I had them VERY BAD this year!
How ever I have found a solution that worked for me. Fly strips are 2$ for 5 at my local, orange big box store.
I hung those things up and the next day most are almost completely full!
Sure it doesn't kill, larva, but it kills them when they reach adulthood. At least I'm not spreading BT all over my soil from Mosquito Dunks. BT will kill lots of the beneficial critters that support the food feb of the soil.
I used strips last year and eventually the predators gained control and I didn't have a problem...
If you poison them, you will poison the predator that hunt them as well.
So far I have not caught one benificial with a flys trip. No Bee's, No butter fly's, No Ladybug's.
Just Thousands of GNATS. If you guys want I can post some pictures, but be warned its sorta a nasty sight to be seen.