Sorry for the late post but better late than never . I had case of serious fungus gnats infestation in the past and I mean it was a jungle dude , so here is my bio method for killing them b*tches once and for all and you can do this each time you suspect a bag is infested with fungus gnats either adults or larvae .
Adult will not cause arm to your crop except dying on the buds , so if you are growing some runtz , you will have some runtz gnats , or fungus runtz
The larvae are a big trouble cause they feed on roots .
So my bio method : get some nematodes Steinernema feltiae or Steinernema Carpocapse ( withstand higher temps than feltiae ) usually you will get the stuff in the form of a powder that you will have to mix with water and water all of your pot with it , Steinernema ar a biological predator of the fungus gnats larvae so they will feed on them .
The second way is to use some mites, stratiolaelaps scimitus they are also a good predator of the fungus gnats larvae ,you will get them in adult form they look like tiny red spiders, you combine the two (mites + nematodes ) and under a week there will be no more fungus gnats larvae .
My bio solution is to kill only the larvae now for the adults get some yellow sticky fly trap , adults fungus gnats love to walk around your pots , so put most of them at the pot level and hang some of them in the corner of your grow tent /cabinet .
The goal here is to kill both the adults and the larvae .
I had 100% success each time with this method .
Good luck and happy grow .