Q. It seems no matter what brand of potting soil, I have to bake it before I can use it. If I don't, I get hundreds of tiny flies 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 in particular are a big problem in potting soils used for houseplants. The younger generations feed off of both decaying plants and soft, succulent living roots. They aren’t very particular about what they feed on, living or dead, so long as it is soft, juicy and tender.
If fungus gnats are extremely happy in their environment they will multiply very rapidly and cause poor growth and stunting. Besides, they are pesky and a nuisance inside the house. If potting soil is sterilized by the manufacturer using a heat treatment it should kill all of the fungus gnats and should pose no problem.
Control fungus gnats with organic pest control products such as beneficial nematodes that go after their destructive larvae and a bacterium is also available with a similar result. You should be able to find these products in your local nursery or garden center.
Yellow or blue sticky traps also work. I received this video on how to make yellow sticky traps from a friend.
Another effective method is to sterilize this potting soil yourself by placing it, moistened, into a clear plastic bag and let it bake in the sun. Temperatures need to get up to about 160 F for at least 30 minutes for good control.
Another option is to apply pyrethrin sprays to the soil and water it in.