Hi everyone! I'm new to this page but I have a lot of happy houseplants

However... I have a really bad problem with fungus gnats. And I have tried almost everything. Does anyone have any advice? I can't stand it any more!

The main problem right now is on my monstera. When I first got it I made sure not to overwater. I watered from the bottom. I then changed pot and soil. I top dressed with small stones. I use neem oil. I use pest spray. I have fly traps everywhere. I've even resorted to covering the rocks/soil with film to stop them getting in!

Nothing I do seems to work. If anyone has any magical tips, do let me know!

Welcome to the Forum!
Fungus gnats are a real nuisance. What "pest spray" have you been using? Killing and trapping the adult flies won't stop them. First of course is to keep your soil as dry as possible. Don't let water stand in saucers or cache pots. I'm not sure if rocks on top help or hurt, it may just give them more cover.
As much as possible, use potting mix with less organic matter. Also use soils and pots that drain quickly.
Yellow sticky traps will catch some of the adults.
You need to get rid of the eggs and larvae in the soil. Stop using products that aren't working, sooner or later you'll damage the plants more than the gnats.
Here are two products that have worked for me:
Least toxic is Bt-i (Bacillus thuringiensis israelensis). It's available in various products, I use Mosquito Bits as I have them on hand to keep mosquitoes out of an open drain. Just sprinkle them on the soil surface and water normally. It takes a week or two to work, and does not kill the adults already flying around.( They only live for a week or two.) You'll need to repeat this every few months. Gnatrol is another product, I'm sure there are others also. Just look for Bt-i in the ingredients. I don't know if this is available outside the U.S.
Most toxic but very effective treatment is imidacloprid. Don't use it outdoors or on flowering plants visited by pollinators. It is extremely poisonous to honeybees. It is available in different products, I use Bonide Systemic Houseplant Insecticide, there are others available in the U.S. Look for imidacloprid in the ingredients and follow package directions carefully. This is very toxic, but I only had to treat one time and the gnats were gone.
Whatever you use, treat ALL of your houseplants at the same time, or they will continue to re-infest each other forever.

I have gotten good results using a combination of sticky flypaper and insecticidal soap (used as a drench for the soil). I believe that is the least toxic of any of the options mentioned so far in this thread. I use Safer brand insecticidal soap, and buy it as a concentrate. I am careful to drench the soil thoroughly when I water with it (which requires watering in more than one pass, waiting a few minutes in between, to fully hydrate the soil).

You cannot control gnats by only focusing on the adults (using flypaper). You have to use a combination of approaches that trap the adults and kill the larvae. Use flypaper placed near soil level as a way to monitor how many bugs are in the area, and assess how well your treatment is working. Treat every single plant in the area when you kill the larvae. Leaving 1 plant without treatment will create a reservoir for future infestation. Remember that all it takes is a couple of flying gnats to start up the problem again. Look at where they might be coming from: soil, other plants you already have, plants outside, new plants that you acquire.

Avoid complicated modifications related to top dressing. What you have described will only trap moisture in the soil, and promote the growth of more gnats.

Ideally the top layer of soil should dry out every time in between watering. Reducing watering is probably the best preventative measure you can take with any serious gnat problem. Bottom watering should yield the same results, roughly, as top watering when you do it right (ie. allowing the bottom layer of soil to get properly wet and waiting long enough for the water to wick upwards).

Be prepared to repeat the soap treatment and replace the flypaper on a periodic basis, but wait in between treatments in order to assess the results by counting bugs on the flypaper. This is definitely a problem that you can solve with care and attention. Once you have beaten down the bugs, just leave a little square of flypaper somewhere near soil level to give you an alert if the situation takes a turn for the worse, before it gets really bad.

Thank you both so much. I will try what you have recommended and hopefully can tackle the problem!

Buy a package of "Mosquito dunks" at the garden center. It is BT. Put one quarter of one in your watering can. This will kill the larvae in the soil and stop future generations. It is all natural and safe. But give it some time to work. Use sticky traps and manually kill the adults.

One thing I found useful was to add ground cinnamon to the soil. I heard this helps discourage them by killing off their food source because it contains cinnamaldehyde

Other things I've found useful are mosquito bits (tremendous) and citronella in different forms. Used as an essential oil, add it to a bowl of water near a heater and let the vapour surrounf your plants, gnats hate it, and it smells nice.

Thank you everyone! I am going to repot and try the mosquito dunks today as that was all I could get hold of in the UK. Wish me luck!

Hi everyone, I may need to make a new post for this but I'll try here first.

So, I did get the mosquito dunks and completely repotted my monstera before I used them. I watered all my house plants with the mosquito dunk water and for a while I was gnat free! However, they have once again come back. I have only been watering my plants with this water and only when they are dry. I have sticky fly tapes still on my plants. But these darn gnats still don't seem deterred!

I really am at the end of my tether now. My poor monstera seems to be suffering and I just do not know what to do, I feel like I've tried everything!

Have you tried using a systemic? Systemics are pesticides that are absorbed into the plant tissue. Anything that takes a bite will be poisoned. They come in liquids or granular - the active ingredient is Imidacloprid.

Imidacloprid will kill gnats but not through the plant. It affects immature gnats in the soil directly, and only while it persists in the soil. I have already explained how insecticidal soap can be used, in much the same way but more harmless to the environment.

I thought Insecticidal Soap had been suggested already. My bad.

But wait, ...

"The systemic insecticide imidacloprid will also kill fungus gnat larvae when applied to the growing medium".

https://extension.colostate.ed....

I'm not too sure what equivalent is to insecticidal soap in the UK?

I've tried a neem oil mix and various different sprays but no luck yet.

I like Safer brand. It's basically a solution of soap in water, specifically potassium salts of fatty acids. This is supposed to be better than the sodium salts found in many other commercial products. Not so much for the leaves but for the roots, as soap on the plant inevitably falls onto the soil below and enters the root zone. I suppose you could spray your plant down with regular soap on the day you would normally water, and use that as an opportunity to flush the soil afterwards.

The soap I use is at about 1-2% of the final volume. You can substitute a mild soap of your choice for this purpose. I use a horticultural product but I've heard from other people who got good results with a household product, used judiciously. The neem is usually suspended or emulsified with soap I think, but I don't know how much would be in your product.

My sympathy - I battled fungus gnats for about 2 yrs before I got it figured out. Cutting to the chase - you mentioned that you tried small stones on top of the soil. Small stones or pebbles leave too big of gaps - the gnats thank you for making nifty caves for them to hide while they get to the soil to dig in and lay more eggs unwatched. SAND. Not pebbles. Not stones. SAND. Add about 1/4" or 1 cm to the top of all plants that have any gnats. All of them. If the adults can't get back to lay more eggs, which they can't with the sand, they'll stop increasing the egg population. Adults die in about 2 weeks. If the larva can't get back up through the sand when they turn into gnats - which they can't - then you'll end that population too. They'll be stuck in the ground - and die. Done. To prevent washing the sand off by watering, consider bottom watering only. I added cut up shoe laces as wicks through the bottoms of many pots. Worked fine. Some plants I could top water with the sand. You just don't want to make bare spots by washing the sand off or making it too thin while you're going through at least a few weeks of this. Make SURE the edges of the soil surface where it touches the pot is covered with sand too. Gnats can see those gaps if you don't cover the edges.

That's it.

You're not going to have to worry about overwatering. You're not going to need any treatments. Just remove their ability to reproduce and they're done.

Seriously. I went through all the hoops people suggested with no luck at all. I even bought Sundew and Butterwort to eat the gnats - which was helpful AFTER I did this by reducing the adult numbers quicker while their last days were numbered. But the sand was the thing that turned tables.

Good luck.

Oh - and any time you buy potting materials (soil, peat, etc.) or potted plants - assume it is infested until proven otherwise.

Has anyone used Stratiolaelaps scimitus(Hypoaspis mites) to control fungus gnats?

No but since I stopped using potting soil with peat in it, no gnats. And I live in gnatopolis, can't go outside w/o gnats tickling me everywhere from ankles to ears, pots are outside for most of the year so gnats absolutely have access to the pots if interested.

gnatopolis!!!!

northerner said: My sympathy - I battled fungus gnats for about 2 yrs before I got it figured out. Cutting to the chase - you mentioned that you tried small stones on top of the soil. Small stones or pebbles leave too big of gaps - the gnats thank you for making nifty caves for them to hide while they get to the soil to dig in and lay more eggs unwatched. SAND. Not pebbles. Not stones. SAND. Add about 1/4" or 1 cm to the top of all plants that have any gnats. All of them. If the adults can't get back to lay more eggs, which they can't with the sand, they'll stop increasing the egg population. Adults die in about 2 weeks. If the larva can't get back up through the sand when they turn into gnats - which they can't - then you'll end that population too. They'll be stuck in the ground - and die. Done. To prevent washing the sand off by watering, consider bottom watering only. I added cut up shoe laces as wicks through the bottoms of many pots. Worked fine. Some plants I could top water with the sand. You just don't want to make bare spots by washing the sand off or making it too thin while you're going through at least a few weeks of this. Make SURE the edges of the soil surface where it touches the pot is covered with sand too. Gnats can see those gaps if you don't cover the edges.

That's it.

You're not going to have to worry about overwatering. You're not going to need any treatments. Just remove their ability to reproduce and they're done.

Seriously. I went through all the hoops people suggested with no luck at all. I even bought Sundew and Butterwort to eat the gnats - which was helpful AFTER I did this by reducing the adult numbers quicker while their last days were numbered. But the sand was the thing that turned tables.

Good luck.

Oh - and any time you buy potting materials (soil, peat, etc.) or potted plants - assume it is infested until proven otherwise.

Great post.

Very thorough and it makes sense, thank you.

I deal with this issue annually.
the few things I do that help greatly-

water from the bottom when able.
Use water treated with BTI (Home Depot)
use sticky traps (I get cheap ones, cut them into strips and stick them near the soil)
at night I turn on a fly trap (Katchy)

Mosquito Bits have not worked well for me.

I have had a bit of success with using cinnamon oil (highly diluted) but it wasn't practical in my situation.

The advice Baja_Costero gives in post 3 in this thread is the same as what I would give today:

Ideally the top layer of soil should dry out every time in between watering. Reducing watering is probably the best preventative measure you can take with any serious gnat problem.

I have also used sand top-dressings, but even then I make it a point to let things dry out between waterings.

Predatorial mites and nematodes are also helpful preventative measures, but cannot be relied upon to eliminate or control Sciarid (Fungus Gnats) outbreaks.

Sciarids are a huge issue with cacti as that the gnats will eat the roots and then the flesh of the cacti, killing them outright. Once the larvae is inside a cactus seedling, sprays and top-dressings do not reach it and all the moisture the little maggots need is in the cactus flesh.

Luckily with many other plants the gnats don't actually eat the seedlings.