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Fungus Gnats: How I Got Rid of Them Permanently (and Organically)


Fungus Gnats: How I Got Rid of Them Permanently (and Organically)

Marie Viljoen February 19, 2024

Ah, the perks of an indoor garden in winter: Fruit, flowers, fragrance, and…fungus gnats? If you are the parent of any indoor plant, the chances are good that you have already met fungus gnats. These small flies are non-buzzing and non-biting, but an annoyance in large numbers because they are attracted to moisture, even if is from your breath, or around your eyes. Swat, swat. Worse, if you do see a lot of the tiny, winged varmints, it means that their even tinier but much hungrier larvae are living in your pots, eating organic matter that includes the roots of your plants, which they damage.

Getting rid of fungus gnats is a three-step process. This is what has worked for me.

Photography by Marie Viljoen.

Above: If only cats would catch fungus gnats.

Fungus gnats are a warning sign: They thrive in moist environments, and their presence indicates you might be overwatering your plants. Long-term, this can lead to their slow death. So take the gnats’ presence as a helpful hint, and then banish them. While the adults do not feed on anything, they do lay eggs. And it is their larvae that do the quiet, subterranean damage.

Above: Adult fungus gnats, trapped.

Where do fungus gnats come from? Like scale insects indoors, they seem to materialize from the ether. A possible source of fungus gnats in your home is the new plant you just bought, whether it’s a seasonal poinsettia, Christmas amaryllis, or your kitty’s fresh wheatgrass from the pe(s)t store. The insects are a common pest in professional greenhouses. Because new plants might be carriers of fungus gnats, if it is practical, keep them apart from your established, unaffected plants for a period of three weeks. (The four-stage fungus gnat life cycle is about three weeks, from egg, to larva, to pupa, to adult fly.)

They could also be present in your growing media, in egg or larval form. Most potting mixes are sterilized, but it’s hard to know for sure.

Above: The pros and cons of indoor growing—fruit, and pesky critter control.

Here are the three steps to getting rid of fungus gnats.

Step 1: Do not overwater.

Above: I water my plants when the meter reaches the red zone.

This is a permanent care-protocol in eliminating fungus gnats: Only water your plants (deeply) when they have come close to drying out. I have been an indoor grower now for over a decade and you’d think I’d know better, but I am still prone to overwatering. A moisture meter’s long probe is able to give me a better sense of what is happening in the soil, not just in the top inch or so.

Allowing your pots to dry between waterings helps prevent a fungus gnat infestation, and it also keep your plants healthier: Root rot is caused by overwatering and can be fatal. It’s hard to turn that around, so the pesky gnats are the canary in the coal mine for indoor growers. Unlike the poor canary that keels over in bad air, they thrive in the unhealthy environment.

Step 2: Use yellow sticky traps.

Above: Ugh, but excellent. Forty-eight (cute) yellow sticky traps are $6.98 on Amazon.

Adult fungus gnats are attracted to yellow. Place sticky yellow traps in your pots. The traps effectively sequester the adult gnats, interrupting their relentless life cycle by preventing them from laying eggs in your soil. I choose traps that are the least offensive aesthetically (pretty shapes!) and change them when I can’t stand seeing the bodies pile up. Aside from catching the bugs, the traps are a good indicator of infestation, even when you have controlled the problem. I keep them to warn me of potential fungus gnat re-emergence.

Above: Fungus gnats are also attracted to dishes of soapy water.

If you can’t find sticky traps or dislike supporting the big A., you can add a drop of de-greasing dish detergent to small saucers of water. The fungus gnats are attracted to the water and drown thanks to the soap, which breaks the surface tension under their tiny legs. It is effective but takes longer than the yellow traps.

Step 3: Bacillus thuringiensis subsp. israelensis (Bti)

Above: BTI for fungus gnat elimination. Microbe-Lift is $18.98 for 2 fl oz from Amazon.

The silver bullet. The tiny stake in the miniature heart. When sticky traps alone did not get rid of fungus gnats, I turned to Reddit, which is where I learned about biological warfare. And that, my plant-loving friends, was the end of that. A liquid form of Bacillus thuringiensis subsp. israelensis (Bti), just a drop at a time, solved my fungus gnat infestation; not overnight, but over three weeks, at the end of which I suddenly realized that I had not seen a single gnats in days.

BTI is most commonly used to kill mosquito larvae in standing water. It is a bacterium that occurs naturally in the soil; commercially, it is produced by fermentation. BTI is toxic only to fly larvae. Added to your regular watering schedule the toxins that BTI releases will eliminate fungus gnat larvae. Because it does not persist in the soil, this is a long-term, repeat solution.

Above: Ripe yuzu in January, around peak fungus gnat season.

While I have found no studies that indicate any attached risk to using this biological control, it is important to use common sense when deploying a living counter-measure. Wash your hands thoroughly after watering, and clean the watering can or jug you use.

How much BTI to use for fungus gnat control? My dose is 1 drop of BTI to a quarter gallon of water (my citrus trees usually drink a half gallon at a time, at about weekly intervals). While it is more than the recommended dose for mosquito control, I have noticed no ill effects on the trees, or on the other creatures that live in the pots (centipedes and some earthworms). Chatter on fungus gnat forums (yes, they exist) concurs with a single drop.

Fungus Gnat Control Recap:

  • 1. Do not overwater.
  • 2. Do use sticky traps.
  • 3. Do apply BTI every time you water.

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