When I get a tough bug, I keep it wet with the cleaner of choice (water with car wash soap, Windex Multi-purpose with Vinegar, or Ice Detailer) and scratch at it with my fingernail. I know it sounds brutal, but a wet fingernail is unlikely to scratch anything, and you can feel when the crud is gone. I have never scratched either paint or windshields with this method, but use it at your own risk. As they say, try it in an inconspicuous spot first.