This is a recipe used at Farm Sanctuary when I was there. It didn't seem that effective, but then nothing seems that effective. They mix up the following and spray it on a rag and rub it on the cows. We're going to try it on our donkeys this summer and see how it goes.
Vinegar — 1 cup
Water — 1/2 cup
Eucalyptus oil — 1 tablespoon
Citronella oil — 1 tablespoon
Tea tree oil — 1 tablespoon
Skin So Soft bath oil — 4cc