How to Get Rid of Fungus Gnats Forever
One sign that you’re on the slippery slope of plant parenthood is the “fungus gnat clap” — the mid-sentence, midair clap trying to kill the microscopic gnat hovering somewhere in front of your face. They’re even smaller than fruit flies, and they breed in wet soil in the pot of your plants.
Fungus gnats? In my soil? It’s more likely than you think.
What are fungus gnats?
Despite being harmless to your plants’ health, your health, and your pets’ health1, they can affect your mental health if the problem gets bad. As a longtime plant parent myself, I’ll have a fungus gnat here or there, but they can take advantage of wet soil or a plant with root rot and get out of control. New plants, old plants that have been outside, or plants that are suffering from health issues are all possible sources of fungus gnats.
They can be an indication of a larger problem in the roots of your plant, like overwatering/waterlogged soil or root rot, but more often than not they are simply taking advantage of the water you are giving your plant. Arid plants are not very vulnerable to fungus gnats because they don’t want their soil to be wet so their substrate won’t host gnats. However, if they start to rot, fungus gnats will take advantage of the exposed wet flesh to feed and lay their eggs.
The plants that are most vulnerable are tropical plants like ferns, the Calathea genus, mosses (if on/in soil), and other plants that don’t want their soil to dry out more than ⅓ of the way. But all tropical plants are vulnerable.
Gnats typically lay their eggs in the top inch of soil. You may see the larvae, like miniscule clear worms, crawling in the soil if the infestation is extensive. In this situation, the larvae, which usually feed on organic matter in the soil, may turn to the roots of your plant for a food source, but an infestation that bad would be immediately obvious due what would probably be a cloud of fungus gnats in your home.
The lil bastards are either in the soil or hovering around your house, probably around your nose, attracted to the CO₂ you exhale and making you feel like a hallucinating Mr. Miyagi. They have life spans of about two to three weeks — including gestation — so they can trick you into a sense of safety but reappear after a couple of weeks when you have nearly forgotten them.
Now for the actual solutions!
Level 1 of Fungus Gnat Eradication: Sticky Traps
Sticky traps are great for preventing infestation in your water-loving plants. Gnats are attracted to the wet soil at first, but once they draw close they notice the trap — the large yellow shape that they are inexplicably drawn to. Whether they think it’s a leaf or a flower or what, I’m not sure, but they land and they don’t get up. Sticky traps are also great for figuring out which of your plants is the culprit when trying to locate a gnat infestation; place sticky traps in all of the suspects and the one with the most gnats trapped is guilty.
The drawback to this method is that sticky traps address only the adult population of gnats that are already flying around your house or newly have their wings. The immature gnats will still be able to mature and reproduce before they get stuck.
Because of the life cycle of the gnats, if you get rid of all of the adults, then you may not see any more for a couple of weeks until the new generation of eggs has hatched and pupated, so be patient or you might get ambushed. Give it a couple of months of not seeing any gnats in the air and no more collecting on your traps. If that gets rid of your problem, amazing. If not, keep your sticky traps around to combat the adults and move on to the next level!
Level 2: Washes
Add a tablespoon of hydrogen peroxide, dish soap, or fungus-gnat-specific insecticide (see Level 2.1) per pint of water when watering your plants to distribute it throughout the pot. A wash like this will kill a majority of the gnat egg and larvae population as well as making the soil inhospitable to egg-laying gnats temporarily.
Doing it regularly (no more than once a month) is important because the leftover eggs and larvae will just continue to repopulate if you don’t, and the adults that are not in the soil (most of them) will be able to lay more eggs. This is why we recommend mixing strategies like washes for the larvae and sticky traps for the adults.
Level 2.1: Nematodes & Other Biological Agents
Most insecticides that are made for fungus gnats are actually biological agents which aren’t chemicals but microorganisms that target fungus gnats. The organism found in most fungus gnat “insecticides” is called Bti or Bacillus thuringiensis subspecies israelensis. The other common fungus gnat predator is nematodes.
Nematodes are related to what we call “roundworms,” which, as you may know and shudder thinking about, are parasitic to humans and animals. Beneficial nematodes commercially sold for pest control are a different species that are completely harmless to us and our pets.2 The nematodes are sent to you as a powder that you mix into water, waking the nematodes up like sea monkeys. Don’t mix insecticide/soap washes and nematode washes, as the bacteria that kills fungus gnats will kill nematodes, too.
Nematodes live in the moist soil and kill pests like fungus gnats and other soil-dwelling larvae by getting inside their bodies and releasing a bacteria that kills the pest. They then use the dead husk of the bug as a place to lay their eggs and to provide food for the larvae when the eggs hatch.
In Insect Hollywood (Holeywood?), there is a movie starring Signattey Weaver called Nematode.
Next level!
Level 3: Bottom watering
That top layer of soil where they party is the place that receives the most water when you top water, so they receive a weekly complimentary shower when you water your plants! Making the switch to bottom watering leaves the very top of soil unsaturated and less inviting to gnats looking for a place to settle down and start a family.
Adding insecticide to the water that you use to bottom water works similarly to a wash: All the moisture that the gnats need is also going to poison them. However, the moisture is still there and an established gnat neighborhood won’t be completely kicked out by bottom watering.
Next level!
Level 4, the Final Boss: Top Dressing
Top dressing can be just for aesthetic reasons. In this situation, it makes the soil inaccessible to gnats for any future gnat family matters. Scoop out the top 1-2 inches of soil — 1” if it's a pot less than 6”, and 2” for a very large pot 14”+ (as long as the roots are still covered) — and throw it out. You’re removing the nearly all of the eggs and larvae.
The adults would simply repopulate if you put down new soil, so top dress the remaining soil with something granular and quick draining like sand, gravel, pumice, pearlite, etc. It will dry quickly — while allowing the soil below to stay moist — and be unattractive and inhospitable to pregnant adult gnats. Sorry, Mary, no room at this inn.
For several large plants with infestations, it can be a lot of work and a lot of top dressing for this method, but it is the most permanently effective step, especially if you add a wash once in a while or continue to bottom water. Many nurseries carry giant bags of perlite, and large bags of gravel are always carried by aquarium stores.
Patience is key with fungus gnats. Start at whatever level you want and give it a few weeks to a month or more before you decide whether or not it has worked.
They are bugs and you will defeat them!
I’m giving you a karate bow but you can’t see it.
“Fungus Gnats,” University of California Integrated Pest Management Program
“Beneficial Nematodes,” University of Connecticut Home and Garden Education Center