Just about any kind of soap will kill bugs. Dawn or shampoo or anything. So for stalls, just about anything works. To put on animals, use the mild soaps like Dawn and cheap human shampoos and not detergents as you are going to leave it on them and not rinse it off.
Before Advantix and the other spot on flea killers for dogs, we’d throw towels in the washing machine after washing dogs and see the fleas dying… from the laundry detergent. Works for flies too. And ants. I pour cheap powdered laundry detergent on ant beds and add water.
You can also mix skinsosoft (cheaper on ebay than at avon) mixed with white vinegar and water and spray horses and stalls. It won’t kill bugs but it will keep them away. I think the cheap perfume smell is the thing that bugs cannot stand.
My friend makes a homemade fly spray with cedar oil but Callie was allergic to cedar so I never got the recipe. Bugs do not like cedar. One time a BO sprinkled cedar shavings in with the pine shavings to keep flies out of stalls and Callie broke out in hives.
Also, the original formula Listerine will not only cure rain rot (and prevent it’s return on legs and face and rump and anywhere), it will keep ants off of horses’ legs and keep flies and gnats and skeeters and ticks off of horses.