I sent away to Bugological (there are quite a few companies that sell these gnats, but I'm a sucker for bad word play). A paper bag arrived with a bunch of sawdust and little black pellets inside. The black nugs were parasistized fly pupae. I held up the bag to the light and looked for hatched gnats every day. And every day was a disappointment.
Finally, on a warm Monday a couple weeks later, a few of the gnats (Muscidifurax zaraptor and Nasonia vitripennis) could be seen flying around the bag wondering, where the hell can I feed on some more fly pupae? Per bugological's instrux, I parsed the sawdust and pupae in my "hot" areas. It'll take 21 days before I notice if it worked or not. The cool thing about this strategy (versus fly strips) is the gnats keep reproducing and feeding on the flies! They're tiny and don't bite, so they won't become a pest themselves.
