What are fly boots?
The flies are really bad around here too. I find most sprays do not last very long. We use fly Wipe, which has a bit more staying power. It is expensive at $45 Cdn a bottle, but will last all summer for a few horses. We apply some on a rag (like a blue J cloth), and wipe it on the horses. The rag is stored in an empty coffee can, and the rag can be used again and again without reapplying new wipe to it each time.
The down side is that this is obviously a strong insecticide, oil based, that is being put on the horses skin. But to compare it to people bug spray, the mild stuff just doesn't work.