I treat ear mites with Diatomaceous Earth.
Drop a little bit (abouta quarter or less of a teaspoon) in the ears - preferrably when the cat is sleeping! You have to be extremely careful NOT to drop any in the eyes, as it IS silica, and will scratch the corneas. I have to say I *have* gotten rid of ear mites using it, when I adopted a sweet little girl who was just left outside when a family moved out of their house. Treated her twice within a month or so, and never had to do it again, and she's never had the dark gunk coming out of her ears again after treating the mites.
She was about six months old, and not spayed....I knew what HER future would be if I did not take her. She's turned out to be one sweet kitteh, and very healthy! Hubby just picked her up for me to bathe this morning, and commented on how much she weighs! A huge change from the long, tall, skinny thing she was when we got her.