I was there a couple years ago in early Aug. Bugs weren’t too bad except at one campsite. I’d expect Jul to be worse, but you might check out some trip reports at http://www.isleroyale.info.
I normally wouldn’t direct someone to the forums over at Backpacker.com (the magazine), but there seem to be a lot of Isle Royale enthusiasts hanging out in the MidWest forum. So you might try asking for advice there.
My trip was mostly along the Greenstone from Windigo to Rock Harbor (with a few detours, including a section of the Minong). I’d think those would be good options for avoiding the bugs as much as possible since you’re high up most of the time or on the nw coast getting the most wind. But I’ve heard the Minong isn’t the place to be in bad weather (we lucked out and had excellent weather the whole week).
My own trip report is at:
http://mcfarren.info/adam/outdoor/backpacking/isleroyale2004/
Hope this helps,
-adam