This may be an advantage if animals are moved at intervals and dust bags or back rubbers are not in place in every pasture or grazing area. Research with insecticide ear tags fitted with a dye showed that the area immediately behind the ears and forward to under the eye and down to the back of the mouth are treated pretty thoroughly. No dye was transferred to the legs, back-line, belly or hips. There is additional spread of the insecticide as the animal grooms itself and mingles with other cattle. Differences in behavior between these two fly species is probably part of the story.
Horn flies spend most of their time on the animal.
They suck small amounts of blood frequently and leave only to lay their eggs on fresh cattle droppings. This constant presence on the animal means they are certain to come in contact with some of the insecticide and be killed. Face flies spend only a small amount of time on the animal. Usually, they are off resting on vegetation. They visit the face, primarily under the eyes and around the mouth.