Absolutely the best repellant/killer I use for all flying bugs is d-limonene. It's concentrated citrus oil. Buy it in spray bottles at wal-mart, the "de-solv it" brand of "contractor's solvent." Spray this on surfaces outside, and flies, skeeters, wasps - anything that flies, stay away. Put it on your skin too.
It also is 100% non toxic to vertebrates, and works much the same way DE does - by dissolving the insects' exoskeleton wax covering on contact. It's exactly what they put in 7-up, Mtn Dew and sprite - to give those their "limon" taste and smell.
http://www.orange-sol.com/household/moreretailers.html