Tuesday, March 15, 2011

Fungus Gnats: Like Puppies That Try To Fly Up Your Nose

One of the things that eventually drove me away from the Garden Web House Plant forum (GWHPF) was the way people posted the same questions there over and over. It wasn't the same person asking the same questions, obviously, but, you know, you see anybody asking something you've answered thirty times already for other people, and you start to wonder why people think it's okay to ask you to type out an answer for their problem for fifteen minutes, but it's too much trouble for them to spend three minutes searching the archives.1 Which leads to irritation.

And then my irritation would bleed through and I'd write subtly hostile, cranky responses, which would just convince the person asking the question that everybody in the Garden Web forums was hostile and mean.2 I suspect I started writing PATSP partly so I could have a place to link to for questions I'd seen a million times already, rather than spending half an hour answering them all over again from scratch.

By far, the most frequently re-asked question at the GWHPF went something like, HELP!!!1!!!1!! I just bought this plant and I don't know what it is but there are all these fruit flies flying around it all the time and this is the worst thing that has ever happened to me so please please please can somebody tell me what to do to make them all die because they are ruining my entire life!!1!!!!!1eleven!! (And yes, some of them were actually that dramatic. You would be amazed.)


First, don't panic. Or, if you'd already started panicking, please stop, 'cause it's annoying to read anything with that many exclamation points.

Second: they're not "fruit flies," (Drosophila) they're fungus gnats (Bradysia). I realize you probably don't see a difference or care, but you have to know your enemy in order to defeat them.

Where do they come from? Most likely, they came in with a recently-purchased plant. If you haven't brought in a new plant, but you've repotted something recently, then your potting soil probably contains gnat eggs. Miracle Gro potting mixes in particular are consistently about 67% gnat eggs by weight,3 but gnats are a risk you take with any high-peat mix.

Why? I dunno. They just like peat, I guess.

What do they want? The adults lay eggs on wet soil, the eggs hatch into larvae, the larvae eat (mostly decomposing organic matter, not necessarily fungus per se), grow, pupate, and then they're transformed into beautiful . . . fungus gnats.4 Adults don't eat,5 so they don't live very long: they're focused on mating and laying eggs. Since the survival of the eggs depends a lot on moisture, adults are very interested in things that strike them as potentially wet. Which means that sometimes, they will try to fly up your nose or in your mouth. Also one will occasionally find them drowned in unattended cups of coffee, or hovering near sinks.

Are they hurting my plants? Probably not. The larvae are capable of feeding on the fine root hairs of plants, but even if they do, this would only harm plants that were very young or very weak. Living tissue is probably not their first choice anyway.

How do I make them go away? You have many options here, with varying levels of ease, expense, and effectiveness.

Personally, I would recommend either watering less often or doing a soil change. If the soil the larvae are in goes completely dry, they die, and then you don't have a next generation of adults. It may take a while, especially if you have a lot of plants, bring home new plants often, or have a lot of plants that will die if they get too dry, but if you're consistent and patient, it should happen.

Letting plants dry between waterings is what I personally do. I bring in new plants often enough that we usually have a few fungus gnats around at any given moment, but considering the number of plants here, a few is as good as none. And honestly, I kind of like them.6

Soil change can be thorough (shake off every bit of soil from the roots and replace with all-new soil from a brand-new bag) or partial (shake off some of the soil and replace it with a faster-drying mix, like potting soil cut with perlite, coarse sand, aquarium gravel, aquatic soil, etc.). Thorough soil replacement may be more stressful for the plant, but it should stop the gnats immediately, assuming the replacement mix is clean. Partial replacement is easier on the plant, but won't fix the problem as fast. Some of the old soil will still be there; it'll just be drying out faster.

Another option: sticky traps. I don't like sticky traps. They sound good in theory -- they're just pieces of yellow cardboard with adhesive on them. Gnats land on the cardboard and then can't fly away again, so gradually you wind up with fewer and fewer gnats flying around.

In my personal experience with sticky traps at work, mostly the traps stick to leaves and hands, not to bugs, and when they do function as advertised, you then get to look at a brilliant yellow piece of cardboard that's covered in dead insects, which . . . makes a statement, certainly, but I don't think it's going to catch on in New York.

Soil drenches are another option. If you're going to go this route, a product based on Bacillus thuringiensis ssp. israelensis, or "BT," is probably the way to go. Some brand names are Gnatrol and Knock Out Gnats; several "mosquito dunks" also use BT and can be used instead, if you don't find gnat-specific preparations.7 BT is a bacterium that produces a protein which screws up the larvae's digestive systems and kills them. It's organic and safe around people or pets. One may have to re-treat once or twice before the gnats completely go away.

Several people at Garden Web swear by the potato method, which is: slice a potato in half. Lay the cut end on top of the soil. The fungus gnat larvae will find the potato, get excited, and start chowing down. After a few hours, you can pick the potato back up, with all the larvae now on it, and throw it away. Or you can slice off a section and put it back on the soil again. I'm not sure this is sufficient to eliminate a gnat infestation all on its own, but it might speed up the process if you decide to reduce watering, and the more larvae you can get rid of, the fewer adults you have to see.

A simple and fairly cheap solution for small houseplant collections: spread a thin layer (maybe 1/4 inch or 6 mm) of coarse sand on top of the soil in your houseplants. The adults can't get through the sand to lay eggs, and the larvae can't get through the sand to become adults, so the problem disappears in one generation. Plants so treated will stay wet longer, since water will have a harder time evaporating from the soil surface, but I suppose one could always scrape the sand back off again after two or three weeks, if it's really a problem.8

Actual pesticides like pyrethrins, imidacloprid, etc. are not, I think, necessary, nor will most of them even be effective, either because of their targets (if the label doesn't specifically say it will kill fungus gnats, it probably won't) or their mode of application (pyrethrins have to make direct contact with an organism to kill, and break down quickly -- they're useless for a fungus gnat larva an inch below the soil). There might be a role for a soil drench of neem oil with dishwashing liquid, if you're going to insist on a pesticide, but I think the other methods will probably work just as well as poison, and some of them are very cheap and easy. I mean, technically, watering less often should save you money. Just not very much of it.

That's pretty much everything I know about fungus gnats and what to do with them. If there's something I haven't covered, or if there's something I did address that you think I got wrong, feel free to leave a question or comment. Alternate methods of dealing with fungus gnats are welcome too.


1 Of course, the people posting questions sort of are being responsible and doing their own research, in a way -- if you have a plant problem, it makes sense to go to a plant-related forum to ask somebody about it. And if you've never been to GW before, you might not be able to appreciate how big it is, you might not be able to locate the search field, you might try to search and wind up getting gibberish back, if GW's code is being glitchy.
Really I was the one with the problem. I wasn't responsible for answering fungus gnat questions, and if I didn't want to answer them, then I could have just not answered them, rather than making it all about me and getting upset.
2 Which, last I checked, was mostly true, actually, but it wasn't always that way, and it certainly doesn't have to be. The dynamics of online communities is perhaps a topic for another time.
That, plus feeling obligated to answer people's questions even though I wasn't, made it sort of a bad place for me to be, overall, and I'm glad I'm done with posting there. I do go back every few months to look around. Sometimes I look to see if I can find a blog topic, though I don't remember that ever actually working out.
3 (Exaggeration for comic effect; I'm sure it's less than 20%.)
4 Not every ugly duckling becomes a swan, yo.
5 (Pretty sure I read this somewhere at some point.) (UPDATE: Aralia left a comment with a link saying that this isn't true; adult fungus gnats do eat. So never mind.)
6 Yes, it's alarming to inhale one accidentally, or have one flying around your face for a few seconds like it's trying to figure out how to get in, but it doesn't actually hurt you. I mean, if you inhale a gnat, you're definitely getting the better end of that experience.
Around here, they come and go -- we are presently having a high-fungus-gnat period, which is why this post -- but I kind of think they're cute. Like puppies! Puppies that try to fly up your nose!
7 The difference, as I understand it, is that mosquito BT is made to dissolve slowly, over a long period, to kill mosquitoes that attempt to lay eggs in whatever body of water the "dunk" is sitting in, while gnat BT is usually either made to be dusted directly on the surface of the soil or made to dissolve more or less immediately in water. I'm fuzzy on this, though, having never used either product, and I've only occasionally seen the mosquito dunks.
8 This might be the place to note that decorative moss or stones on top of the soil will also keep the soil wetter longer. If you have such items on your soil, you should take them off at least until the fungus gnat problem is resolved. (I would actually advise not putting them back on again ever, but that's just me.)


strangemouse said...

As someone who breeds tropical snails(!) I am rather familiar with fungus gnats and I found this is a really useful post, thanks! I'm glad I'm not the only only one who dislikes sticky traps, they usually end up stuck in my hair. Not cool.

Ginny Burton said...

Liza, over at GoodtoGrow, recommends covering the top of the plant in plastic wrap for a week.

Like you, the gnats don't bother me much. We always have a bunch of fruit flies anyway, and those little white moths (?) that hatch out of the bird food, so gnats just add to the air show.

Claude said...

good stuff... but if you have a whole lot of the things flying around... you can try a half glass of cheap red wine sitting there while you're letting your plants dry out. The adults make a b-lne for it. And it looks better than the yellow sticky things. Kinda. Maybe...

Jeane said...

I had fungus gnats. From the Miracle-Grow (I don't use their soil anymore). Tried less watering, soap in the water, and the potato trick. What finally worked for me was putting out little dishes of cider vinegar. The gnats would fly into the vinegar and drown. After just a few generations they were all gone. And ugh, yes, they do try to fly up your nose! very annoying.

Anonymous said...

Thanks for the great post. This actually raises a question I have been trying to answer recently: I need to buy some coarse sand to mix with potting soil for a few succulents I have. However I have come across some conflicting opinions of what sand to buy. Some articles recommend builder's sand while others say to buy sand that has been washed and sterilized. Do you have any thoughts or preference on the type of sand to use for potting?

~Gardener on Sherlock Street said...

If there is just one fungus gnat in the house, it will want to be in my face especially while I'm working at the computer. My husband is good as slapping his hands together and killing them. I have a fly strip by my overwintering plants because, yes, I have fungas gnats. I had not heard the potato trick before so thank you for your post. I have a hard time letting my plants dry out enough to kill them all.

Dreamybee said...

The title of your post almost had something flying out of my nose! Thanks for the laugh.

mr_subjunctive said...


My impression (NOTE: NOT HARD FACT) is that builder's sand should be okay to use, but it can't hurt to wash and sterilize it first. (Which could be done in one step, by pouring it into boiling water.)

Justin said...

I've found lots of information on orchid forums (being an orchid collector) without ever starting a single thread. In fact, most of the information that I want is already out there, either on the forums, on some other website, or in a book. This is all to say that I understand your frustration with the forums.

Tamra said...

Thanks for your post! I'm an amateur house plant owner. Every winter we take our plants downstairs and put them under the grow lights until it's warm enough to take them back outside (we don't have enough natural light in our house for all the plants I want to own). Inevitably, by the end of winter we have those little bugs. I'm happy to know that they aren't really harming my plants!

Paul va said...

Mix diatomaceous earth into your dirt. I don't have any FG's anymore. I never overwatered but once you have enough plants or have plants like african violets there is always somewhere wet for them to breed. Once i started using DE they went away.
A carnivorous plant next to the wetter plants is not a bad idea either

Shawn said...

I can vouch for simply watering less often doing the trick. A friend of mine recently left me a nearly waterlogged Dracaena Marginata that had gnats, and knowing that this species needs to dry out between waterings from having read your blog earlier on, the problem quickly resolved itself before I ever got around to doing anything about it...Excellent blog!

allandrewsplants said...

I've got a couple of those yellow traps around. They're covered in fungus gnats. They work well but I don't like them.

Personally I'm waiting until my pitcher plants get back to having pitchers to see how effective they can be at controlling them.

Other than being really annoying I don't have a huge problem with them.

Ivynettle said...

There was a time when we had so many of them that I could spend hours prowling around the house, growling, 'kiiiilllll gnnnnaaaatssss' and doing just that whenever it was possible. Ended up with mouldy squished gnats on all the windows, which was kinda gross.
It's not been that bad in a couple of years, though, for which I'm grateful - waking up in the middle of the night to drink some water and swallowing a mouthful of gnats is also kinda gross.

Generally, they're not a problem to me now, but I still get a little nervous when I see them around any 'baby' plants, after we lost a bunch of [i]Ageratum[/i] cuttings to them at work. ([i]Ageratum[/i], of all things... so ridiculously easy to root, and yet not fast enought to outgrow those larvae.)

Ficurinia said...

Thank you for saying all the things I dare not say!

Anonymous said...

After my junior year of college, I had a terrible fungus gnat infestation, even after re-potting all of my plants. One of my roommates went into hysterics about them, so after attempting to eradicate them with vinegar--with only limited success--I used a spray bottle full of rubbing alcohol, spraying them when they landed. I could only kill one at a time, but it was effective, and now I only have 3 or so at a time. I wouldn't call this a 'recommend' remedy, but if you're desperate it does work.

Jazz said...

Per my recollection of GW of some years ago, the major exception to the grumpiness rule was the Compost Forum. Among the self-proclaimed "Compost Whackos."

Something about decomposing plant matter does it. I dug up the bottom of my heap a couple of weeks ago and held a big handful up to my nose for a sniff -- and was giddy with glee for the rest of the afternoon.

Anonymous said...

I'm just glad I'm not the only person who thinks fungus gnats are cute.

Terrific blog, always a delight to see a new post.

hydrophyte said...

I mostly only ever see fungus gnats when I have been watering a lot, and then they disappear when I back off and let stuff dry a bit more.

I don't at all understand the wild, hysterical over reactions that people (especially Americans) have to insects. I have heard people express such panic that they sound ready to burn the whole damn house down because they saw a snowy tree cricket or what have you stroll across the kitchen floor. Where does that crazy thinking come from anyway?

Aralia said...

I've read from here: (in finnish) http://www.biotus.fi/DowebEasyCMS/?Page=Harsosaasket, that fungus gnats eat also as adults different kinds of algae, fungus and all kinds of degrading matter. The larvae eats all that, too, but also the fine roots and callus of plants. I keep that link pretty reliable, since they study all plant related problems and they have to keep the damn things alive for that.

Rameen said...

I have a Drosera capensis which helped my gnat problem so much I considered buying another one. It's possible it got overfed since (I estimate a hundred gnats?) because it's not doing so well now...

Kate Hinds said...

We have a couple of infested Christmas cactuses (cacti) and we refer to it as "gnat TV," because it makes the cat so very happy to sit on the windowsill for hours and watch them, head all a-bobbing. So I don't have the heart to get rid of them -- it'd be like taking her version of General Hospital off the air.

themanicgardener said...

The wine trick recommended by Claude definitely works; they love the stuff. Not that I've seen it cut the population; but there's a weird, morbid satisfaction in seeing the evidence there in the glass.

What has actually drastically cut the number of insects bobbing in front of the computer screen or flying up my nose is beneficial nematodes, which eat the larvae in the soil. But they need to be kept damp, so this method can't be combined with your dry-earth method.

Just thought I'd mention that I'm not afraid of snakes, frogs, spiders, or insects in general, but I don't like gnats flying around my face, which is why I try to get rid of them.


Sofia Mac Mullen said...

I live in Londo and Im covered in them and they seem to have enjoyed killing my coriander, wich was also covered in a lillt green insect, ill asume its a young version of the gnat. and now my thyme and basil are suffering, carrots where a no go but I dont know if its the same reason. I read that sprayng them with nettle is a good solution? but ill try the sand thing, it seems to be the easiest one.

mr_subjunctive said...

Sofia Mac Mullen:

The larval form of fungus gnats lives in soil, not on plants. Your green insects may be aphids. I'm not terribly familiar with what insect pests are common in the UK.

Fungus gnats are probably not responsible for your plants dying. Aphids can kill or stunt plants.

I have no idea what spraying plants with nettle would involve and have never heard of that before.

Unknown said...

Sterilize potting soil in the microwave oven before using it to repot a plant.
I cut the top off a one gallon water jug and fill it with potting soil, put it in the microwave for 2 minutes on high.
Don't walk away and leave it alone, I've had some batches start smoking if they were very dry, so it could be a fire hazard if you are careless and don't watch it the whole time it is being nuked.

mr_subjunctive said...


The plastic jug doesn't melt? I tried to sterilize potting soil in the microwave in a glass container semi-recently, and I was surprised by how hot it got, and how long it took to cool down. That was really wet soil, granted. But still.

Anonymous said...

I read somewhere that using adding hydrogen peroxide to the water in a 1:4 ratio will kill the gnats and not harm your plants.

Has anyone ever tried this?

Cynthia Wiser said...

I am currently sitting on my couch with my shirt over my face. This is the third time a fungus gnat has tried to fly up my nose today. I am at wits end and am near the flamethrower phase of this infestation. I think I overwatered my plant and now I have a bunch of little bugs all over my apartment. They fly into my food when I am eating and the other day I was drinking from my water bottle and one even flew up my nose during that. I spit water out all over the place and made quite a mess. Well I am going to stop rambling now. I would just like to say, thank you for your article and all of the information!

Anonymous said...

I found this at another blog site regarding fungus gnats. Cinnamon sprinkled over the surface of the soil works great. Apparently, it's an anti-fungal and interrupts their habitat. I tried it and no more gnats!

jam said...

i had a super bad case of fungus gnats. then i got a couple of carnivorous sundews and they've taken care of a lot of the problem. now i also look at any new gnats differently, because they're serving a purpose :)

Anonymous said...

Nematodes are a really good option. Theyre microscopic things and they kill the larve in a suprisingly brutal way lol. Theyre totally safe. I used them and they worked well, except gnats found a way into my house with all the windows open. nowhere near me carried them so i got it off amazon for less than $20, and the coverage is really good. You just have to make sure you get it for fungus gnats, but the advertising is pretty specific. You just take a sponge with the lil nematodes on it (it sort of creeped me out knowing what was on the sponge, but buy looking at it you wouldnt know) and stick it into a gallon of water or whatever size you get. you do it again a week later and that's really it. Some people with a lot of infested areas may need to do more than 2 treatments, but its so simple and can even go into a sprinkler system. This comment is really late but i hope this helps someone! (once again, you literally can not see the nematodes and they;re only interested in the gnat larve. I feel a little bad but once they've eaten all the larve they just die i assume).

also with cinnamon you have to be careful it doesnt touch the plant at all-i never really tried the technique though

Rachael Modafferi said...

I am very happy to have found this site. I do have a question pertaining to my recent gnat problem and my aloe vera plant. This aloe plant has been in my family for over 25 years and a few months ago I re-potted it with Jolly Gardener Plotting Mix. Now I have a gnat problem and they have been making their way into my bedroom. I was wondering if I were to use the sand method, what particular sand can I use? I have black aquarium sand from my fish tank, is that acceptable? I rarely water the plant so the soil is very dry. Maybe I made the wrong choice in soil.

mr_subjunctive said...

Rachael Modafferi:

Is it sand, or is it gravel? The reason for asking is because the amount of space between the individual particles is the relevant factor: if there's enough space for a fungus gnat to crawl through to lay eggs on the soil (or a larva to leave the soil), as there would be between pieces of gravel, then it might help, but it's unlikely to cure the problem. You need something that packs tightly enough to prevent adults from reaching the soil and larvae from reaching the air.

I'm not familiar with your brand of soil, so I couldn't say whether it's suited to growing Aloes or not, but it does look like the gnats came in with the soil, so you could probably do better. I wouldn't necessarily change the soil the plant is in now, but if it's water-retentive enough to harbor fungus gnats, then it's probably water-retentive enough to rot Aloe roots too, so at the very least you need to be really careful about when and how much water you're giving the plant.