I've used flowers of sulphur one hot summer on my horse when he was bitten by tiny red ants which was in our soil. Successful but unfortunately, most chemists in my area don't seem to stock it anymore. I was told to sprinkle the powder on the backs of the legs which seemed to do the trick. Not sure if its successful in all cases. Trial and error I guess.
It is supposed to help reduce reactions by lowering the blood temperature (according to my trainer's client who is a doctor) so therefore reduces reactions to things like fly bites.