Wednesday, December 22, 2010

Fungus Gnats on Houseplants: How To Control Them

 Winter's rain, snow, wind and cold tends to turn a gardener's attention to the indoor jungle, and the creatures that are hiding there.
From the garden e-mail bag, Nickie asks: "I have a question for you about the little gnats that seem to come from some of our house plants. How do you get rid of them? Why do the plants produce them?"
Those are fungus gnats that live in the soil surrounding the house plant, although the adults sometimes can be found on the plant leaves. They thrive in soil that is overly moist, filled with partially composted materials and rich in organic matter. You may notice that these critters tend to come up to the surface when you water the plant thoroughly. If there are a lot of fungus gnats, you may even notice a slime trail on the top of the soil.

The larvae of fungus gnats chew on plant roots, causing stunting or plant decline. The adults can spread other plant disease problems.

How did the fungus gnats get to your potted plants? Although they possibly flew to that location, more than likely they were in the soil when you either purchased the plant or repotted it in a bigger container with more soil, especially if that soil contains homemade compost or moist, backyard dirt.

         If fungus gnats are bugging you, take them to a shady area outside for a short time on a nice day (or the garage, on a not-nice day). Water the plant thoroughly in a deep pan. When you see the gnats rise to the surface, scrape off the top inch of soil into the pan, then to the trash. Replace that soil with fresh, bug-free potting mix. 

Then, apply the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti). This will control any fungus gnat larvae that are still in container media. You may need to repeat this application every five days until the problem is under control. Commercially available nematodes (especially the Steinernema feltiae nematode) can also control fungus gnat populations. Among the sources for nematodes: Peaceful Valley Farm Supply.

 To control future fungus gnat outbreaks:

• Make sure your window and door screens are secure.

• Let the surface of your houseplant soil dry out between waterings.

• Make sure the container has good drainage.

• Get rid of any standing water in the saucer below the pot. 

• If you are using homemade compost in your potting soil mix, heat it in the oven first to kill off any fungus gnat eggs.

 Colorado State University Cooperative Extension offers these suggestions for using heat to sterilize the soil:

Oven Method - Spread soil not more than four inches deep in non-plastic containers, such as seed flats, clay pots and glass or metal baking pans. Cover each container tightly with aluminum foil. Insert a meat or candy thermometer through the foil into the center of the soil. Set the oven between 180° and 200° F. Heat the soil to at least 180° F; keep at this temperature for 30 minutes. Do not allow the temperature to go above 200° F. High temperatures may produce plant toxins. After heating, cool, remove containers from the oven and leave aluminum foil in place until ready to use. The heated soil will give off an odor.

Microwave Oven Method
- Microwave soil for 90 seconds per kilogram (2.2 pounds) on full power. Don't use metal containers and aluminum foil when using a microwave.

Or, get new potting soil. Use only pasteurized container mixes for your house plants.Then, be sure to store any excess potting soil in closed containers. 

• Avoid fertilizing with excessive amounts of manure, blood meal, or similar organic materials. 

• Adding an inch of coarse sand on the top of the soil can also deter fungus gnats.

• A  Yellow Sticky White Fly Trap, placed upright in the container, is another way to monitor the adult fungus gnat population.

• What about synthetic insecticides? Sprays that include active ingredients such as bifenthrin or permethrin are often effective and persistent. However, they are more toxic to beneficial insects. Because of that, some experts advise that when using these on houseplants, move plants outdoors for treatment and wait about a day after application before bringing them back inside. Um, frankly, do you want your houseplants outside, overnight, in the winter? Not a good idea. It's a shock to their system to be outside in the first place. And you don't want to be risking that the spray might drift onto pets or children indoors. So, bypass these sprays.

         The Integrated Pest Management experts at UC Davis offer this fun way for any junior scientists in your household to help you monitor future fungus gnat populations in your plants:

• Bury one-inch cubes or slices of peeled, raw potatoes into your house plant soil, about 3/8 of an inch deep.

• Once or twice a week, unearth and examine the underside of each potato and the soil immediately beneath it. Keep a chart of the numbers of larvae found before and after any treatment to determine whether larvae are being controlled.

• You'll know if they are the fungus gnat larvae if they have a shiny black head and an elongate, whitish to clear, legless body. 

Adult fungus gnats, on the other hand, are about one-eighth of an inch long with wings, slender legs and antennae that are longer than their heads. 

• Discard the potato along with the fungus gnats; add a new piece to continue the experiment.



  1. Another good method is to mulch your houseplants with cedar shaving pet bedding. It's inexpensive to boot; the gnats retreat from the odor of cedar and watering causes the cedar oils to spread into the soil, where they kill the gnat maggots.

    As a side bonus, my cats can't abide the smell either, and won't dig in the potted plants!

  2. If you can tolerate the infestation at bit, assuming it isn't too bad, just correct the condition of the soil by letting your houseplants go a little on the dry side for awhile and they will eventually die off. Takes patience though and sometimes results in a gnat up the nose now and then! In the past I used malathion to kill the larvae (killing the adults isn't enough) but it's toxic and smelly. Now I just do as I mentioned and they are gone in a week or two.

  3. Thanks for sharing such an informative post.

  4. What about cinnamon powder? I have heard that it works to sprinkle some over the topsoil from time to time.I think it works by deterring the adults from landing on it and dessicating any larvae and eggs that it comes in contact with (I'm not able to verify this).. but I'm sure it doesn't harm the plant at the very least.