We have used this spray for years. Just spray directly on the insects or spray in the path of flies and it kills them.
Seems to work well during the fly season. I Use it all spring and summer. Easy to mount in pipe corral with the holder.
I have a Country Vet battery operated fly misting system for my stall that uses these refills. They last a full 30 days and keep my horse comfortable when he's in the stall for part of his day. Corro's price is the best I've found.
Prices for certain "must have" items have gone way up, but the price for this brand of metered fly spray is very reasonable.