Fungus gnats (Bradysia) do not cause serious damage to house plants. They do however, eat at the root hairs and the base of the plants, causing house plants to weaken. Fungus gnats are also a nuisance in the home. Fortunately, you do not have to go out and get an insecticide because there are several home remedies you can try to kill and get rid of your fungus gnats. Use a combination of treatments for the best success.
Replace the soil in your house plant. Outside, gently tap the sides of the pot and turn your plant upside to get it out. Then wash off the roots of any remaining dirt and replant in new potting soil to the same depth as it was planted before.
Stop watering your house plant so much. Soil that is too moist is the main reason for fungus gnats. Allow the soil to dry out about an inch or more deep prior to watering it again. Also, importantly, do not allow water to sit in the drainage container. This will keep the soil too moist.
Clean up the organic matter (old leaves and flowers) that accumulates around the base of your house plant. Throw it outside in the trash. Fungus gnats eat decaying organic matter and if they don't have anything to eat, they won't thrive.
Make your own yellow sticky trap. Buy yellow cardboard and smear it with petroleum jelly and hang it near your plant. The fungus gnats will stick to the trap. Discard and replace as needed.