How do i make a good fly repellent for my horse who has just developed sweet itch?

my pony is 10 and has never had sweet itch before, but in the last few weeks has started to get it on her tail and under her mane any ideas of a home made fly spray which will help. ive been using citronella and eucaliptis oil mixed with water but doesnt last long.How do i make a good fly repellent for my horse who has just developed sweet itch?
I use natural fly spray and its been great.

clarins