When the flies are bad I use dirty engine oil. It is liberally wiped on the hair, not rubbed in. The bugs do not like it. It doesn't harm the horse and it lasts about three days. After an oil change I leave the oil sit uncapped for 24 hrs to dissipate any gasoline that might be in it. You can also use chain saw oil which contains a lot of sugar and no they don't try to eat it. It's thick and also doesn't harm the animal. Saw oil is red.