I hate using chemicals (of any kind) for bug protection. In the past, I've used windshirts, rain jackets and The Original Bugshirt for bug protection. The Original Bugshirt is fairly heavy, while the windshirt and rain jackets don't breathe that well (although not too bad). So, here are some options:
1) Get a really breathable windshirt. Unfortunately, it is really hard to find the most breathable windshirt because (unlike rain jackets) companies don't print the numbers or even brag about their breathability. This makes sense because generally speaking, the most breathable windshirt also offers the least protection against the wind. In my case, I'm willing to sacrifice that (and use my rain jacket for wind protection). So, if someone can tell me about the most breathable windshirt, I'm all ears. Bonus points if the windshirt has a hood, as I've found that it helps to have a hood, even though I always wear a headnet (it's just easier to cover everything with a tucked in hood).
2) Buy a button down (or some other style) hiking shirt. A lot of companies offer these, and they seem to be bug resistant and breathe really well. Unfortunately, many of them come pre-treated, which sort of defeats the purpose in my mind (if I'm willing to use chemicals, then I'll just use chemicals on my existing T-Shirt).
3) Some other option I haven't though of.