Fungus gnats: they’re here, they’re there, they’re everywhere! Photo: Akchamczuk, depositphotos
By Larry Hodgson
For the past few days, I have been getting a lot of questions from readers about fungus gnats. Now might be a good time to republish this article from 2018 for the benefit of all.
You’re sitting quietly in your living room and, suddenly, a tiny black fly zigzags in front of your face. You chase it off with a wave of the hand. Attracted by the CO2 in your breath, it soon comes back. You manage to squash it this time, but soon another one appears.
What is this insect? And where does it come from?
Many people mistake this tiny pest for a fruit fly (Drosophila spp.), and it is about the right size, but this particular little creature has no interest in fruit. Instead, it seems mostly to hang out around your firewood or your houseplants and seedlings … that is, when it’s not flitting about your face.
The pest in question is a fungus gnat, a fly in the Sciaridae family of which there are some 20,000 species worldwide. Most of those that infest our houseplants are in the genera Scatella, Bradysia, Orfelia or Sciara, a rather useless bit of information, because it’s unlikely you’ll ever know the name of the exact species that shows up in your home.
A Family Portrait
The adult fungus gnat is a very small fly, rarely more than 4 mm in length, and usually black with transparent or grayish wings. You’ll often find several of them in the vicinity of a pile of firewood or around houseplants or trays of vegetable seedlings, but it also roams all over the house and, as mentioned, seems particularly interested in the air you breathe out.
The larva, which resembles a whitish or translucent, black-headed worm about 4 mm long, needs moisture to survive and will be found in some sort of moist organic medium, such as damp bark or potting soil.
How do fungus gnats get into your house? Well, adults may not be the world’s greatest flyers, but they can certainly be blown indoors or travel in on a pet or clinging to someone’s clothes. And it only takes one fertilized female to start a new horde!
That’s not the most common method, though. Most of the time, you (and yes, I’m pointing my finger in your direction!) brought eggs into the house by accident, either on firewood or when you bought a new houseplant or a bag of contaminated potting soil.
Most gardeners assume potting soils and seed starting mixes have been sterilized, but in fact, that’s almost never the case. Potting soil manufacturers instead use ingredients that are unlikely to harbor pests, but that do include some beneficial organisms coming from the organic ingredients, like peat, chopped bark and coir, that make it up. They don’t want to sterilize their blends, as that would kill the beneficials. However, even if the potting mix is shipped out sans fungus gnat eggs, that doesn’t mean it will stay free of them.
When the bags of soil make it to the retail store, they often sit there for months and any tiny hole in the bag, even one no larger than a pin prick, is enough for female fungus gnats to lay their eggs in. Since garden centers inevitably host at least a small resident population of fungus gnats (they’re pretty much ubiquitous in humid environments), it’s not at all rare to bring home a potting or seed-starting mix already contaminated with fungus gnat eggs.
Good News, Bad News
The good news is that fungus gnats are usually quite harmless.
Other than annoying humans by their mere presence (who wants to see Aunt Mathilda swatting at gnats when she’s over for a visit?), adult fungus gnats cause no damage. Yes, they have mouth parts, but they neither bite nor sting. In fact, many even make themselves useful by pollinating flowers!
Even the larvae are usually not very harmful to plants. They mostly consume decaying plant matter they find in the soil mix as well as small fungi and algae. They’re especially attracted to the types of fungus found growing in overly moist soil. And here too, they can make themselves useful, because by digesting these products, they produce mineral-rich droppings plants can feed on.
On the other hand, especially when there is nothing else to eat, the larvae of some species (but not all) do attack plant roots. On a healthy plant, this is rarely very harmful, however. To start with, most houseplants produce more roots than they really need and in fact, a bit of fungus gnat pruning often stimulates the plant to develop a denser and more efficient root system! Plus, fungus gnats generally prefer dead or dying roots to live ones, especially tending to feed on roots that have been immersed in water too long and have started to rot. A mature plant has to be in pretty bad shape for fungus gnats to do any major damage.
Their effect on seedlings, on the other hand, can sometimes be more serious. Young plants have very limited root systems and rather weak ones at that. In addition, adult flies can carry spores of harmful fungi, such as the dreaded seedling disease known as damping off. Plus, the small wounds fungus gnat larvae leave can serve as an open door for rot to set in. So, you really don’t want them chomping on the roots of your seedlings.
That’s why, even if the fungus gnats are not always very harmful, eliminating them is still the best policy … and if you discover they’ve found your seed trays, you need to get rid of them as soon as possible.
Know Thine Enemy
Indoors, fungus gnats can be present in any season and produce several to many generations a year. Although the adult seldom lives longer than 8 days, the female still has time to lay about 50 to 200 tiny eggs in moist potting soil or organic debris, preferably in the presence of fungi (it’s not for nothing they’re called fungus gnats: the odor of fungus attracts them). The eggs hatch in about 5 days and the larvae then feed for another 14 days. Next they pupate for 5 days. Following that, the adults emerge and the cycle begins again. The entire process takes about a month under normal indoor conditions, but can take place in as few as 17 days in a hot, humid greenhouse.
How to Control Fungus Gnats
Reduce Your Watering
The first key to success in controlling fungus gnats in indoor plants is to understand that they absolutely need constantly moist potting soil during their larval stage. If you make a habit of letting your plants dry out a little longer, this will often solve the problem completely. In fact, many experts consider fungus gnats more a symptom of overwatering than a problem in itself.
Try applying the golden rule of watering. That is, water deeply, enough to moisten the entire root ball, then allow the soil to dry before watering again. Many gardeners discover that careful watering keeps fungus gnats fully under control.
Don’t be discouraged if this doesn’t seem to work at first: it takes at least a month to before the treatment will have reached all the stages of the insect’s life cycle.
A Barrier Between Soil and Air
Another possibility is to make the surface soil unfit for fungus gnat larvae. Larvae live near the surface of the soil, no more than 1 ¼ inch (3 cm) deep, as they have to regularly move to the surface to breathe. Plus, they need moist, organic soil to live in. If you cover the soil with a ½ inch (1 cm) layer of sand or pebbles (there are even commercial products especially developed for this purpose, such as Gnat Nix), the larvae will no longer be able to live there … and besides, adults won’t be able to lay their eggs on such a dry medium either.
Trap the Adults
To control adults, you can try setting out yellow sticky traps or, better yet, a light trap. I find the latter works like a charm!
Bring On the Insecticides
It is also possible to drench the soil with an insecticide. Might I suggest one with few adverse effects on the environment, such as insecticidal soap or neem oil? It is also possible to make the soil less attractive to larvae by mixing diatomaceous earth or cinnamon powder into it.
Predators to the Rescue
A totally different direction to go in would be to use the fungus gnat’s natural ennemies agains it. Do note though that these predators will only be effective when fungus gnat larvae are present in the soil. They won’t have any effect against free-flying adults.
There are, in fact, several nearly microscopic predators you can use to control fungus gnats, including beneficial nematodes like Steinernema feltiae or predatory mites like Hypoaspis miles. The nematodes can wipe out a population of gnat larvae in only 48 hours!
You can order these predators on the Internet from such sites as Lady Bug Phytoprotection. Just apply them according to the instructions on the packaging.
Then there is another biological larvicide: Bti* (Bacillus thuringensis israelensis). This is a bacterium usually used to manage mosquitoes, but it will help control fungus gnat larvae as well. Make a solution and use it to drench the potting soil. Larvae quickly consume the bacteria and die within 24 hours. It will likely be necessary to repeat the Bti treatment weekly for 3 or 4 weeks in order to eliminate all the pests as they cycle through the various stages of their life.
*Do not use Btk (Bacillus thuringensis kurstaki), designed for use on butterfly larvae, on fungus gnats. You need Bti, specific to diptera larvae (flies).
There are formulations specifically developed for this purpose, such as Knock-Out-Gnats and Gnatrol, two professional products you can find on the Internet. However, you can also take MosquitoDunk type pellets, likely available locally, and crush them into powder to make a solution.
And finally, you can also use carnivorous plants to catch fungus gnats. You can learn more about them in the article Grow Your Own Sticky Trap! Personally, I find butterworts (Pinguicula spp.) especially efficient!
Fungus gnats: they’re often more annoying than really harmful and fairly easy to eliminate as well. Don’t lose too much sleep over them!
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In greenhouse production, fungus gnats spread some diseases. They are harmless on their own, but the diseases that they transmit in greenhouse environments can be major problems. Fortunately, such diseases are not much of a problem in home interiors.
Oh! Is that what the little buggers are. I just spray the soil surface with normal fly spray. Although I have Australian Gnats and use Australian fly spray so I can’t guarantee it’ll work for you too. It also won’t work in the case of hydroponics!
Excellent article. So many new-to-houseplants freak out when they see these little creatures and are ready to use the nukes. I always find your articles encouraging people to slow down, learn more and only reach for the pesticides as a last resort. Kudos!
Thank you! ?
Being a fairly new subscriber, I think it is great if you give yourself a day off once in a while and republish some of your most important work. Put your feet up and enjoy spring, Larry.
Thanks for understanding, Marian! ?
Something you didn’t cover that I’ve had personal experience with is in hydroponic systems they can be far more damaging, but I have controlled them using gnat nix and sticky traps
Thanks for the added info! ?