What kind of tarp are you bringing? I think that by the beginning of September there shouldn't be much of a bug problem....especially if you are smart about where you set up. I would be more cautious about those late summer thunder storms that can roll through. But...if you have a large enough tarp to offer full protection from the rain, I think you could manage with just the tarp.
Having said that...it has been a VERY heavy snow year, so there might be mosquitoes around later in the season than usual...