What makes tomatoes grow sweet?

Our tomatoes are beginning to ripen-a lot of green ones are still out there. It seems that as soon as they develop that rosy/yellowish blush as the ripining begins small gnat like bugs start to bore holes. By the time we pick them for final reddening the bugs have ruined the fruit. They start to fly around my windowsill after a few days. We have a totally organic garden and any recs as to a safe way to repel these bugs would be most appreciated. .