how did you come to the ID???? .. if nothing works.. and one presumes you targeted your threat ..... then one has to question the ID .... [maybe you took the sticky traps to a professional????]

one might suppose there are a multitude of gnats??? .. if in fact.. they are gnats ....

have you talked to your county extension office ... perhaps they can offer some wisdom.. as i doubt you are the only yard in the county dealing with such ...

i presume we arent going to get a picture of them ... so i am searching for more info ....

good luck

ken

ps: bt takes a very long time to act ... like multiple seasons.. if in fact.. it does work ....of which, i am not convinced ... and i hope the person who ID'd the threat.. wasnt the dude who sold you the bt ....

I am also curious about how you came to the identity of these insects. Do you live adjacent to a wetland or boggy area?

In an outdoor setting, fungus gnats aren't usually an overwhelming problem. They wouldn't be the problem for landscape plants that they can be for plants confined to a container.

Could the issues with your plants be caused by excess moisture? These little insects don't bite, by any chance?

Fungus Gnats are known problems in house and greenhouse plants but not in the general garden unless one has a bog garden. They need a soil with fairly large amounts of organic matter that is quite wet most of the time, since one simple means of control is to withhold water.

Here is a link that might be useful: About Fungus Gnats