There are quite a few people at our barn who use Pyrahna and it has worked really well... but after time flys do get resistant to them. So this year it hasn't worked quite as well as it has in the past. (Bronco does absolutely nothing for our horses btw). No reactions, etc. Plus, it smells great!
Pyrahna = awesome. It's a MUST at paint shows here in NJ... everyone uses it, because it's AWESOME.
Also, Bronco--it's basically water in a bottle, that's why it doesn't work. Also why it's so cheap. lol!