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Permethrin
Best thing I have found so far for ticks. I ride through some cattle fields / heavily deer populated areas that are just full of ticks, and this is the only thing I have found that really works.
My horse never gets bites if she has been treated with it, but I WILL find dead ticks on the surface of her hair, down on her legs - where they started to make their journey, and died quickly.
Now Permethrin is an insecticide, and not a repellent. To repel and kill, you can use a two way attack recommended by the CDC and use DEET in addition to Permethrin (DEET is a repellent only).
Deep Woods Off and similar products are 25% DEET. You can also buy 100% DEET at camping stores (Jungle Juice is a common brand).
Permethrin is in many fly sprays, but you can also but 10% concentrate. This is what I do, and I add additional Permethrin to store bought spray to up the effectiveness.
http://www.cdc.gov/ticks/avoid/on_people.html
Edited to add - “Equispot” is 45% Permethrin[/QUOTE]
This bears repeating. I am in the epicenter of the Lyme disease epidemic. You can try equispot, it never worked for me. I use concentrated permethrin (bought at Tractor Supply) which I then dilute. Very cheap and very effective.
If it were my horse, I would absolutely not rely on any UNPROVEN treatments/repellants etc. That means, I would not rely on garlic etc. If you want to go all natural, Rosemary Oil has been studied by the Maine Dept. of Agriculture and found to be effective. However I am not aware of any place you can buy it in large enough quantities, or exactly how you would apply it to horses.