Wont work... I made this mistake myself once. The Safer stuff is Bacillus thuringiensis kurstaki, which is effective for caterpillers/moths (Lepidoptera), but with gnats/blackfly you want the israeli sp. as they produce different toxins. One time I bought the stuff you refer to, not noticing the sub-species myself. After a couple weeks I was wondering wtf was going on because the problem didn't seem to be getting any better.
Mosquito Dunks are probably the most commonly available Bti product, sold at virtually every hardware store and garden center. Obviously they kill mosquitoes, too, I use them in my rain barrel for this reason as well as for fungus gnats. You can use it before you even get gnats, say about once a month.
Nematodes are effective, you have to re-apply eventually though and for future crops. Instead, I keep Hypoaspis predator mites around, in containers and in soil aging/recycling totes. Had them for years and just keep introducing them to new containers and back. They eat fungus gnats eggs or larva/pupal stages living in the soil, as well as a plethora of other soil-dwelling pests, springtails, other mites, as well as dead vegetation (microbes) if that is all there is. Other benefits come with them being a part of the soil food web.
