I get questions like this occasionally:
"When I took the lid off the bin I noticed tiny little black flies flying out... I don't think they are fruit flies because I put out a dish with mixture of cider vinegar, water and a little mild dish soap which always takes care of fruit flies but these guys could care less about it! Please help."
Most commonly these are be fungus gnats (see image). Fungus Gnats are very small 1/32 to 7/16 inch long, long legged and mosquito-like and are usually black, gray, or brown. The larvae feed on moist decaying organic matter and fungi growing in the soil.
I have found the best treatment of fungus gnats to be BTi (Bacillus thuringiensis israelenis) mosquito rings/dunks. To treat an infestation, make a solution of 2 cups of water for about 1/4 of the dunk. Remove the newspaper layer and spray about 1/2 of this solution on the soil daily for 10 days (make fresh solution every 2 days-- one piece of ring lasts 2 or 3 times). Vacuum any adults your can when you open
the lid (dust buster or shop vac work well). Keep the newspaper layer off your bin during the treatment period and do not add food. That should take care of them.
To prevent fungus gnats, be sure to bury any food scraps. If you see mold starting to grow in your bin, replace the newspaper layer and and hold food back until the worms catch up.