Yep as Ed said DEET does nothing to them. On the "PA Fish-In" last year we were lucky enough to be there right after they released all them SOB's and I was slathered up in 100% DEET to the point of having birds falling out of trees within a 100ft radius of me and yet the flies still swarmed me! Those things were driving me nuts last year on that trip, I almost slapped myself silly tryin to kill them.

Steve