Fungus gnats are annoying little creatures that like to live in the moist potting soil used for house plants. While not particularly dangerous, once they've taken hold in a pot or two, they can be a real nuisance to get rid of.
These gnats are tiny flying insects which resemble fruit flies or other gnats. Being airborne, they can enter the house through open windows and doors, but are also extremely common in bagged potting soil used for new plants. They thrive in wet soil and tend to appear in the largest quantities after watering.
Having dealt with the gnat menace successfully in the past, here is my experience in dealing with the issue.
Unsuccessful
Yellow Sticky Cards - The gnats land on these and get stuck. It kills some of them, sure, but the coverage is not sufficient to exterminate them, and you're just forced to stare at the bodies.
Insecticide - Commercial insecticides will definitely kill the gnats that come in contact with it. Unfortunately many of the products available are toxic to people or animals and may not be the best things to spray heavily around the house. Furthermore, it may take a number of successive applications to be successful. Even if you kill one batch of gnats, the spray may wear off before the next generation hatches.
Successful
I had my best success using a larvicide called Mosquito Bits. This product is designed to be spread in water where mosquitos breed. By killing the mosquito larva, their life cycle is interrupted. As it happens, it works well on the moisture-breeding fungus gnats too.
Be sure to use non-chlorinated, luke-warm water. Distilled jugs from a store, a rain barrel, or just neutralized tap water all work. The larvicide itself needs to make it into the soil intact to be most effective.
Fill each gallon jug with 2-3 spoonfuls of mosquito bits 15-30 minutes prior to watering. Allowing the bits to soak gives them time to activate and distribute into solution.
To keep things simple, have an extra empty gallon jug handy. Once the bits have had time to disperse, use a funnel and sieve to strain the bulk of the bits out and discard. Now, simply water the plants as normal.
Keep in mind that larvicide kills the larvae, not the adults. You will need to tolerate them for a few more weeks while the existing gnats get old and die. Patience is the key, and you should see the numbers dwindle with each watering.
Sand
The gnats need access to the soil to reproduce. I've also had luck spreading a thick layer of sand across the potted plants, forming a barrier between the air and the soil. This may help frustrate the gnats, but also changes the aesthetics and soil composition. May be worth trying in cases of heavy infestation.
Watering
Soil should generally be allowed to dry out between waterings. Although all plants have different needs, you should realize that constantly wet soil will breed both fungus and attract gnats.
Prevention
Gnats love to hitch a ride into the house on fresh potting soil. They infect it while it's still in the bag (or earlier), and the larvae sit and wait for you to pot a plant and provide the water they need to hatch. You can head the nuisance off at the pass by sterilizing any fresh potting soil immediately upon first use.
Heat - You can literally bake the fresh soil to kill off organisms living within it. I suggest a Google search to determine the necessary time and temperature to use. I've tried this, and although it seems to work well, it may smell up the house.
Larvicide - I use the Mosquito Bits larvicide for the first several waterings of all new plants. Any larvae will be destroyed before they grow up to reproduce.
Good Luck!
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