A farmer my son knew had a sick calf that he didn't have time to deal with and gave it to my son, hoping he could nurse it back to health and then keep it. So this calf had some kind of parasite/bugs on the face. I mixed up tea tree essential oil and water (1 T. oil to 1.5 cups water) in a spray bottle. My son sprayed this on the calf in the face a couple of times a day. It did seem to rid her of the parasite. We also noticed that flies were no longer on her face. If we sprayed her whole body she would be fly free for hours. I guess this would be an expensive long term solution (and not very convenient)but for a sick animal who needs to be spared the further irritation of flies, it works.
By the way, my son has had the calf for 6 weeks or so by now. She is doing well. I think she is going to make it! She is very sweet and friendly and hangs out with our big cows.
~Mama to Many~