I've been to Isle Royale twice: once in August, and once the week after Labor Day. Bugs were not a problem either time. They were there; I didn't try sleeping outside my tent, but they weren't horrifyingly bad. (I've had a lot worse in Kentucky in August.)
Late July through Labor Day is a wonderful time to go. The crowds aren't too bad; after Labor Day, they're non-existent. That trip, we camped mostly along the inner harbor at the "popular" sites, and never had more than 12 people in the site (4 of them were us.)
Enjoy the trip.