Q. It seems no matter what brand, no matter where I buy my potting soil I have to bake it before I can use it.  If I don’t, I get hundreds of tiny fly that hatch and swarm.  I put out water to catch and drown them. I spray insect oil on top of the soil several times a day. I’m so afraid I’ll kill the plants. A. Yes, fungus gnats are a big problem in potting soils and they can create damage to plant roots. They feed off of both decaying plant parts and soft, succulent living roots as well. Potting soil does not smell very good if you have to put it in your oven at 150° for 15 minutes to kill these nuisance critters. The larvae can cause damage to new, tender roots of plants. If they are extremely happy in their environment they can multiply very rapidly and cause some severe damage. Besides that, they are pesky and a nuisance inside the house. This is Garden Gourmet potting soil and I have never seen fungus gnats in this product. However, bagged potting soil that is not been heat treated will carry fungus gnats. This product sells for $5 for one cubic foot. If potting soil is sterilized with a heat treatment by the manufacturer it should kill all of the fungus gnats. If the soil is introduced into an environment where there are fungus gnats present then it will get reinfested again. Here is what the University of California says about them They can be controlled with some organic pest control products; nematodes that go after these larvae and a bacterium which does the same thing. You should be able to find some of these products in your local nursery or garden center. I have never used them so I am reporting only what has been reported on the internet using these methods for fungus gnat control. However, cooking them does work. Where to get beneficial nematodes Where to get beneficial bacteria  It says for mosquito control but it is the same product used for fungus gnats and will be included on the label. Another method is to sterilize this potting soil yourself by placing it moistened into a clear plastic bag in full sunlight and let it bake. Temperatures need to get up to about 160 F for at least 30 minutes for good control. Keep in mind that if you introduce this back in the environment where a fungus gnats are present and they will reinfest this sterilized potting soil. Another option is to apply pyrethrin to the soil and water it in.