It sounds like technology at its best: devices emitting ultrasound waves that will drive away insects so your horse can have some peace. They’re available as room/stall units or even small medallion type units you’re supposed to braid into the mane or tail, or hang from the halter. With the attach-on devices only costing around $10, and many indoor units well below $50, what more could you ask for'
For starters, they need to work and there’s no evidence that they do. While a few species of large nocturnal moths seem to react to, and try to avoid, the ultrasonic radar given off by bats hunting for their dinner, not even all of these moths react.
It’s been found that moths of the same types that come from areas where there have never been bats don’t show this behavior. Bugs that communicate by ultrasound, like crickets and katydids, may also react to ultrasound devices, but only in narrow and species-specific wavelengths and frequencies.
Research has shown that the only insects that exhibit the ”acoustic startle response” to ultrasound and might avoid it — in the right wavelength and frequency — are insects that fly at night.
This means that claims of ultrasound devices repelling anything and everything, including ticks and flies, goes against everything known about insects and there are no studies that prove these devices work on that type of bug. Mosquitos, of course, do fly at night, but there are at least a dozen studies specifically with mosquitos that show ultrasound doesn’t repel them.
Even if the electronic ultrasound repellents did work, the small units you’re supposed to hang on the horse wouldn’t. The signal would have to encircle the horse from nose to nail, ground level to top, but these devices only work directly below and around the device itself. One hung on the bottom of the halter would only influence the area along the front of the neck and chest, and underneath the jaw. Put that $10 to better use.