OP, I hope you find something that works for you! I don’t think people are saying what they use is better than what others use. Rather, I think they are trying to help you find an alternative that may work in your situation. It really does seem to depend on climate, what has been used heavily in your area previously, the conditions of your property and also neighboring properties, and even the individual horses (coloring, skin secretions, sweat production, and such).
What I’ve found is along the same lines as others who have responded. What works well for a period of time becomes less effective at some point. I assume it’s because the insects develop a resistance?
I had GREAT results w/ Eco Vet up until this year. It’s still working well for black flies, horse flies, deer flies, and ticks but mosquitos, stable flies, and house flies are not as bothered by it anymore.
Clac (also called Deolotion) has been my go to for years to repel mosquitos and to boost up the effects of the Eco Vet if need be. Now, the house flies are not deterred. I’m having better luck by layering Outsmart under the Eco Vet or even the Absorbine Black or Red in very hot/humid weather. The Outsmart makes my beasts all want to roll so it must tingle.
I did try a product this year that appears to be similar to Eco Vet called Fly Bye Plus. I was hoping I could rotate that into my system for a year and then return to Eco Vet. Unfortunately, one of my horses had a reaction to that one although it does repel the bugs on my other two. It is greasy, you spray more than you do of the Eco Vet, and it leaves a film, so it’s not my favorite. It may work really well for others though.
Oh, and I saw really good results w/ Essentria a few years ago when mixed w/ mineral oil and wiped on the horses. Another of my herd was allergic to that one and that was the end of that. I was using the remainder of the concentrate as a stall spray (mixed w/ water instead of oil) but it seemed to lose it’s effectiveness very quickly.
Sorry I couldn’t point you to any studies, OP! While I can’t offer you scientific data, I did try to give you some options that have worked for me in the past with the hopes that one of them might work for you for a period of time as well.
Best of luck.