Insects that bite horses or suck the blood of horses rely mainly on smell to find and home in on their prey. Although some insects will to some extent use other senses, such as sight, smell is the critical factor for homing in on their prey.
In addition to the importance of smell in finding horses, it is also important in terms of selecting a horse. In fact, if a fly finds a horse but it smells bad, the fly will virtually never bite the horse or suck its blood.
All fly repellents, whether artificial or natural, chemical or organic, are based on these two facts. Their work by releasing strong scent modules, which confuse the fly (the smell masks the smell of the horse, so they cannot find it) or is so unpleasant that the fly does not want to bite (as if someone poured sewage over your dinner plate), or a combination of the two.
Note that not all strong smelling substances will work in this way. For example, an old practical joke fly repellent for horses among hunters and fishers is to advise their younger counterparts to wear perfume in the forest to mask their smell. However, the smell of the perfume is not unpleasant to biting insects and it does not mask the smell of the person but in fact makes it easier for them to find the person, resulting in the person wearing it getting bit even worse. Consequently, a successful fly repellent needs to be not only strong but also be tested to see if it is effective.
In addition, smell does not affect all flies in the same way; what is offensive to one species will have little effect on another. For this reason, I use different repellents at different time of the year depending on which type of fly is predominant. The most extreme case of this I’ve found is with biting midges (the source of horse ‘summer itch’), where there is only one product of all the products I’ve tested which is highly effective against them, but this product is ineffective against all the other local biting flies. Consequently, it is not accurate to say a fly repellent is effective or not. Rather, one should say that a given fly repellent is effective against certain types of flies.
Another big consideration for horse owners is the duration of effectiveness. In my experience this varies from half a day, up to a week, depending on the repellent. A long duration repellent is obviously preferable as one does not need to reapply it as often.
Most repellents are sprayed on the outside of the horse, where they remain and release scent until they are used up. An alternative approach is to add something smelly to the horse’s daily feed (garlic is a popular choice), which travels through their body and is released in sweat, where they release scent molecules to repel flies. The effectiveness of the latter method varies from horse owner to horse owner, with some reporting very good results and some reporting poor results. In any case, before adding anything to a horse’s feed, one should first discuss with a veterinarian if the material and quantity is appropriate from a health perspective.