How To Get Rid Of Fungus Gnats – What Works And What Doesn’t

Fungus Gnat

Just before the first frost of the season, we bring all our potted plants indoors.  This marks the beginning of our seasonal battle with indoor fungus gnats.  They’ve been busy colonizing all our pots all summer, and within two weeks of coming inside they are pretty much everywhere.

The adult fungus gnats don’t really fly around a lot and initially aren’t that noticeable unless we disturb a pot by moving it around or when feed watering.  We’ll see them fly a short distance to another pot, or maybe spot a few running across the surface of the soil while frantically fluttering their wings.  But once the infestation is in full gear, the adult flying gnats are everywhere.  The adults can’t do any damage to plants (they don’t eat), so they are more of an annoyance.  But their presence is a clear sign that there is an infestation underway inside our plants’ soil.

We use heavily composted soil for all our plants which is heaven for the hungry gnat larvae.  We water all our indoor plants using capillary mats, which is convenient for us because we have so many plants and don’t have to worry so much plants drying out between waterings like we did when we watered each plant individually.  But bottom watering makes soil conditions even more ideal for the gnats because the plant’s soil is kept evenly moist.  Plus plants use less water in the fall and winter because of the decreased length of the day and growing period, which means the soil stays wetter longer.

For the older, hardier, more established plants we have never really seen any damage caused by the gnats.  But plants such as herbs, impatiens, hoyas, and vegetable seedlings seem to really suffer once the fungus gnat larvae take hold in the soil.  Seedlings fail to thrive.  Other plants drop leaves or succumb to fungal infections.  Apparently the larvae are capable of carrying pathogens that can cause real damage to vulnerable plants.

Instead of relying on random recommendations on the internet which didn’t worked for us in previous years, we decided to research and test out different methods to see what was most efficient and economical.  Keep reading if you want to see the specifics of what we tried and whether (or whether not) they worked.  If you just want to know what ended up working the best, jump to the bottom of this page.

Some Quick Fungus Gnat Facts

Size

Adults are about 2.5 mm in length.  Larvae are tiny when hatched and can grow to 5.5 mm in length

Lifespan

Adult fungus gnats live seven to ten days.  It takes a hatched egg approximately 18 days until it pupates and emerges as an adult.

Biology

Adult fungus gnats do not eat; their only purpose is reproduction.  Over the course of her short lifespan, a female fungus gnat can lay 100 to 200+ eggs.   A female gnat will deposit up to 40 eggs at a time on the surface of moist soil, near decaying organic matter, or hidden closer to the stem of a plant.

Within 4 to 6 days, the eggs will hatch and begin feeding.  For 14 days, the larvae will feed on any organic matter on or in the soil.  They primarily focus on eating the soil’s organic matter, but in the case of severe infestation, they will also begin feed on the finer root hairs of a plant, the plant’s roots, and possibly the crown, stem and any leaves of the plant that are located near the surface of the soil.  Larvae are usually located in the top 2-3 inches of soil, but we have observed larvae as deep as 6 inches in potted soil as well as around a pot’s drainage holes.

After 14 days the larvae will stop eating and begin their pupal stage to emerge as adults in 4 to 6 days.  A female can mate shortly after emerging from her pupal case, and the cycle begins again.

Fungus Gnat Control Methods

Apple Cider Vinegar

Wasn’t effective in catching adult gnats; was effective in killing the plant

We tried placing a shallow glass containing apple cider vinegar near affected plants and, at best, had mixed results with minimal catches.  The few catches we had were probably accidental.  This method might work for fruit flies but did not seem effective in reducing adult fungus gnats.

We’ve seen a few sites that recommend adding dish soap and vinegar to your plant’s soil to kill larvae.  DO NOT do this.  You might kill a few larvae, but you will definitely kill your plant.  Vinegar contains acetic acid, which kills a plant’s cell structure and acidifies the soil.

Diatomaceous Earth

Is not effective in killing larvae in the soil


Diatomaceous Earth is made up of very tiny crystals of silica.  To humans, it looks and feels like a fine, white powder.  To small crawling insects, it feels and acts like giant shards of glass.  As insects crawl over the dry Diatomaceous Earth dust, the sharp silica causes rips and tears to their body, which leads to dehydration and eventual death.

There have been suggestions that adding Diatomaceous Earth into the soil as well as top-dressing the soil will help eradicate fungus gnats.  However, as soon as you read the directions on a bottle of Diatomaceous Earth, it becomes obvious why this won’t work to kill larvae in the soil and will likely not kill adult gnats.

The DE has to stay dry.  Diatomaceous Earth is not effective when it is wet.  It will not work when the soil is damp or the environment is humid.  Diatomaceous Earth may work in killing existing insects if you are pre-treating dry soil before potting up a plant.  But when treating an existing potted plant it’s soil will be moist, rendering the DE ineffective.  As for adult fungus gnats who are flying around, their bodies would have to come into direct contact with dry DE in order to cause any real damage.

Mosquito Bits

Worked for killing larvae in the soil

Mosquito Bits are made up of granules containing a biological larvacide Bacillus thuringiensis israelensis (BTI for short).  It contains a natural bacteria that is specifically targeted to kill certain larvae, and is considered to have a low toxicity for non-target species of insects.

When the larvae eat the BTI, the bacterial toxin is activated by the alkaline environment in their gut causing infection and internal gut damage.  Depending on how much they consume, the larvae can die within a short hours, or more gradually over the course of a week or two.  In order for the BTI to work, it has to be eaten by the target species.  For this reason, it won’t kill pupating larvae or adult fungus gnats because they do not eat.  You will still need to use a method to catch the adults.

Although the product label says to sprinkle the granules on top of the soil and then water well, you probably don’t want to do this for your indoor potted plants.  The granules are actually bits of corn cob, which will eventually decay and get moldy.  You really just want to extract the BTI from the corn cob bits by soaking them and then use the strained water to water your plants.  The BTI usually only penetrates the top couple of inches of soil, so you will want to water generously to distribute the solution as deeply as possible.  Or press a wooden spoon or dowel into the soil to create holes that will allow the solution to penetrate deeper into the soil

You can also use a wooden dowel (or pencil) to make 4-6 inch deep holes in the plant’s soil, which will allow the BTI solution to be absorbed and distributed deeper into the soil.

How Much Mosquito Bits To Use Per Gallon Of Water?

For fungus gnats, the manufacturer recommends using 4 tablespoon, or 1/4 cup, of Mosquito Bits per one gallon of water.

Add water to a one gallon jug and pour in 1/4 cup (or 4 tablespoons) of Mosquito Bits.  Shake the jug well and let the granules soak for around 6 hours.

Plan on using the solution within 24 hours for maximum effectiveness.

How Long Is BTI effective In The Soil?

Because the BTI is activated by alkaline environments, it breaks down faster when used in acidic soils.  But typically, you should expect it to remain effective for several days to two weeks.  One study reported that it remained effective underneath the soil’s surface for four to twelve months.  If fungus gnats emerge 2 weeks after treatment, you’ll need to reapply the solution.

Sunlight also breaks it down faster after it is activated.   Once you’ve made a batch of the solution, keep it out of sunlight.  A study reported that sunlight can reduce the effectiveness up to 60% within 24 hours.

Mosquito Donuts

Worked for killing larvae in the soil

Mosquito Donuts are similar to Mosquito Bits with the exception that they are made to be slow release so that the BTI is effective up to 30 days.  The donuts are much more concentrated than the bits, so if you only need one gallon, you would use (approximately) 1/16th of the donut.  And because the donut is slow release, you should let it soak in water for at least 24 hours before applying the solution drench to your plants.

Permethrin

Didn’t see a noticeable reduction in numbers of larvae or adult gnats


It is possible to use a permethrin spray on the adults (mosquito spray, flea/tick spray) and mix permethrin granules into the soil.  However, it’s been discovered that some gnats can develop a resistance to a pyrethroid insecticides making it ineffective, especially if you have previously treated plants or soil with permethrin.

We used a permethrin spray on the adults after agitating them into flight by shaking plant pots.  They seemed pretty annoyed at getting sprayed, but we didn’t notice any dead gnats a few hours after spraying so we can’t say it worked.

Hydrogen Peroxide

Did not work for killing larvae in the soil


It’s been suggested that by drenching the soil with a hydrogen peroxide mix will kill fungus gnat larvae.   It’s important to keep in mind that if your soil is fairly compact, hydrogen peroxide will only penetrate the top few inches of the soil.  As soon as the hydrogen peroxide interacts with organic material (when you see fizzing) it begins to break down into water and oxygen.  Depending on how much organic material is in your soil, even if your soil is loose the hydrogen peroxide will likely break down before getting deeper into the soil.

However, larvae that are close to the soil surface would likely be damaged or killed by the hydrogen peroxide.  You would still need to find a way to address the larvae further down in the soil.

Another thing to keep in mind is that the same action that might kill the larvae is also killing beneficial bacteria in your soil.

The strength of hydrogen peroxide is also important.  People recommend using a 1:3 percent ratio of peroxide to water.  Using this ratio, a 3% strength which is already 97% water would be severely diluted and less effective, and a 35% strength would likely be too strong and could damage the plant’s roots.

Imidacloprid

Does work for killing larvae in the soil

Imidacloprid is a systemic insecticide that works similarly to nicotine.  When larvae or insects eat a plant or material containing imidacloprid, their nervous system is disrupted and they eventually die.

For the liquid version to work in potted plants, the soil must be allowed to dry out enough so that the soil absorbs the imidacloprid solution without any water runoff.  For the next 10 days, any watering should be minimal to prevent leaching the imidacloprid solution from the soil.  This ensures that the soil maintains the right ratio of imidacloprid during the 10 day period the plant needs to fully uptake the insecticide.

If you use granules, you would mix them into the top 3 inches of soil and water as you normally would.  The product label says that the slow release granules remain effective for up to 8 weeks.  We didn’t try using imidacloprid because we are also growing vegetables indoors and the product label stated it was to be used for non-edible plants only.  However, multiple product reviews claimed it was highly effective in getting rid of fungus gnat larvae in the soil.

Neem Oil

Did not work for killing larvae in the soil


Neem oil is great, especially for aphids and lantana lace bugs.  However, Neem oil doesn’t directly kill bugs.  It is used as a drench, which the plant absorbs into its vascular system.  After an insect eats a portion of the treated plant,  it disrupts the insect’s eating and mating signals.   It can cause insects to stop feeding, stop growing, and reduce sending out pheromone mating signals.

The problem here is that the larvae have to actually eat the plant.  The active ingredient in Neem oil is absorbed by the plant’s roots and taken up through the vascular system to the plant’s leaves.  If the soil itself is rich in organic material, the larvae will have plenty to nibble on other the actual plant itself.  We unfortunately didn’t see any significant difference in the number of gnats when using Neem oil.

Sticky Traps

Worked for catching adult gnats


Sticky traps are definitely effective in catching adult fungus gnats.  They won’t do anything to solve your gnat larvae problem, but they will reduce the number of annoying little flying gnats everywhere.  Fungus gnats don’t seem to big fans of flying far unless they’re disturbed, are looking for water, or are searching out a mate.  We see them more often than not just hanging around either on the soil’s surface or the rim of pots.

By placing sticky traps in every pot near the rim of the pot, you increase your chance of catching newly emerged adults as they crawl up to dry their wings.  You can increase your catch by disturbing the pots (tapping on the sides) and adding a circulating fan (to hopefully blow them into the traps).

While sticky traps won’t get rid of your gnat problem on their own, they are definitely helpful when combined with other methods that address the larvae in the soil.

Top Dressing

More of a prevention than a cure


Top dressing potted plants with a 1-2 inches of coarse sand or small pebbles actually seems like a great way to deter adult fungus gnats from laying eggs.  In theory, the layer of sand or pebbles will remain relatively dry despite waterings.  Even if the gnat were to lay eggs on them, the eggs would likely dry out before being able to hatch.

This seems to be a great way to deter the gnats if you don’t have a gnat problem yet and only have a few plants to top dress.  It can get expensive if you have a lot of plants.

This might be a good option after you’ve gotten your infestation problem under control, but it really won’t help in getting rid of the problem if there are already eggs or larvae in the soil.

Plant Isolation Or Encapsulation

Works as a way to contain a problem, when used with other methods

This method can either temporarily protect an uninfested plant from infestation (while affected plants are being treated) or isolate a plant during treatment for an infestation.

Once you’ve treated a plant, enclose the planter in a small clear trash bag or inside a clear storage tub to prevent flying adults from laying eggs in the soil while you continue to treat your other plants. This method alone will not address an infestation.  The gnats seem perfectly capable of living and continuing to reproduce in the enclosed bag because they still have access to all the resources they need in the plant’s soil.

The clear bags will provide a barrier, preventing any new flying gnats from from spreading out to other plants while allowing you to visually monitor the plants for any signs of infestation.

You can find inexpensive trash bags at your local dollar tree.

Beneficial Nematodes

Works in killing larvae but was too expensive for continued use (because we have too many plants)

Beneficial nematodes, specifically Steinernema nematodes, have been proven effective in addressing fungus gnat infestations.  An added benefit is that the nematodes will reproduce and continue to seek out larvae once they have established themselves in the soil.  They are added to soil using a drench and will remain effective as long as the soil remains moist and temperatures remain in the 60-90 degree range.

The two downsides to using Steinernema nematodes is that they are shipped live which makes them susceptible to very warm weather and very dry humidity conditions during shipping (them might arrive dead) if they are not shipped with a cold pack, and the nematodes tend to be on the expensive side, especially if you have a lot of plants and have to do repeated applications to build up the population in the soil if the initial application didn’t fix the gnat problem.

After the first application, we noticed a definite reduction in the numbers of emerging adult gnats but we weren’t successful in building up enough of a nematode population in every planter to keep the larvae in check.  We probably needed another application or two.

Drowning

Partially works on some larvae but plants weren’t happy

There are suggestions that submerging the pot of a potted plant in a bucket of water for a few hours will drown the fungus gnat larvae.  This approach didn’t work for us at all.  So we tried letting the soil soak overnight (10 hours).  Again, didn’t work.  There were still living larvae in the soil.

The only modest noticeable results of attempting to drown the larvae was after a full 24 hour soak in water.  A rough guesstimate is that around 50% had drowned.  The other half were motionless, but started to revive shortly afterward.

Keep in mind that if you are treating the soil of a plant already in distress or already experiencing root issues, exposing it to even more moisture or depriving the soil (and plant roots) of oxygen too long might also kill your plant.

Bounce Fabric Dryer Sheets

Works to repel 2/3 of adult gnats from laying eggs in soil

There has actually been a study conducted to evaluate whether dryer sheets are effective in repelling fungus gnats.  The sheets contain ingredients that are know to repel insects;  linalool,  benzyl acetate, and beta-citronellol. This study concluded that Bounce dryer sheets were effective in repelling up to 69% of adult fungus gnats.

This method is much easier than top dressing your soil with coarse sand or small pebbles.

Greenhouses cut the sheets up into strips and add them to the soil.  But you can also lay the sheet on the surface of the soil to create a barrier.  Keep in mind that this will only repel roughly two thirds of the adult gnats, so you would still need to combine other methods to completely eradicate the fungus gnats.

Cinnamon

Didn’t work on larvae at all

The theory is that the cinnamon is a deterrent in egg laying, and that it also kills fungus in the soil that is attracting the gnats.  We didn’t observe any difference in adult gnat activity from the soil of plants with cinnamon sprinkled on top of the soil to gnat activity on the soil plants without cinnamon on the surface.  The cinnamon didn’t really seem to deter the adults from laying eggs.  We also mixed cinnamon into the top 2 inches of soil, but we continued to see larvae.

Plants actually rely on fungus in the soil.  The fungus colonizes in the soil and around plant roots, which then supplies the plant’s roots with access to nutrients as the fungi breaks down organic material in the soil.  Any changes made to the biological makeup of a plant’s soil, such as cinnamon to inhibit fungi or hydrogen peroxide (which can kill beneficial bacteria) should include a plan to restore these elements by incorporating a sterilized compost back into the plant’s soil.

Drying Out The Soil

Didn’t kill larvae deeper in soil, and plants weren’t happy

This really should fall in the “killing two birds with one stone” category.   Drying out the top half of a plant’s soil will most likely kill quite a few larvae, but it could also likely kill your plant because you would have to do this several times to completely get rid of the larvae that burrowed a little deeper into the soil.

Some have suggested turning the top few inches of soil and adding circulating fans to dry the soil surface out faster.  This may work in killing the larvae closer to the surface but won’t address those that burrowed deeper.

Potato Slices

Didn’t catch enough larvae to make enough of a difference

Potato slices might be helpful if your soil doesn’t have a high organic content, if you only have a few plants to treat, and are combining this with other methods.  Because larvae feed on organic matter, they might be drawn to the potato slices that are sitting on the soil’s surface.  You would just toss out each potato slice every other day, and hopefully a few dozen feeding larvae along with it.

We tried this on two plants, and presumably because we used composted soil our larvae had plenty of other organic matter to feed on and weren’t interested in the potato slices.   Using potato slices wasn’t an option in potted plants were the plant growth completely covered all the soil’s surface (peppermint plants, dragonwing begonias, etc).

Bottom Watering

Didn’t work; our fungus gnats love bottom watering!

The theory is to water your plants from the bottom so that the soil’s surface is drier.  This may help somewhat along with other measures, especially for larger planters (1 gallon size) if you can control how much water the plants receive so that the surface soil stays fairly dry.

However, most of our plants are in smaller 6 to 8 inch pots and we use capillary mats and watering spikes which consistently keeps the soil evenly moist.  Bottom watering seemed to create the ideal fungus gnat environment.  We noticed that adult fungus gnats will drink water from saucers and capillary mats.

Also, the adult gnats would just congregate near the drainage holes of planters (probably laying eggs there too) for the few plants in larger planters that had a drier surface soil.

Replacing The Top 3 Inches Of Soil

Didn’t address larvae located deeper in the soil

Again, this may be feasible if you’re only dealing with a few potted plants and are also using other control measures.  It might reduce the number of larvae located near the soil’s surface of a larger planter, but it won’t address those that are located deeper inside the pot or those that are located near the pot’s drainage holes.

What Methods Work to Eradicate Fungus Gnats

Before bringing your plants inside:

Pretreat all your potted plants with either a Mosquito Bits or Mosquito Dunks solution one week before you bring your plants indoors.

For treating a fungus gnat problem:

If fungus gnats start to emerge after you’ve brought your houseplants inside, you will need to use one method to kill the larvae and one method to catch flying adult fungus gnats.

  1. Use an insecticide or nematodes to kill the larve
    1. We use Mosquito Bits
    2. Imidacloprid granules and beneficial nematodes also work
  2. Use sticky traps to catch adult fungus gnats

Sources

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3868951/
Source: https://mrec.ifas.ufl.edu/Foliage/entomol/FUNGNAT/fungnat.htm
Source: http://pss.uvm.edu/ppp/pubs/el50.htm
Source: https://extension.colostate.edu/topic-areas/insects/fungus-gnats-as-houseplant-and-indoor-pests-5-584/
Source: https://content.ces.ncsu.edu/fungus-gnats-indoors
Source: https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053864
Source: https://wiki.bugwood.org/HPIPM:Fungus_Gnats
Source: https://union.ces.ncsu.edu/2019/02/fungus-gnats-a-common-problem-of-overwatered-houseplants/#:~:text=If%20the%20plant%20is%20of,for%20control%20of%20fungus%20gnats.
Source: http://npic.orst.edu/factsheets/btgen.html
Source: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7448.html
Source: http://legacy.picol.cahnrs.wsu.edu/~picol/pdf/WA_archive/54755.pdf.2011-11-03
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4553482/#:~:text=Fungus%20gnat%20larvae%20not%20only,22%2C23%2C24%5D.&text=Furthermore%2C%20larvae%20of%20the%20fungus%20gnat%2C%20B.

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