Thanks for the suggestions. We have some wild birds (they flock around the stock pond). We regularly feed DE, but not garlic -- I'll try adding that. There is alternate shade, but they have to walk down to the barn, and they prefer staying out in the grass most of the day.
Tea tree oil really does seem to work. I got a small bottle and poured it into a little sprayer (travel-size hairspray

). For the horses that wouldn't let me touch their ears, I just sprayed the oil in. Not only are the sores slowly healing, but I saw one tick crawl right out of an ear after I had sprayed it, and we haven't seen new ones, which makes me think ticks might not like the oil.
Would orange oil sting the open sores? I've never used it.
Thanks
Judith