I recently wrote this article for my greenhouse forum, and I thought I would share it with you because fungus gnats are a common greenhouse pest. (I am sure some of you are very knowledgeable about greenhouse pests; this article is aimed at the beginner to intermediate hobby greenhouse grower.)

Fungus Gnats in the Greenhouse

You hear it all too often, "Something is killing my plants!" There are countless problems that cause diminished plant health in the greenhouse, and one of the most common of these is pests. Of the numerous pests that can be found in the greenhouse, fungus gnats come in near the top of the list.

The warm and humid environment of the greenhouse makes an excellent habitat for fungus gnats. Adult gnats are small, dark, mosquito-like flies with grayish wings, about one-eighth of an inch long and are usually first spotted flying around the surface of greenhouse potting soil rich in organic matter. These adult flies are rather innocuous, nothing more than an nuisance, but their presence indicates that their larvae are already at work undermining the health of your plants.

The adult female lays her eggs in soil that is warm, damp and full of decaying organic material. The eggs hatch into the larval stage and, depending on the species of fungus gnat, quickly set off feeding on the beneficial fungi and algae in the soil or on the roots of the plants themselves. Either way, this infestation leads to diminished health in the plants in the form of stunted development or disease resulting from the pathogens carried by the fungus gnat larvae.

If you have flying fungus gnats, you can bet on the fact that you have fungus gnat larvae lurking in your greenhouse soil. The larvae are dark in color, but very difficult to see without the aid of a 10x-20x hand lens. However, the adults are visible to the naked eye. Once identified, you can mount your response to control or eliminate them.

The most effective way to thwart fungus gnats is prevention, and the easiest prevention method is to water your plants properly, especially during the winter. Potting soil that remains moist for extended periods of time attracts fungus gnats because the warm moist soil provides a steady supply of food for their larvae. Also, keep the surface of your pots clean of any dying, dead or decaying plant material.

Insecticides are not warranted in controlling or eliminating fungus gnats as there are so many effective natural remedies. Here are but a few:

· Allow the top layer of soil to dry, and then water as normal with a 3% hydrogen peroxide to four parts water solution. The larvae will die on contact and the peroxide will quickly break down into harmless oxygen and water molecules. You can repeat this as needed. Don't worry about the fizzing; that will subside within a few minutes and is harmless to the plants.

· Top dress your pots with one-quarter inch of sand. Sands drains very quickly and will discourage fungus gnats from laying their eggs.

· Fill small containers half way with apple cider vinegar or cheap beer and a few drops of dish soap. Replace the tops or use plastic wrap to cover the containers. Poke small holes through the tops and place near the infested plants. As the old saying goes, they check in, but they don't check out.

· Place yellow sticky traps horizontal to the soil surface to capture large numbers of flying adults. They are attracted to yellow and can be easily disposed of before they lay more eggs.

· Products that contain Bacillus thuringiensis var. israelensis, (i.e. Microbe-Lift or Mosquito Bits) is a highly selective biological pesticide that kills the larvae of many greenhouse pests. It is safe and has no harmful residues.

· Beneficial Nematodes. Nematodes are microscopic round worms that penetrate fungus gnat larvae, as well as other soil-borne pests, then release a bacterium that consumes the larvae from the inside out. The long-lasting nematodes are safe for use.

These are the most popular natural weapons you have in your arsenal for defending your plants against the pesky and destructive fungus gnat, an unwelcomed but common guest in the greenhouse.

Thanks, Jim. I haven't had the problem in my greenhouse, but I'm sure fighting them in my houseplants. I had several plants that were badly infested and I'm trying to get rid of them now using the peroxide and water method. I do have the yellow sticky traps but they aren't very nice to hang in the house!

I have them in my worm composter, and they are very hard to get rid of because most methods will also hurt the worms. It's a constant battle.

Thanks! Of all.places, we have gnats in our box turtle habitat!

The little buggers are annoying! I'm constantly killing them in the kitchen. Funny thing though, our worm composter sits right next to our indoor kitchen garden, but I never see the gnats in the kitchen garden, just the composter.

Thanks, Jim. Good information. If I have seen the use of hydrogen peroxide mentioned elsewhere, but have never understood the science behind it and its effectiveness. Can you provide references on hydrogen peroxide, how it acts on gnat larvae and how well it has been tested for effectiveness?

Thanks.

Hi Will. I searched for about 40 minutes, and while I could find countless articles on how to use hydrogen peroxide to kill fungus gnat larvae, I could not find a single reference concerning the action of hydrogen peroxide on the larvae or the science of how it kills them other than references like it "poisons them". Hydrogen peroxide in high concentrations, much higher than hydrogen peroxide found in most medicine cabinets, can be an irritant and/or toxic to humans if ingested. My assumption, while not very scientific, is the hydrogen peroxide is either poisonous or "burns" the larvae without having any detrimental effect to the plant. It breaks down quickly into water before any harm can come to the plant, but not before it kills the fungus gnat larvae.

Thanks, Jim. I know that the peroxide does break down almost immediately and that has made me question whether it would really be effective on gnat larvae. Using it also adds that much more moisture to the soil and that is a plus for the larvae. If it were truly effective, I would think it would be easy enough to do a controlled study of it and that professional growers would use it. Until I see better evidence, I will not recommend it as treatment.

I understand where you are coming from @WillC, but there is an overwhelming amount of articles, blogs and forum entries on the internet where people swear by it, so it must do something. I understand that it does add moisture to the soil which is a plus for the larvae, but I wonder if within that short period before it does turn to water, it does have a positive affect on killing the larvae. I, too, think it is strange that there are no controlled studies out there, that I can find, and I hope to find at least one at some point. In the meantime, based on all the anecdotal "evidence", I feel comfortable recommending it.

If you find any literature regarding the use of hydrogen peroxide regarding fungus gnats, please do post it here. I will do the same.

Jim

I tried hydrogen peroxide for a year or so and had no effective results. There is something seductive about all-natural products that we can find at home and can safely use on plant pests. Dish soap and various oils (vegetable, mineral, etc.) are among those that I use. But in those instances, their is a scientific logic behind the way they work, such as clogging and smothering breathing apparatus. Cayenne pepper causes small insects to overheat and die. But I have never understood the rationale behind hydrogen peroxide.

I find it interesting how easily unproven information gets picked up and reproduced and passed along to the point where it becomes conventional wisdom or truth. I know of a dozen or more "plant myths" that are very commonly believed and often lead folks down the wrong path.

Perhaps you are right. There are a lot of wives tales out there in many areas and subjects that folks have just become to accept as "truth" and they are totally false. I don't wish fungus gnats on myself, but if it occurs, I will try the hydrogen peroxide and see if it works. If it doesn't, I'll be knocking at your door for help...and give you credit when I update my article...fair enough?

@WillC -- I see where you said that you use dish soap and various oils to kill -- what? fungus gnats? their larvae?
You also mentioned cayenne pepper for small insects so I'm guessing maybe for the fungus gnats but not their larvae.

So anyway, I would be most appreciative if you could elaborate a little. Like with the dish soap and vegetable oil -- what type of vegetable oil or does it matter? (I have a lot of oils including 'Happy Jalapeño Olive Oil'.) And what ratio of soap to oil? And do you use this concoction to kill fungus gnats and/or their larvae?

I'm thinking about repotting some indoor plants that are in 4" pots (and will probably remain in 4" pots). Would still like to know if there's a preventative step when I repot. For larger houseplants I wouldn't like the idea of repotting them.

Thanks for any help you can provide. I'm just starting to see some of the little boogers and really want to stop them in their tracks.

Karen 😀 @plantmanager
Also, @tx_flower_child
I'll tell you ! What works !
Let plants dry as much as possible. Water with a drench of Neem oil water. Then a sprinkling of Diatomasous Earth, on soil.
Then, repeat in a week.

My sister and I, didn't need to do a third application.

We did this after we tried all the, afformed mentioned treatments that didn't do DITTLY SQUAT !!!

TTFN !
😎😎😎

Good to know Phil. I have both in stock if those little buggers appear in the greenhouse. Thanks.

Same here. They are a pain. I have some in the house now, flying around in front of my screen.

Neem oil makes me nervous. I guess it depends how one uses it, and not having a light touch, it can have unintended results. I've read on the Facebook street not to use it on epiphyllum branches because it will smother them. Of course other people recommend it. So I dunno.

I think DE works tho I haven't tried it. But would it kill the larvae or just the adults? I might try some BTi because I have some. I know I keep a good supply of it in the 'dunken donuts' form and might still have some in liquid form. Wonder if beneficial nemetodes would get the larvae? I'm about to get some for grubs and could sprinkle a few in potted (🤔) plants. Wouldn't buy them solely for fungus gnats because they're not cheap, there are different types and a little goes a long way.

Right now I don't think I have many so I'm repotting in clean pot (🤔) with fresh potting soil. Rinsing roots in the process. No proof that anything I've said in this post is true and will work.

You may have something there, Texas Girl 👍.
DE, mixed with water, and then water plants ?🤔??? Might just kill larva in soil but ! I kind of doubt it, as bugs need to come in contact with it.
I believe if you kept top of soil dusted for the gnats 10 day life cycle, you would be rid of them.
To distribute DE on top of soil. Put DE in an old plastic ketchup bottle, the kind with plastic lid, that has a flip lid with smaller hole in it. Then you'll have better control and accuracy, at applying.
Apply after each watering.

Main thing is letting 1 or 2 inches of soil dry out. Also, I would allways keep a regular o'l sticky fly tape trap, and a yellow sticky trap hanging.

Ttfn there, Texas gal. 👍👍
😎😎😎

Sorry Philip but while DE might not work, it's really important to kill the larvae. By the time you kill the gnats, the larvae will be growing and keeping the cycle going. You have to kill both the gnats (the ones you see flying around) and the larvae (that you don't see).

WillC said:Thanks, Jim. Good information. If I have seen the use of hydrogen peroxide mentioned elsewhere, but have never understood the science behind it and its effectiveness. Can you provide references on hydrogen peroxide, how it acts on gnat larvae and how well it has been tested for effectiveness?

Thanks.

I should first state that I am *not* an expert on this matter, but you were looking for studies that might indicate how hydrogen peroxide kills fungus gnats and this *might* offer some indication to that effect. This study is not directly about how peroxide kills fungus gnats, but rather how it kills cells: https://www.ncbi.nlm.nih.gov/p...

This may be a loose connection at the moment, but it is my understanding that most pesticides work by attacking insects' central nervous systems and they discuss how H2O2 can kill cells in the central nervous system.

It looks like there may be an issue of concentration, though. How weak is too weak and how strong is too strong?

"In addition, experiments presented show that exposure to high concentrations of H2O2 (100 microM) causes increases in intracellular free calcium within 3 h, suggesting that increased intracellular calcium may be associated with some aspects of H2O2-induced cell death. However, at intermediate concentrations of H2O2 (30 microM), intracellular calcium remained stable during a 3 h exposure, during which time membrane blebbing was observed in ultrastructural studies."

..and, again, this wasn't in relation to fungus gnats, so take from this what you will...

(For the record, it isn't my goal to get into a debate about it...if anyone chooses to nix the idea that this article can be related to how effective H202 might be on gnats, that's perfectly fine with me, LoL! My feelings won't be hurt. I'm simply trying to find some connection as to *why* this long-practiced remedy might work.)