Dealing with Fungus Gnats!
One of the most common questions we get is: how to deal with fungus gnats, aka Bradysia spp. Often confused with fruit flies, these small, dark flying insects thrive in moist environments and are commonly found around potted plants. They are closely related to other small flies in the Sciaridae family and can be a nuisance in both indoor and outdoor gardening settings. If you have indoor plants, you’ve probably noticed them. Read on for more information about these annoying little gnats, why they’re so difficult to fully eradicate, how to prevent infestations, and how we deal with them in the shop.
The short answer / TLDR…
Fungus gnats are relatively harmless unless they are highly infested in your plant’s soil. If this is the case you will see hundreds of larvae crawling around the soil, and you’ll often see large numbers of adults congregating on windows/windowsills. You would 100% know you have a problem if you experience this…!
If an infestation is happening, your plants can be damaged from fungus gnat larvae feeding on organic matter like root hairs.
If you’re just seeing a few fungus gnats fly out of your soil when watering, or an occasional adult flying around your home, you don’t have much to worry about. Smaller populations won’t damage your plants, and will be more annoying than concerning.
Scroll down for treatment recommendations!
Soil should be alive!
To start, I want to make it very clear that growing plants in soil within our homes brings inside more than just plants and dirt. And, for your plants to be healthy, you want the soil to be alive and biodiverse. If you analyze healthy soil found outside, it will contain life in the form of invertebrates (insects, worms, nematodes) and microorganisms (fungi and bacteria). Plants co-evolved with these life forms and depend on them for optimal health. The needs of indoor houseplants are no different to the needs of plants growing outside. The fun part of indoor gardening is that you become the end all be all when it comes to providing for your plant’s needs, nurturing soil that is healthy, biodiverse, and nutrient-rich. The problem with most indoor houseplants sold in nurseries is a lack of soil biodiversity. This means that fungus gnats and other soil-born pests do not always encounter predators to reduce their populations naturally.
Fungus gnats are ubiquitous.
Fungus gnats can be found all over the globe, their populations most numerous in the warmer months. Unless you live in a state of the art fortress, or you’re very diligent with fungus gnat prevention methods, they will find their way into your home and into the damp soil of your houseplants. Without introducing natural fungus gnat predators like Hypoaspis soil mites, rove beetles or beneficial nematodes, fungus gnats will enjoy a predator-free lifestyle, breeding as much as they please. These predatory creatures are unlikely to find their way into your plant’s soil unassisted. Though, you can purchase these predators online nowadays.
A reality check…!
Over the last ten years working with indoor plants, I can’t remember a single time I’ve unboxed a shipment of plants and not seen a fungus gnat fly out. For a while I stressed about them, especially when they showed up inside closed terrariums. I assumed something was wrong with my soil, and that I wasn’t doing enough to prevent or deal with them. It became quite frustrating and I took on a lot of worry and stress that people would be dissappointed with my plants or terrariums. As time has gone on, I’ve had to accept that fungus gnats and other pests are completely out of control within the houseplant industry.
There is absolutely no way for me to completely prevent them from being in my shop, in the plants I sell, or in the soil we use. We receive plants from dozens of different growers every week. It’s unrealistic for us to repot every single plant into new soil, but we do thoroughly check and treat all of our inventory every single day for more serious pests like spider mites, mealy bugs, thrips, scale etc. It’s MUCH easier to control fungus gnat populations and other pests in your home than in a nursery with constantly rotating inventory. I’d much rather say that my shop is fungus gnat free, but this would be a lie and this would also be very very difficult to achieve. Anyone with a plant shop or nursery that says they don’t have fungus gnats in their plants either grows their inventory themselves in a controlled environment, or…they’re lying to you.
How we deal with fungus gnats!
With that said, we do employ various methods to prevent fungus gnat infestations in the shop, and you can easily replicate these methods at home. All methods to deal with fungus gnats require diligence and regularity. If you aren’t doing something to counter their numbers, they will eventually repopulate. So heres’s how!
Reduce how often you’re watering your plants - since fungus gnats breed in the top few inches of damp soil, letting this portion of soil dry out more will be helpful.
Remove/replace the top couple inches of soil.
Regularly aerate soil with a chopstick, or add a top layer of rice hulls or gravel to create a physical barrier to prevent breeding.
Bottom watering is also helpful as it reduces soil compaction and maintains good air flow, which prevents soil from staying damp for extended periods (which gnats love!).
Best way to treat larvae and pupae:
1 part 3% hydrogen peroxide to 4 parts water, soak your plant when it’s time to water. Repeat until you start to observe their numbers declining, and do so on an occasional basis as a preventative measure. Peroxide is toxic to fungus gnat larvae.
Sprinkle Bios fertilizer (sold in shop) on the surface of the soil. This product contains a beneficial bacteria, Bacillus thuringiensis var. israelensis (Bti). Bti is a strain of bacteria that produces toxins specifically lethal to the larvae of fungus gnats and other dipteran insects like mosquitoes and black flies. When ingested by the larvae, Bti disrupts their gut cells, leading to paralysis and death. This bacteria is also found in the product Mosquito Bits.
Apply a soil drench with Sacred Elements soil tonic which smothers larvae and disrupts reproduction, all while boosting your plant’s immune system.
Best way to treat adults:
Yellow sticky traps- adults are attracted to yellow and will fly onto the traps, get stuck and die. It can also help to place some of the traps in windows as the adults are also drawn to the light.
We strategically hide yellow traps all over the shop (bc they’re ugly) and we put out even more traps when the shop is closed.
Introduce predatory insects:
We mix predatory Hypoaspis soil mites into our soil that we use for repotting and terrarium making. Though not available to purchase in shop, you can find some sellers online.
Thanks for reading and please leave your questions in the comments or reach out to us at plantcare@bwillow.com
-Liz Vayda, B.Willow Owner
