I'm gonna go get a bottle of the 15% stuff tomorrow. Black flies are more of a problem to me than mosquitoes and if it works for them I'm satisfied. And I know what you mean about deer flies. That is, if what I call "horseflies" are the same thing. I hike on a horse trail and those jokers know how to bite and draw some blood. I swear sometimes I feel like a small animal came out of the bushes and took a bite out of my leg.