I have a solution for you.
My father is an entomologist of 30+ years, he's highly decorated and his name is known by most in the field. His house has been the guinea pig for all kinds of experiments over the years, and there was one product that really stuck out. It's called Termidor.
http://www.termidorhome.com
It is a spray that is put down once a year around the outside of the house. You wouldn't think this would be effective, but believe me it is. There is no bug that will cross that line without dieing. So summer comes, all the bugs overwintering in your walls (theres thousands of them you dont know about) go outside, die in the process. then fall comes, and they start to come inside, but never make it when they hit that spray. You'll never see spiders again (that is what he tested on. he had a BAD yellow sac spider problem, gone with one application before fall)
Long story short, your garage will remain bug free if you treat the outside of your hosue with this stuff. the only way they will get past it is if you leave your garage door open (like all of us do) with lights on and the bugs fly in. then this method is useless. but if you keep the door shut, they will not make it into your garage. This stuff works wonders.
It's the "proper" way to treat for insects before they become a problem. It is marketed for termites, but works for all insects. you'll have to have a professional come out and do it, as this isnt something you can pick up at home depot. but believe me, this **** rocks.