posted 13 years ago
I have them as well, and I'm pretty sure they are fungus gnats. If they are gnats not fruit flies, they breed in the top few inches of soil that stays consistently moist. I think once you have them, they will be hard to get rid of completely. However I have had good success with keeping a dry layer (newsprint, straw, etc.) as deep as possible over the food/bedding. Another way to kill the larvae is to let the soil they are breeding in to dry out completely, but that's not really possible in a worm bin.
They are decomposers and help the composting process along so I can't completely hate them. They are just annoying flying around my basement, and sometimes they will migrate to the soil of your houseplants, potentially feeding on roots but I've never had them kill anything.