I recently restarted compost in the tumbler.  After one week in the tumbler, it was very wet out, and I had some knats inside.  They swarmed out like a cloud when I  opened the lid to check it.  The knats don't worry me, as most of them will die when I tumble the composter.  The problem was, that most of them escaped when I opened the lid.  The material was a little too wet, so I did not add any water, just gave it a lot of tumbles (6-7).