Looking for a way to kill fungus gnats naturally? Sooner or later you will have to deal with fungus gnats infesting your houseplants. Fungus gnats are the tiny flying pests that can swarm like an annoying black cloud whenever an infested houseplant or outdoor potted plant is moved.
Fungus gnats (Bradysia species) are small, mosquito-like insects that commonly infest the soil or growing medium of potted plants. When outdoor plants are brought indoors for the colder months, fungus gnats in the potting mix are brought indoors. That’s when they can quickly infest the soil of other houseplants.
Fungus gnats thrive in moist potting soil, where they lay their eggs. After the eggs hatch, fungus gnat larvae live in the top two to three inches of the potting soil. Fungus gnat larvae can cause plant damage by eating plant roots. After about two or three weeks, the fungus gnat larvae pupate in the potting soil. Then they become the tiny black adult gnats that fly erratically when a potted plant is moved.
Let the potting soil dry out between plant waterings. This will help control fungus gnats. But sometimes more aggressive measures are needed to kill these insect pests. Fortunately, there are now natural, non-chemical ways to naturally kill fungus gnats and control fungus gnat infestations.
Kill Fungus Gnats Naturally– Control Larvae with BTI
To kill fungus gnats naturally, use BTI. Fungus gnats are closely related to mosquitoes, so fungus gnat larvae are effectively killed by BTI. (BTI is a naturally occurring bacterium that is deadly to mosquito larvae AND fungus gnat larvae.)
BTI, which is short for Bacillus thuringiensis israelensis, is deadly to mosquito larvae and fungus gnat larvae but harmless to other living things. The bacterium is nature’s way of keeping fungus gnat and mosquito populations down. But fortunately, BTI has no effect on other insects (such as bees and butterflies). Nor does it harm animals, fish, birds, plants, pets or people.
BTI is the active ingredient in top-selling Mosquito Bits®, which is approved for the control of fungus gnats. To kill fungus gnat larvae, simply shake the granular Mosquito Bits® onto the potting soil in houseplants. Also use it on other container-grown plants—including vegetables and herbs.
Soak Mosquito Bits in Water
Here’s another easy way to kill fungus gnats naturally with BTI. Make Mosquito Bits tea! Just mix 4 tablespoons of Mosquito Bits® in 1 gallon of water. Let the Bits soak for 30 minutes to remove the BTI, then skim off the floating granules. (You can also place the Bits in a nylon stocking prior to immersing them in water. That makes it easier to remove the granules, which can be composted or discarded.)
Now stir or shake the water to make sure the BTI is thoroughly mixed in. Then use the “BTI tea” to water the soil or potting mix of houseplants and container plants. This treatment controls fungus gnat larvae present in the soil at the time of application. For best results, use your Mosquito Bits/BTI tea solution within 48 hours.
Repeat this process weekly for three weeks. If fungus gnats reappear at a later time, simply repeat this treatment again. Buy Mosquito Bits here.
Because fungus gnats can be an ongoing problem, keep a container of Mosquito Bits on your Potting Table with your other gardening supplies.
Kill Fungus Gnats Naturally with Horticultural Oil
There is also an environmentally friendly, organic way to kill fungus gnats after they become adults. Summit® Year-Round® Spray Oil is a lightweight horticultural oil that controls a wide variety of insect pests including fungus gnats. Summit® Year-Round® Spray Oil is OMRI Listed® by the Organic Materials Review Institute and approved for use in certified organic production, and it’s perfect for indoor use.
Unlike many other pesticides that kill insects with toxic chemicals, Summit® Year-Round® Spray Oil literally smothers insects to death. The oil spray blocks an insect’s breathing holes (called spiracles), causing insects to die from asphyxiation.
Summit Year Round Spray Oil can be used any time during the year to kill fungus gnats because it won’t harm tender plant buds, shoots or leaves. This lightweight oil has no odor, and it will effectively control pests on even sensitive plants such as tropical houseplants, roses, impatiens, citrus trees, and ferns. Summit® Year-Round® Spray Oil naturally kills fungus gnats. It also kills a wide variety of insect pests including aphids, whiteflies, spider mites, mealybugs, scale, thrips, lace bugs and more. Unlike chemical pesticides which can become less effective after repeated use, there is no known build-up of insect resistance to horticultural oil sprays.
Mosquito Bits® and Summit® Year-Round® Spray Oil are available at home centers, hardware stores, garden centers and at www.SummitResponsibleSolutions.com.
Can BTI Be Used to Control Mosquitoes?
The short answer is Yes! In fact, BTI has been used to kill mosquitoes for more than 30 years. Bacillus thuringiensis israelensis (BTI) has been called “Mother Nature’s mosquito control” because this bacterium kills mosquito larvae and virtually nothing else. BTI also kills fungus gnats, because fungus gnats are genetically very closely related to mosquitoes.
The best-selling BTI product in the world is called Mosquito Dunks. Just place the handy donut-shaped Dunks in birdbaths, rainbarrels, ponds and anywhere that water collects. The BTI in the Mosquito Dunks will kill mosquito larvae for 30 days or longer. For more information, read Use BTI to Kill Mosquitoes. To check the price and availability of Mosquito Dunks, click here.
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I have a ton of fungus gnats in my yard and I think it’s because for two years I had a leak in my inground pool. I have a garden along the back of my house about 100 feet long by 3 feet wide. I do not have any plants, but there are rocks and dirt underneath. I’m thinking it’s drenched under there and that’s where they’re coming from. I noticed a ton of them there the other day, and they’re constantly by my patio door. We fixed the leak in the pool and I’m hoping to finally be rid of these pests. I’ve read solutions on how to get rid of them, but I’m looking for a solution where I don’t have to move the rocks and use something of the large area. Suggestions please, thank you.
Laura–
It does sound like your leaky in-ground pool created a wet breeding ground for fungus gnats. You need to buy Mosquito Bits (the large size!) and sprinkle them throughout the affected area. The BTI on the surface of the Mosquito Bits will kill the fungus gnat larvae in the soil. (BTI is a naturally occurring bacterium that kills the larvae of fungus gnats and mosquitoes but is harmless to people, pets, beneficial insects, birds and other wildlife.)
With the infestation you describe, apply Mosquito Bits every 7 days throughout the summer. You should start seeing WAY FEWER fungus gnats soon! Click on the Mosquito Bits jug photo above and you will be taken straight to the Summit Responsible Solutions store on Amazon where you can buy Mosquito Bits.
Randy Schultz
Ugh! These fungus gnats are taking over my house. I’ve used mosquito bits two times now in two weeks. Now I’m noticing more flying around opposed to just seeing them in the plants with an occasional one flying around. Is this normal?! Does it get worse before it gets better. Help me please! I’m ready to throw out all my beautiful plants 😢
Alana–
It’s important to remember that the BTI in Mosquito Bits only kills the larvae of fungus gnats. This interrupts the life cycle of these pests and kills the next generation of gnats. To kill the adults, use Summit Year-Round Spray Oil. Please re-read that section of the story.
Also, we just added info about how to create “BTI water” by soaking Mosquito BIts granules. Please see that section of the story for instructions.
Don’t give up on your plants quite yet!
I’ve read elsewhere that Mosquito Dunks have a higher BTI concentration than Mosquito Bits. Can you advise how to make “BTI Water” (BTI Tea) using Mosquito Dunks?
Tom–
It’s not that Mosquito Dunks have a higher concentration of BTI than Mosquito Bits. The difference is, because of the size of the Dunk, it dissolves slowly in water. Therefore, it releases the BTI over a period of 30 days or more. If you want to make BTI water or BTI tea, we recommend using Mosquito Bits, because all the BTI is removed from the Bits very quickly.
Randy
Is there a difference between sprinkling the mosquito bits on top of the soil versus using it as mosquito bit water? Is one more effective than the other?
Amber–
Both methods of using Mosquito Bits are effective. One of the advantages of soaking the Bits in water and then drenching the soil is the BTI (the active ingredient in the Mosquito Bits) gets thoroughly distributed throughout the potting soil.
Randy Schultz
You’re doing God’s work here, my friend! This gnat treatment has saved my sanity a couple times now. After my first experience with fungus gnats I’m very careful about not over-watering, but recently a friend of a friend gifted me several cuttings/starter plants and soon here I go again with the fungus gnats! But your BTI tea does the trick, and the sticky trap things work well on the live ones! I’ll put the sticky traps out as soon as I notice the gnats even if I don’t have time right then to make the tea.
What is the difference between BT and BTI?
There are several strains of BT, but the most commonly used are the kurstaki strain (BTK) and the israelensis strain (BTI). Each strain is specific to the insect larvae it controls. BTI is effective against mosquito and fungus gnat larvae. BTK is effective against the caterpillars of certain moths and butterflies.
I have a Mosquito Dunk mixed up in water, Would it be ok to apply it with a small sprayer like the one in the illustration at the top of the page? It would be an easier way to dampen the soil in my situation. Thanks!
Sure. That should work fine.
Thanks so much! Take care.
I’ve soaked 4 tbsp of bits in a gallon of water as directed. I don’t use up the entire gallon in a day, so how long does the mixture keep until it is not effective anymore?
After soaking the Mosquito Bits in water, the solution should be used within 24-48 hours.
Hi! If I don’t use all the Mosquito bits tea do I need to dump it out or how long does it last before going bad?
Thank you!
Hi Shanelle! When you make Mosquito Bits tea, you need to use it on your plants within 48 hours.
Which season or month is the best time to apply BTI or Mosquito Bits?
Is fall a good time, or in spring of the year?
Jeff– When using BTI to control fungus gnats in potted plants, there is no specific “right time of the year.” Fungus gnats can infest a houseplant any time of year. So, feel free to use Mosquito Bits whenever you see fungus gnats flying around your houseplants. Also, using Mosquito Bits proactively to PREVENT a fungus gnat infestation is also a good idea.
Will this hurt my two dogs if I put granules on grass? It says on bag avoid direct contact to skin. What if they ingest?
Mark– No, it won’t hurt your dogs. Mosquito Bits and Mosquito Dunks are safe for pets and people. BTI is toxic only to mosquito larvae. Even if dogs ingest Bits or Dunks, they will be fine.
Will BTI also kill no-see-ems and outdoor gnats? We’ve set up mosquito trap buckets using Mosquito Dunks to control mosquitos, but biting gnats are also a problem in our yard. Wondering if the mosquito bucket traps will work for the as well. Thanks
Sue: The BTI in Mosquito Dunks will kill mosquito larvae AND fungus gnat larvae. But it will not kill no-see-ems.
Is it more beneficial to allow mosquito bits soak overnight?
Christa–
You can soak the Mosquito Bits overnight if you wish, but it only takes about 30 minutes of soaking in water to remove the BTI from the bits.
I’m growing peppers to eat . Is it safe to eat the peppers after doing this.
Bill–
Yes. The BTI in Mosquito Bits and Mosquito Dunks will do no harm to humans or animals. It only affects fungus gnats and mosquitoes.
Can I spray the leaves of my veggie plants, and should I wait to eat things like greens? Thanks!
Yes, you can spray Summit Year-Round Spray Oil and the BTI solution on edible plants, right up to the day of harvest. Then, give fruits and vegetables a good rinsing with water before your eat them.
I have leaf mulch in all my flower beds. I think that is where the gnats are coming from. Will it be effecient to just sprinkle the BTI granules on the mulch and water in? How much should I apply?
Just follow the instructions on the Mosquito Bits label.
Unfortunately I bought a large bag of well known commercially available potting soil that is contaminated with fungus gnats. I’m using the BTi bits soaked in water to treat the house plants. I transplanted with the contaminated potting soil. Can I add this solution of the bits directly to the potting soil bag? Any suggestion of amount of solution to add to ~2 cu. ft. of potting soil to be most effective. Would it be better to scoop out the top 3-4 in. and treat this separately?
You can also sprinkle Mosquito Bits on the surface of the potting soil of any infected plant. Then water thoroughly. If fungus gnats return, re-apply the Mosquito Bits every 7-10 days.
After applying mosquito bits solution to outside soil, is it necessary to keep it moist to be effective?
Robert–
We don’t recommend using Mosquito Bits solution in outside soil– unless you use it to water a potted or container plant. And yes, the soil must be moist for the BTI solution to be affective against fungus gnat and mosquito larvae.
I used the Mosquito Bits about a week and a half ago. My plants still have those pesty gnats. Looks like a few little ones are making their way to the top of the soil. I wanted to do another treatment but my soil is still very wet from last time. Is it ok to do another treatment?
Marlena–
It’s OK to repeat an application of Mosquito Bits every week for a total of three weeks in a row.
Randy