your going to be doing that 2 times a day, and you are only displacing the live ones, the eggs will hatch and the cycle will continue. Go get yourself an organic pesticide. Azamax is your best choice, its expensive, but it can be applied upto the day of harvest without leaving any residue. Get down to your hydro store and pick some up, or go online and get it (ebay would be my choice). Good luck.