There's been some discussion about fungus gnats and after a recent bout of them in my home, I would like to provide the SCIENTIFICALLY PROVEN TREATMENTS and also comment on what NOT to do.
1. DO put yellow sticky cards in the pots on some of your plants so you can always know if fungus gnats are present. By the time you see them flying around, it means the infection is bad. Most likely, you won't see them flying around, you will notice leaves on your most sensitive plants turning yellow. Or, in the case of my tricolor dragon tree, the leaves will suddenly wilt, lose their color, and there's no reviving it at that point.
2. DO NOT throughly rinse the roots of a plant that you suspect may have fungus gnats. You can inspect for root rot and if you find it, use dilute hydrogen peroxide. Simply cut off the rot and dip the infected root into the peroxide solution, then replant. There was a website that advocated thoroughly rinsing the roots and I'm sure that stress is what killed my beautiful Calathea White Fusion! Big chunky roots like a palm tree or bird-of-paradise might tolerate the rinse, but not a plant with smaller, hairy roots.
3. DO NOT pour neem oil into your plant pots or dilute pine soap. The former requires soaking the whole pot in a bucket for an hour and you will need to do this again in a week. Maybe if you have water-loving plants this won't kill them, but I can't imagine soaking most plants like that. Dilute pine soap poured into a pot is a single application, it might not kill all the gnats, but it will surely irritate most plants. Both of these have been recommended by plant stores. Fortunately, people who actually tried them posted comments about their experiences.
4. DO buy Mosquito Bits if you live in North America or the UK. This is an inexpensive product that you can find at a lot of shops or purchase on Amazon. It contains the beneficial bacteria BTI (bacillus thuringiensis serotype israelensis). You simply sprinkle the mosquito bits on top of the soil in each pot and give it enough water to moisten, which activates the bacteria.
BEWARE that you need BTI, where the "I" is for "israelensis". There are other bacillus thuringiensis subtypes for other pests and you want to make sure you get "israelensis".
5. DO buy SF nematodes (steinernema feltiae) if you live in a country where you can't get Mosquito Bits. In Europe, a good source for this product is Koeppert. SF nematodes are very tiny worms that come as a fluffy white powdery substance in a sealed plastic bag that must be kept at a temperature between 2 - 6 C (35.6 - 42.8 F). There are different size boxes, depending on how many plants you want to treat and the default size is 25 or 50 million. DO NOT buy this product on Amazon because there are too many complaints about receiving a package with no cold packs and the nematodes didn't cure the fungus gnats. That's because the nematodes arrived dead.
To use a SF Nematode product, you open the sealed bag, spoon the amount you want to use onto a clean dish and let it rest for 30 minutes. If you don't use all of it, you can reseal the bag with tape, put in the box or a plastic container, and store it in your refridgerator for 1 - 2 weeks.
After waiting 30 minutes, you transfer the powder from the dish to a bucket and add water. In my case, I mixed a batch of 25 million nematodes, so I added about 2.5 liters of water (3.3 liters is one gallon). Then, I lightly watered all my plants. The nematodes in the soil will release bacteria that kill the fungal gnat larvae.
THE BENEFIT OF MOSQUITO BITS OR SF NEMATODES IS THEY REMAIN IN THE SOIL FOR MONTHS! That means you will get rid of the gnats and they won't come back for the rest of the season. In my case, I noticed a few adults on the yellow sticky cards the same day I applied the SF nematode water. The next day, I saw only a couple. Three days later, no adult gnats.
6. DO understand that female gnats have no wings. When the eggs hatch, the females stay in the soil. Male gnats have wings. After mating, a female gnat can lay up to 100 eggs. It takes about 4 weeks for the complete cycle to go from an egg to an adult gnat. So, it's good the bacteria/nematodes stays in the soil and keeps working until there are no more female gnats around to lay eggs.
7. DO put a thin layer of sand on top of the soil in each pot. Use sterilized sand - preferrably play sand or horticultural sand - not sand for builders or construction. Leave it on the pots for a couple weeks. If you have to water your plants, spoon off the sand and then replace it after you pour in the water. Alternatively, you can water from the bottom by resting the pot for 20 minutes in a dish with water and then you don't have to spoon off the sand.
Why sand on the top? Because it will keep more gnats from climbing into the pots and it will prevent males gnats from mating with females that are already in the pots. Gnats like moist soil, not dry sand.
8. My Personal Experience:
If you're worried about sand on the top of the soil in the plant pots, you don't have to use it, but here's my experience with it. My Porthos Snow Queen, which is has been a really fussy plant, suddenly stopped being fussy. Same for my Spider Plant (which had a bad case of gnats and I thought it would die). The Croton Iceton, Draceana, Palm Trees, Tradescantia, Tineke (variegated rubber tree), Cala Lily, Peace Lily, all my Catatheas, Diffenbachia, velvet leaf Alocasia and Anthuriums, Coffee Arabia, Blue Grass, Syngonium, African Violet, and even the Ostrich Fern from the forest were okay. HOWEVER, the Arborica Schleffera "Charlotte" and Strelitzia Reginae had slight reactions - the former dropped a few leaves and the latter had leaves starting to turn yellow (the smaller leaves, not the top). Whether this is from the sand or stress from the fungal gnats, I'm not sure.
The plants that died from the gants (before treatment) developed yellow leaves and wilted. They were very healthy plants and they died within a few days of the leaves turning yellow (the tricolor dragon tree simply lost color, it didn't actually turn yellow). One calathea (orbifolia) developed yellow leaves, so I chopped it into 3 parts, removed the baby bulbs on the roots, put each section of the calathea in a separate pot and put all the babies in separate pots. All of them are still alive and I'm keeping my fingers crossed that at least one section will survive. So far, no more yellow leaves and the wilting is less. Every leaf is supported by a stick (I use pipe cleaners to tie them, which doesn't scar the stems). My biggest loss was the Calathea White Fusion. This plant did not have root rot, but the stress of inspecting the roots by removing the dirt and rinsing with water was just too much for it. The only reason I looked at the roots is because some of the leaves were turning yellow. Upon inspection, the roots were white, healthy, and hairy. Another plant lost was my dwarf coconut palm. I was so proud of that one, growing well and developing new leaves. Upon inspection, I found root rot, cut off the bad parts, and repotting didn't save it. Since it was well-cared for, I presume the root rot was caused by the fungus gnats. Also showing root rot was one Strelitzia. However, I've got 2 of them and both are showing some yellow leaves 2 days after treatment. I hope they make it!
Next week, I'm replacing the Calathea White Fusion (it came with a 1-year warranty) and if the Strelitzia don't show improvement, I'll get another one. Both will be treated with SF Nematodes and quarantined for a couple weeks in another room.
Also, I'll update this post next week and report if the sick plants survived or if any of the others developed yellow leaves. Some of the healthy-looking plants are already getting new leaves, so I think the treatment worked very well.
One final note: my trandescantia (purple jewel) was in really bad shape. It went from being a big plant with lots of stems & leaves to a few stems with pale leaves. Initially, I thought it from lack of sunlight (it's dark now in Finland), so I set up a plant light for it. Then, I picked through the plant and realized that most of the stems had no roots! I put some in water for a few days and the ones with roots stayed in the pot. When I got the SF Nematodes, I put all the ones that had been in water back to the pot (just made a hole with a chop stick, put in the stem, and pushed soil around it). Most of the stems had one or two leaves, but no roots. 24 hours after applying the nematode water, the plant started to thrive. Three days later, it has bright shiny leaves, a lot of baby leaves have appeared, and no more gnats on the yellow sticky card.
In closing, I've been a plant enthusiastic all my life, but after the pandemic, I decided to build a small jungle. There's lots of articles about spider mites, mealy bugs, root rot, and leaf fungus, but I never heard of fungal gnats before. If I hadn't seen that tiny black bug on the tricolor dragon tree, when I cut off the top to see if it could be saved, I still would not know what the problem was. In my case, the TELLTALE SIGN OF FUNGAL GNATS were healthy plants losing color in their leaves or leaves turning yellow. The calathea orbifolia was the most dramatic. It went from a slightly yellow leaf to several yellow leaves and wilting stems in 24 hours! Most articles about houseplants note that yellow leaves are usually a sign of over-watering. If you notice this, put a yellow sticky card in the soil and check it in a few hows to make sure it's not the dreaded fungal gnat!