Flies can be gone in a DAY!
Matins Permethrin Insecticide, 13.30%,
https://www.tractorsupply.com/tsc/product/control-solutions-permethrin-133-32-oz?cm_vc=-10005
PZD Sweet PDZ Stall Refresher
horse stables spray permethrin for flies and works like a charm. You should be using it anyway for mite, lice, ant control. spray everything, chickens too. The ground should be good for ten to fourteen days after spraying. A light broad cast of the PZD will do wonders but both together is a sure fix. Now it doesn't mean you shouldn't be keeping good house keeping the area dry and clean but this will flat out make you go WHY DIDN'T I DO THIS BEFORE! lol.