Fungus Gnat Larvae: How To Kill Them Off Quickly (2018 Update)

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One of the most frustrating pests in many indoor gardens is the dreaded fungus gnat and its young, the fungus gnat larvae. These little buggers can absolutely destroy your plants if you’re not vigilant — and they can do it quickly.

The primary way that fungus gnats affect your plants is through their larvae. They lay eggs in your growing medium. Once they hatch, the larvae will attach to the roots of your plants and drain them of nutrients.

Although the larvae are the main negative actors, adult fungus gnats can carry disease, especially fungal diseases.​ These can be deadly on their own, but that’s not all. They also lay hundreds of eggs fast, which will devour plant roots!

It’s absolutely essential that you stop these pests before they can take hold, whether indoors or out. The last thing you need is for your plants to succumb to a horrible fate.

I’ll help you to overcome your fungus gnat woes, though. Let’s go into detail about fungus gnats and their larvae, and learn how to demolish them before they can take over!

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Fungus Gnat Overview

Fungus gnat
An adult fungus gnat. Source: andybadger
Common Name(s) Fungus gnats
Scientific Name(s) Multiple, but the dangerous ones to plants are Sciaridae family spp.
Family Sciaridae
Origin Worldwide
Plants Affected Indoor/nursery/greenhouse plants, some outdoor plants. Particularly susceptible are plants such as carnations, African violets, geraniums, and poinsettias, but they can strike any weakened plant in the right conditions.
Common Remedies Hydrogen peroxide soil drenches, neem oil, other azadirachtins, pyrethrins, beneficial nematodes, Bacillus thurigiensis var. israelensis, sticky stakes/sticky traps, and mulches

Types of Fungus Gnat Larvae

There are six different families of insects which make up the broader category of fungus gnats. These six families include the Sciaridae, Mycetophilidae, Ditomyiidae, Bolitophilidae, Diadocidiidae, and Keroplatidae.

Most fungus gnat species are not harmful to our gardens, but the dangerous ones are in the Sciaridae family.

While adults don’t cause any lasting damage, the fungus gnat larvae of that family will move on to plant roots once their preferred foodstuff is gone. The larvae will chew holes in the roots and can cause yellowing, wilting, and even plant death in large numbers.

Not only do they damage roots, but the Sciaridae can spread disease. Since their preferred food is fungal growth, they can easily pick up spores from infected plants or soil and spread leaf spots, scabs, cankers, rot, and more.

Fungus gnat adults are often confused with mosquitos as they look similar in shape, but are much smaller. These tiny black gnats rarely get any larger than an eighth of an inch long.

The larvae themselves grow a bit larger than the adult gnats. Fungus gnat larvae have a black, shiny head with a white to clear body that can reach almost a quarter inch in length. They’re hard to locate because they tend to remain under the soil’s surface, out of view.

Life Cycle Of Fungus Gnats

Fungus gnat larva and eggs
A fungus gnat larva and some eggs. Source: myriorama

The life cycle of the fungus gnat is made up of four stages: egg, larvae, pupae, and adult.

The adult fungus gnat can lay about three hundred eggs in its short lifespan. They typically only live for about a week as adults, so they make the most of their time! Eggs are laid in rich and fertile, moist soil at the base of plants.

About four to six days after the eggs are laid, the larvae emerge. These larvae will be extremely tiny at first. During the roughly two-week period in which fungus gnat larvae are growing, they rapidly increase in size by eating their way through organic matter in the soil.

Once they’ve achieved maximum larval growth, there will be a 3-4 day pupal phase. The pupae will be hidden under the soil’s surface, and when the adult emerges, this cycle begins again.

Because of this quick life cycle, many generations of fungus gnat can be born in quick succession. In addition, many stages of the life cycle may be present at any given time. Not only do you need to get rid of the annoying gnats themselves, but you’ll need to deal with the larvae and find a way to sterilize the eggs.

Common Habitats For Fungus Gnat Larvae

Moist and rich soil, especially soil that tends to be a bit overly-damp, is perfect for fungus gnat larval development. Planting mixes that contain a heavy amount of peat moss or coconut coir will encourage them to move in and take up residence.

Avoid overwatering, especially during the cooler winter and spring months. Too much moisture will speed their development as it can cause roots to begin to decay, making a perfect food source.

Greenhouses stay warmer through the winter months than the outside garden, which makes them a perfect place for fungus gnats to try to overwinter. It’s especially risky as they typically have richer soils that are regularly watered, making for prime fungus gnat real estate.

In addition, people who have houseplants may find fungus gnats trying to invade their homes during the winter. Needless to say, that’s something that you absolutely want to stop as soon as possible!

What Do Fungus Gnat Larvae Eat?

Fungus gnat on flower petal
A fungus gnat on a flower petal. Source: Arthur Chapman

While most fungus gnat larvae prefer decaying organic materials and fungal growths, any weakness or rot in plant roots can make them appealing targets. Similarly, if there’s a lack of organic material in the soil, the gnat larvae will feast on your plants instead.

Especially at risk to fungus gnat larvae are African violets, geraniums, poinsettias, and carnations. These four plants not only can develop root rot quickly, but they tend to live in a nearly-ideal soil type for the gnat larvae to colonize.

However, nearly any plant that is weak can succumb to fungus gnat larvae. Keeping your plants strong and healthy and free of decay will help protect them from this pest.

When Fungus Gnats Strike

Dark winged fungus gnat
A dark-winged fungus gnat. Source: John Tann

You’re most likely to get fungus gnat problems around the fall. As the weather cools, they seek out warmer temperatures, and your house or greenhouse are prime locations. Once they get to your soil and start laying eggs, they can damage or destroy your plants rapidly.

If you have plant problems and suspect it might be the work of fungus gnats, be on the lookout for yellow leaves that otherwise appear normal, or extremely slow growth. Both are symptoms of fungus gnat larvae in the soil.

If you spot either of these symptoms, or see adult flies buzzing around your garden, you need to react immediately.

Treat all your plants, not just the ones closest to the flies! It’s very hard to know if larvae are on plants that look alright, so you should cover your bases and treat everything. It’s better to be safe than sorry.

Plants recovering from fungus gnat problems still face the risk of disease problems. Fungus gnat larvae can spread fungus spores that are dropped by adult gnats to your plant’s roots, possibly causing a number of common plant diseases.

As a precaution, treat any affected plants and those in the area with fungicide a day or two after pesticide application. This ensures that if your plants did have contact with potentially dangerous disease spores, they shouldn’t contract the disease.

A root drench is more effective than spraying the plant’s foliage, as that’s where damage would be done.

How To Get Rid Of Fungus Gnat Larvae

Fungus gnat on leaf
An adult fungus gnat on a leaf. Source: epitree

If you use the prevention methods below, you may never see fungus gnats or their larvae. But if you do, here’s how to wipe them out!

Organic Fungus Gnat Control

Interestingly enough, a common household item is one of the top recommendations that I have for controlling these pests. Hydrogen peroxide (the standard 3% topical variety) can be used as a soil drench.

Mix one part peroxide with four parts water, and pour it through the soil at the root zone until it begins to come out of the base of the pot. The peroxide kills fungus gnat larvae on contact.

Neem oil is also an effective soil drench to combat fungus gnat larvae. Dilute the oil with water per manufacturer’s directions and directly drench the soil at the roots of the plant. You can also spray the upper portion of the plant to keep adult gnats at bay.

AzaMax is a higher-strength concentration of the azadirachtin which naturally occurs in neem oil. It’s safe in hydroponics use as well as in greenhouses, gardens, and indoors. Use it per manufacturer’s directions in the same way you would use neem oil.

Pyrethrin sprays are also effective against fungus gnats and their larvae. I recommend Garden Safe Houseplant & Garden Insect Killer.

To use pyrethrins, lightly mist all plant surfaces and the top of the soil. You don’t want the plants dripping wet, a thin mist will be enough.

If there’s fungus gnat larvae in the soil, spray the soil directly to thoroughly moisten the top, then avoid watering until the soil has dried to at least a 2″ depth.

Environmental Fungus Gnat Control

One particular form of bacteria will destroy fungus gnat larvae. Bacillus thurigiensis var. israelensis. It’s not in most commercial BT sprays, but it is available as part of a product called Microbe-Lift BMC Fertilizer.

Use this to fertilize with, and you should see a decline in your fungus gnat problems.

Another way to get this bacteria in your soil is by sprinkling Mosquito Bits over the surface and watering them in. You can use these both indoors and outdoors. They aren’t just for mosquito killing!

As they break down, they release Bacillus thurigiensis var. israelensis into your soil, where it can get to work killing larvae.

Beneficial nematodes can also play a major part in eradicating the fungus gnat larvae. You won’t be able to see these microscopic soil-dwellers, but they will take out the fungus gnat larvae along with hundreds of other soil-dwelling pests.

For people with indoor plants, you can add these nematodes to your soil with one of these Nema-Globe Pot Poppers. Larger garden or full yard coverage can be achieved using Dr. Pye’s Scanmask, which disperses the nematodes evenly with water.

Be aware that you cannot use beneficial nematodes at the same time as your hydrogen peroxide soil drench, as it will kill the nematodes! Wait for at least a couple weeks after the infestation and then add nematodes back into the soil.

Preventing Fungus Gnats

Inspect potential plants before buying them. Check at the base of the plant, gently looking through the soil to find signs of the clear or whitish fungus gnat larvae. Avoid any plants with visible adults around them.

To be doubly sure that new plants are pest-free, keep them quarantined from other plants for at least 2-3 weeks. This gives you plenty of time to spot newly-emerged adults, as well as establish control methods before they can spread.

Avoid overwatering your plants. Also, if fungus gnats or their larvae are in evidence, avoid watering until the soil has dried to at least 2″ deep.

Be sure to use sticky traps to find adult gnats. Since it only takes one gnat to lay potentially hundreds of eggs, you want to keep the adults at bay!

For indoor plants, these butterfly-shaped sticky stakes work well and are smaller in size (although they’ll still look big next to smaller plants). Outdoors, you can use these double-sided sticky traps.

Finally, mulching has good effect against fungus gnat infestation, as the mulch keeps the adults away from the soil. This prevents them from laying their eggs.

You can use a thick layer of a stone chip mulch over your soil, or opt for a product such as GnatNix.

Made from recycled glass, GnatNix prevents emerging adults from getting out of the soil. It also keeps adult fungus gnats from laying their eggs in the first place.


In the end, the best solution for fungus gnats is prevention. But there are many options you can take if they catch you unexpectedly. Have you ever fought against fungus gnat larvae or adults? What methods did you choose? Share your stories with us in the comments below!


The Green Thumbs Behind This Article:


Kevin Espiritu
Founder

Lorin Nielsen
Lifetime Gardener

Fungus gnat larvae and the adult flies can destroy your garden. Learn how to treat and prevent fungus gnats in a few simple steps.
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53 thoughts on “Fungus Gnat Larvae: How To Kill Them Off Quickly (2018 Update)”

  1. I am thinking of getting the nematoads but following the link
    There are 8 MILLION of them
    Won’t I then be dealing with a different infestation ?

    • There are different types of nematodes – beneficial ones cause no damage to your plants, root knot nematodes are a bad type. Anything you’d buy from Amazon / elsewhere will be of the beneficial kind, so there’s nothing wrong with having a ‘good’ infestation 😉

  2. So, I have heavily planted terrariums. One is only for test growing plants. The other my crested gecko lives in. Some point last spring, they contracted fungus gnats. Obviously, using chemicals is not an option, including neem oil and it’s relatives, both because of my gecko, and because many of these plants are sensitive to chemicals. I have tried nematodes. I actually used way more than I needed, mostly to justify the shipping cost (insanely high). The treatment knocked them out like a nuke!! Gone.

    But the little s.o.b.’s are back. I can only let the terrariums dry out a bit, as the gecko needs higher humidity. There is no standing water in the bottom. I can see the bottom and the terrariums have a false bottom layer and a small layer of drainage material. Also, while I did install emergency drains, they are not designed with extreme ease of use in mind. (So I could potentially do a peroxide drench, but it’d be a massive pain in the arse.)

    I purchased a nematode box from a local store, but I knew there was a good chance it was probably really old and dead. It didn’t do anything. So….I need to come up with a way to kill these pests, that won’t hurt my gecko, and not cost a fortune like my first purchase of high grade nematodes. Removing, cleaning, and replanting the terrariums is not an option. The planting media is a personally designed custom blend (coir fiber, various fine barks, actual sphagnum moss–not peat) that was costly to make. Also, most of the plants have been growing for at least a year and are very well rooted in and mixed in with each other, so pulling them out could likely kill them.

    I’m thinking Bt-i. might be worth a try. Also wondering about a Castile soap spray? As mentioned above, I could potentially do a soil drench, but removing the excess liquid would be a major pain.

    Any suggestions?
    Thanks,
    Barbara

    If I had dart frogs, I would not have a problem…well maybe fat frogs.

    • Nematodes from local stores are hit-and-miss because often they don’t care for them properly. They should be sold & used within a week or two of arriving at the store, and most of the time they need to be stored in refrigerated environments. If your local store doesn’t have them in a fridge, don’t waste the money!

      Having said that, there are some cheaper options through Amazon for nematodes at this point. The Nema Globe Pot Popper contains 4 million beneficial nematodes and is surprisingly reasonable — just a smidge more than using the BT-i would set you back. The biggest problem with nematodes is that to maintain a consistent population, you really need to release two batches with a one-week break in between releases. As beneficial nematodes generally only live for 2-3 weeks, the second application gets you another generation of breeders to help maintain the colony of beneficial soil-dwellers.

      Neem oil is a natural derivative from pressing neem seeds. The seeds contain a natural substance known as azdirachtin. While it’s not toxic to geckos normally, it apparently can create some very minor indigestion if they eat large amounts of neem on the leaves of your terrarium plants. I wouldn’t use AzaMax, but if the gecko typically leaves your plants alone and aren’t snacking on them constantly, diluted neem oil -should- be okay on the plant surfaces themselves, and definitely should be fine on the substrate.

      I understand the desire for caution, though! BT-i should work fine and won’t harm your geckos. Sprinkle Mosquito Bits in the tank and work it lightly into the substrate, as that should work to prevent the gnats. They’re made with BT-i on corn cob pellets, so they should be safe even if the gecko decides they’re snack food.

      Castile soap sprays may also work to help smother the eggs, but it’s a bit trickier. Not all castile soaps are made equally, and some may contain uncommon oils (even sometimes neem oil) which may be problematic for your gecko! As long as you check the ingredients, they should work to at least eliminate the eggs in your substrate. But it may not do much against the gnat larvae, which is why I think doing Mosquito Bits may be your best bet right now.

  3. Amazing and concise info on fungus gnats. Best info I’ve found online.

    Just curious, which of these methods work for which stages of their life cycle? Are there overlaps? Like does peroxide just kill larvae or does it kill eggs AND larvae? Same with the nematodes; which do those handle? This is the only thing I didn’t quite pick up on in the article.

    I want to make sure I’m handling all stages otherwise I don’t think I will ever win this battle. So far I did a peroxide drench (although I didn’t quite drench 100% of the soil; I only bought one bottle of peroxide so the 1:4 ratio only mixed up enough to water maybe 90% of the quantity of soil in my Fiddle Leaf Fig pot; it ALMOST drained out of the bottom though and I got all of the top of the soil drenched completely where most of the larvae should be anyway, right?). I also sprayed Neem oil all over the leaves of the plant and thoroughly wetted the top of the soil.

    FINALLY, once the neem dried a little I sprayed a little bit of an organic insecticide spray I had on hand (based on plant oils; cinnamon, thyme, peppermint). Brought the plant inside and a couple hours later I saw gnats on the outside of the pot and still see them daily.

    I hope I get this under control 🙁

    • The peroxide drench will deal with hatched larvae and will damage the eggs, although a few eggs may still hatch.

      Neem oil as a soil drench will also deal with hatched larvae, plus will coat any eggs with an oily residue that will slowly kill them off as well. On the leafy surfaces, neem oil acts as a gnat preventative. Sprinkling a little neem cake meal on the surface of the soil will also work well to prevent fungus gnat adults from landing on the soil – neem cake meal is a byproduct of the manufacture of neem oil.

      AzaMax is in essence a purer, more potent version of neem oil. You can use it exactly as you would neem oil.

      Pyrethrin sprays are effective against adult fungus gnats, but it can be hard to catch all the adult gnats as they do fly. You might find yellow sticky trap stakes in your pots to be a better preventative measure against the adult gnats.

      Both forms of bacillus thurigiensis var. israelensis mentioned in the article are effective against the larvae themselves. The BT will also damage the exterior of eggs, making it far more unlikely for them to hatch.

      Beneficial nematodes are extremely effective against both larvae and eggs, but less so against the adults (again, the adults fly, and the nematodes are soil-dwellers). You should not use a peroxide drench if you’re using nematodes, however. The nematodes will be killed off along with the gnat larvae.

      Coating the surface of your soil with neem cake meal or a thick layer of mulch is a far easier way of protecting the soil from adults depositing their eggs there than trying to kill off the adult gnats themselves. They will move on to easier targets if you use deterrent measures on your plants.

      Best of luck!

  4. I’ve tried both neem oil/soap/water and Hydrogen Peroxide/water solutions (mixed at the proper amounts) with NO success to kill fungus gnat larvae….I’ve tried multiple times with no success..

    • The larvae can be stubborn little pests! If you’re still having larval issues, you’ve got two choices.

      If it’s been a while since you did a peroxide soil drench (and you’ve watered consistently afterwards), you can release beneficial nematodes into the soil. These microscopic soil-dwelling worms will eat the fungus gnat larvae and are harmless to humans or our pets as well as to the plants.

      Alternately, you can use Bacillus thurigiensis var. israelensis, a specific strain of the BT bacteria which is very effective against fungus gnat larvae. It can be found in Mosquito Bits or in a product called Microbe-Lift BMC Fertilizer. Both options will work, they just require different applications.

  5. I know this information is for getting rid of fungus gnats. But I have 2 questions: I’m not sure what type of flying insects I have, by they sometimes come from the belt purchased organic soil I bought AND the mulch for cacti.
    So I assume they’re fungus gnats.

    These winged bugs fly out of soil from plants without mulch as well as ones with mulch.

    So my first question is, can I still use the problem solvers you mentioned such as the hydrogen peroxide soil drench, nematodes & neem on the leaves?

    Second situatuon: (which jogged me to this page) is, regarding the very tiny little cotton ball-like beads on my indoor China Plant.

    You suggested to another reader that these white dots are most likely mealy bugs. They’ve cause many leaves to fall off my plant (or maybe I just didn’t water it enough). I snipped off those branches most covered then used soapy water to spray- saturate the plant & the soil.

    The problem still exists. This leads me to my 2nd question: what’s the best approach to did this problem. Seems like some type of systemic spray would be best; Neem or that stronger pyrithin?

    • The peroxide soil drench should be effective on nearly all flying, gnat-like insects that lay eggs in the soil. However, you’ll need to avoid using nematodes for a while after a peroxide drench, as the drench will kill off the nematodes as well.

      If they’re coming out of mulched plants, I’d be more inclined to assume you had whiteflies than fungus gnats. Our detailed pest guide for whiteflies will help you to figure out how to treat those.

      On the second situation, I’m going to aim you at our guide on eliminating mealybugs for treatment methods!

  6. Is there a maximum amount of times you can use the peroxide solution before it will harm your plant? I was planning on using it every time I watered along with the sticky strips until the gnats are gone, but I don’t want to kill my plants either. Thanks!

    • Do NOT use the peroxide drench every time you water. You’ll be causing more harm to the plant than it’s worth.

      I recommend doing the peroxide drench no more than once per month. If you’re having continual problems, opt to use beneficial nematodes in the soil instead of doing peroxide drenches, as the nematode population will eat the fungus gnat larvae.

    • Hydrogen peroxide drenches are safe as long as you use them sparingly. Don’t do this treatment every time you water, and even once a month may be too much for some plant types. This is an extreme method that will effectively kill off the fungus gnat larvae, but it’ll also kill off your beneficial nematodes in the soil and other soil-dwelling organisms you might want there.

      If you’re concerned or have a plant that’s in less than perfect health, I recommend adding beneficial nematodes to your soil instead. These microscopic soil-dwellers will eat the larvae and deal with your problem, plus they will eat any eggs laid in the soil and prevent future gnat infestations.

  7. I have noticed gnats in my house which are flying at the windows. I don’t have any plants in my house. How can I find the source to then take action?

    • While fungus gnats are the most common indoor gnat pests, they really require the presence of potted plants to survive. The soil is required for them to lay eggs.

      It’s far more likely that you’re experiencing fruit flies. Adults can live for a few weeks in the house, and you can catch them by placing saucers full of vinegar around. They will be attracted to the vinegar and will drown. Larval fruit flies are usually found in overripe fruit or vegetables, so dispose of anything which matches that description.

    • Nematodes are a form of microscopic worm that lives in the soil at the base of your plant. There are thousands of different types, and they’re too tiny to see without a microscope. These live in soil around the world naturally. Some are good, some aren’t.

      Adding beneficial nematodes to your soil is a common method to reduce pest populations. The beneficial varieties will take up residence in your soil or potting mix as long as it remains moist, and they will kill off the larval stages of many other pests.

      Harmful nematodes such as root knot nematodes exist, and are common dangers to many types of plant. Beneficial nematodes often eat the harmful types so they don’t attack the root systems of your plants, too.

  8. For the hydrogen peroxide soak are you removing all the soil and soaking the roots and then proceeding to buy new soil for the plants?

    How long should the roots be soaked for?
    Thanks!

    • I used the wrong terminology there, and I’m glad you pointed it out — the actual term is a soil drench, rather than a soak! I’ll have to fix the text there. Thank you!

      Once you’ve mixed the peroxide thoroughly with water, pour it over the soil’s surface slowly, being sure to evenly coat the soil to get it through the entire contents of the pot. Once the excess moisture starts coming out the bottom, stop and leave it to drain on its own.

      Do not remove the plants from their soil, do not soak the roots on their own!

  9. My sister gave me a freshly potted plant and it didn’t take me long to notice that it had gnats. I put it outside on my back steps so they wouldn’t spread to my other plants. I kept it out there for about 2 months and the gnats are completely gone. My back steps are small and there are quite a few spiders that have webs between the rungs of the railings. So it seems that nature took care of the issue and I have repotted it and moved it inside, gnat free.

    • I love it when nature fixes a problem on its own. Eliminate the adult gnats, and they can’t lay eggs and create more larvae! Sometimes, spiders are useful little creatures, aren’t they?

    • If you’re spraying the top of the soil, you can actually spray it fairly regularly, 2-3 times per week if you want. If you’re spraying the plant’s leaves and stems, it’s better to space it out to every 5-7 days. Neem oil breaks down quickly in the soil and is absorbed as a fertilizer by the plants themselves.

  10. These are a lot of options. Thank you! Which would you recommend doing first, second, third and for someone who has many houseplants huddled together?

    • If you have a lot of houseplants grouped together, I’d start by breaking them up and trying to determine which plants are infested and which aren’t. Segregating them should help you to determine your problem plants and then work towards a solution.

      Once you know which plants are housing your gnats, I would start with a peroxide soil drench to kill off the larvae. Use the standard 3% hydrogen peroxide, and mix one part of that to four parts of water. Soak the soil with that solution at the root zone (avoid getting it on the plant’s stems and leaves). That should kill off the larvae.

      To back that up so that gnats don’t linger around the foliage, use neem oil to spray all surfaces of the leaves and stems. That should keep the gnats away from the plant itself.

      That should clear up your issues very quickly and then you can go ahead and re-arrange your plants. You may actually want to just treat all of your plants with the peroxide soil drench and neem oil to be on the safe side!

  11. After fighting fungus gnats in my houseplants – brought on by a NEW bag of potting soil with gnat eggs in it! – I’ve finally got them on the run, and wanted to share some things I’ve found that helped.
    Fungus gnats are hard enough to eradicate when you have no way of putting your plants outside for natural predators. We, however, live in an old apartment building, and a couple of our nearby neighbors are lazy about their housework – unless pest control’s been by lately, their homes are always full of gnats, flies, spiders, and oddly enough, STINK BUGS. (…I don’t even wanna know.) Even after we’ve eradicated gnats in our plants and even when we keep our unit pristine, all it takes is one of those neighbors’ little ‘friends’ hitching a ride in on someone’s hair or a food delivery bag and the problem starts all over again.
    BUT! There is hope. Read on for details.

    Repellents: FAIL. A lot of people told me that gnats are repelled by some plants like basil, mints, lemon-scented plants, and etc. Baloney. The gnats saw my sweet-mint, peppermint, and lemon balm plants as a delicacy and slaughtered them all. I lost four sweet basil plants and two dozen seedlings to the gnats, and the remaining ten fully grown Basil plants were nearly killed as well. That doesn’t include the multitude of seedlings, houseplants, and previously healthy plants the gnats killed or sickened. I’ve also tried the old ‘block’em off at the pass’ technique – covering the surface of the soil with gravel or something similarly unfriendly. The gnats had no difficulty worming their way past the gravel to lay their eggs and, because that gravel kept the soil from drying out quickly, their larvae thrived in the dirt to the point of building civilizations, developing their own communication systems, and building little gnat governments. By the time they were considering elections, I chucked the affected plants entirely; those plants were beyond hope and no one wants their home stunk up by insect politics.

    Treatments: DOUBLE FAIL. Neem oil didn’t do a thing and the Neem oil drench I was instructed to try killed an entire window-box of catnip plants. (My cats still hold a grudge.) Soapy water didn’t even phase the larvae even when sprayed directly – I swear the little worms just looked up at me and asked “What, that’s supposed to hurt?” None of the soaps, detergents, or dish soaps I tried did anything but weaken my plants, no matter what water-to-soap percentage I used. None of the pesticides I tried were effective at more than killing adults – nothing killed the larvae and nothing prevented the eggs from hatching. Drying out the soil, even down to the bottom of the planter, was completely ineffective at anything but killing my plants. Every time, I eventually had to water my plants to keep them from keeling over completely; every time, I checked the soil the next day and found hordes of larvae thumbing their tiny black noses at me. Finally, I read many tips suggesting to grind up a “mosquito-dunk” (tiny compressed disc of mosquito killer meant to be dropped in standing water to kill larvae long-term) and water the plants with it to kill larvae. Nope. The larvae drank it up, belched, and hollered for another round. How rude.

    Product search: Even bigger fail. A lot of sites promote treatments and tools and claim “you can find these anywhere!” Apparently large cities in the Midwest don’t count as “anywhere.” Over a matter of months, I developed a new and unpleasant reputation in my hometown: “that loopy cat-lady who asks about stuff we don’t sell, doesn’t get the meaning of ‘let it dry out,’ and is probably growing pot because she won’t take her plants outside.” (I’m not growing pot – I live in an apartment and have had plants stolen after being left outside for five minutes to attract aphid-killing ladybugs. Joke’s on the idiot who stole a planter of dying aphid-infested peppers.) No one around here carries yellow sticky traps, even the specialty hardware and gardening shops – they only have white traps and yellow or orange flypaper rolls. Asking about predatory insects resulted in “Check the pet stores, they have scorpions.” (Scorpions aren’t insects.) We found predatory mites for sale online – about $25 for a tube – but the delivery charges would have brought the full price up to over $75. It was nearly impossible to find pesticides that include gnats or sciarid flies on the labels. More often than not, anytime I asked for advice at a store, botany shop, garden center, etc, I got the same old song and dance: “Dry it out and spray it with soapy water. It’s not rocket science.” Thankfully it’s not rocket science, because if it was, we could be sure the moon landing was faked. The closest thing I could ever find to ‘gnat control’ was little plastic bulbs with bait – bait the gnats decided was chopped liver compared to my Zinnias. It took literally months and a couple dozen different stores to even find mosquito dunks, another product recommended for gnat killing.

    By the time I FINALLY found a system that worked, my precious houseplants, veggies, and herbs were over 2/3 gone – twenty carrots, 30 rosemary in plants and seedlings, countless mint and lemon balm sprouts, two fully-grown mint and Lemon balm plants, and seven catnip plants and dozens of sprouts, AND the orchid my partner gave me were all dead. Half the basil and Zinnias were killed and the Columbine sprouts I was nurturing for a relative were double-dead. My two aloe vera plants, previously HUGE and healthy, are barely clinging to life after numerous repottings due to larvae-infested soil. Now I’ve FINALLY figured out a system that works – my basil plants are completely gnat-free and I’m making major progress with the other plants.

    The verdict?
    Never buy soil if the bags are kept where they can be rained on, and never if the bag feels damp or cold; wet soil attracts gnats. Don’t mix peat moss in with the potting soil – gnats are attracted to it and breed in it. If you can’t find yellow flat traps, you can suspend yellow flypaper rolls above the plants horizontally to trap adults; gnats hiding in the foliage will be disturbed and flee to their deaths above if the planter is jostled or watered from the surface. I’ve hung a length of that flypaper over the planters on my grow table, right between the grow-lights and the planters, and I’ve also strung a length between two wire supports above my basil plants; over time, these ribbons have been catching fewer and fewer adults and I’ve been seeing far fewer larvae in the soil. Water plants from the bottom up if at all possible, because damp soil on the surface attracts gnats. My ten surviving basil plants are in a long plastic window-box with drainage holes drilled in the bottom, and that planter is nested in another identical one. When the soil’s dry a few inches below the surface, I pour about 2″ of water into the bottom planter and slowly submerge the top planter to let it soak in. The mosquito dunks didn’t work when used as directed, but they do work for long-term – I chopped up a dunk into six pieces and put a piece in all six depressions in the bottom planter – the ‘feet.’ There’s always a little water remaining in these depressions even when the plants are nearly dried up, and the dunks dissolve just enough each time to dose the fresh water with enough poison to deter adults and kill any new arrivals. Planters that aren’t nested and have openings for watering – like self-watering planters – are also vulnerable but a piece of mosquito dunk in those openings or reservoirs works wonders. When starting seeds, I’ve taken to keeping plastic wrap over the planters until the sprouts are almost big enough to transplant; in some cases, I’ve moved smaller, weaker plants (like the latest lemon balm sprouts) to makeshift terrariums to protect them from gnats until they’re stronger.

    In summary: Mosquito-killing tablets can provide long-term treatment and deter new arrivals when used correctly. Water plants from the bottom up or make sure there’s no damp soil left on the surface; after watering, carefully disturb the surface soil to cover any damp soil and encourage it to dry up. Use slow-dissolving pesticides in water reservoirs to kill larvae with every watering, and around drainage holes to prevent gnats from creeping in. Yellow flypaper strung horizontally captures more adults than when hung vertically and helps you keep an eye on the population. Some gnat larvae are resilient to chemical pesticides, adverse conditions, and tried and true treatments, especially if you’ve fought an infestation for a few hatchings. Choose your potting soil carefully to avoid purchasing infected bags and avoid gnat-friendly soil additives. Protect seedlings and sprouts until they’re stronger, and consider protecting delicate plants long-term. If the gnats are coming in from an outside source, for instance a neighboring apartment, contact your landlord and take measures to keep invading pests out.
    All joking aside, I hope the experience and tips I’ve shared will help others get their bugs under control.

  12. Great and informative article. 1 week ago I split my 4 stem fiddle leaf in one pot into 2 stems in 2 pots. It’s been doing well except for the newly sprouted growth stalled out and one of the bottom leaves was dropping. Came home today and the bottom leaf was dead. Plucked it off then watered both pots. Gnats flew out of the soil when I watered. I didn’t have these before I repotted. I suspect they came from the new soil I bought. UGH! I’m going to pick up some neem oil, sticky traps and Mosquito Bits tomorrow. Should I throw out this soil and report fresh with new soil? I hope I can save my precious fiddles. They are 5 feet tall and gorgeous.

    • While you could throw out the soil and repot it with fresh potting mix, you might want to try a hydrogen peroxide soil drench first. Use the standard 3% hydrogen peroxide sold for medical use, and mix one part of that to four parts water. Apply it to the soil beneath the plant to drench the soil (but avoid getting it on the foliage/stems). The peroxide will kill off the gnat larvae in the soil.

      Neem oil should keep them off the leaves as well!

  13. I had a gnat infestation in multiple plants (it was a result of already infested Miracle Gro soil). I took the plants out of the soil and put them in glass bottles full of water to continue growing and threw away the soil. I haven’t seen any more gnats since doing this and my plants are doing well in water, as that’s how they were propagated in the first place, but I’m wondering: have the larvae died being submerged in water for about 2 weeks now? or will they hatch and become adults once I take the plants out of the water and repot them in new soil? I haven’t found any information about this online elsewhere.

  14. Can i mixed in a 1:4 peroxide to water and spray on the of soil. I need to spray very wet
    or not. Can i add peroxide to reservoir. Is it the same ratio.

    • A soil drench is meant to completely soak the soil. Doing it with a spray bottle is a tricky solution, as you should avoid getting any of the spray onto the leaves or stems. I’d do the 1:4 ratio of peroxide to water and then gently pour it directly onto the soil’s surface, being sure to get all of the soil thoroughly moistened.

      Do not use this peroxide/water mix as a replacement for normal watering, however. Do your soil drench between normal waterings so that it can kill off the gnats, and give it a day or so to do its job before your next watering. Avoid adding peroxide to your water reservoir.

  15. Thanks for the tips. Any advice about gnats and microgreens ? Is neem oil safe and, if so, at what stage would you apply it?

    • Gnats shouldn’t be a huge issue w/ microgreens provided you’re growing them in clean soil every time and not planting too densely. If you do decide to use neem, I’d thoroughly wash your micros post-harvest.

  16. Apparently, our gnat popualation in Louisiana is hardcore….the larvae can (and do!) live in completely dry soil. Ive tried cinnamon, peroxide, sprays, and letting the soil dry out between watering. I think I’ll try nematodes next. Any other suggestions?

  17. i followed your link to the peroxide, purchased and mixed in a 1:4 peroxide to water ratio. the link is to 35%, which is way too strong and needs to be mixed down to 3% before the 1:4 ratio is correct. I burned my plants and made a huge mess. The pots got hot, the soil foamed over and the started steaming. Luckily I only watered 5 of my 23 house plants before I realized the huge mistake I had made.

  18. if you add the nematodes first will the hydrogen peroxide kill the nematodes as well as the gnat larvae? can they be used together or not?

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