Is it safe to use both Gnatrol and Nematodes Sf at the same time?
It is not a good idea to use any pesticides with nematodes or any other beneficial insects. Gnatrol is not a pesticide, so it is probably okay to use with nematodes. However, why would you want to? Pick one or the other, try it and see if it works. If it does not, then try the other one. Gnatrol is less expensive and more readily available so that is the logical one to start with. Using both simultaneously provides no added benefit.
Poor soil quality, unnecessary repotting and keeping soil too moist are the primary causes of fungus gnat problems. It is important to address these issues in addition to treating the larvae in the soil. One of the most effective things you can do is to remove all loose surface soil not in immediate contact with roots and discard it. This is where many of the larvae live. In addition, removing the excess soil will allow the soil in the root zone to dry out sooner. Dry soil is the enemy of gnat larvae, so keep the soil as dry as possible.