If you have gnats hovering around your houseplants, they are probably fungus gnats. There are loads of DIY fungus gnat remedies promoted online, but most fail for one simple reason. Learn why a two-pronged approach is necessary to conquer these annoying pests once and for all.
If you have houseplants, you might be all too familiar with this scenario. You see small black dots hovering over your houseplants and occasionally elsewhere in your home. Sometimes they even buzz your face.
These are fungus gnats (Bradysia spp.). They get into your home and plants by hitching a ride with new houseplants. Sometimes they are living in bags of potting soil. And once in your home, they fly from plant to plant, especially if you have a lot of plants in one room or area.
These annoying pests spend most of their lives in houseplant soil, where they rapidly multiply. And once you’ve got an infestation, they can be frustratingly difficult to get rid of.
They are usually more annoying than harmful, but too many can damage your plants causing wilting, stunted growth, iron chlorosis and leaf loss.
There are many remedies that seem to work… for a while. But before you know it, they’re back.
Let’s jump right in with what works. Then we’ll look at preventive measures you can take. And finally, you can learn more about the remedies that you should skip because they don’t work and why.
Two-Pronged Approach That Works
The lifecycle of fungus gnats has four stages — egg, larva, pupa, and adult. The first three occur in the soil unseen. The final, adult stage is the one we’re all familiar with.
To get rid your plants of fungus gnats, you need to get rid of the adults and the egg/larvae/pupa stages. And to do this you need a two-pronged approach.
Amazing Fact!
Each female adult gnat lives only
a few weeks but can lay 300 eggs.
Sticky Traps for Adults
Getting rid of the adults is pretty straight forward. Attract and capture them with yellow sticky traps. These work quite well. They are easy to find and inexpensive, especially if you buy large squares that you cut to size. The cutesy shaped like flowers or butterflies are much more expensive and not all that cute once they’re covered in gnats.
There’s no need to mess with bowls of vinegar and soap, an oft-recommended DIY remedy. The smell of vinegar gets sickening fast, and I didn’t find that it worked any better than putting out a dish of water.
Caution!
Don’t use sticky traps outside.
They attract and catch beneficial insects
along with the gnats.
Another DIY attractant is raw potato slices. You can use this if you have a gnat problem in your outdoor potted plants.
Bt or Systemic for the Rest
Let’s take a look at two proven ways to get rid of the soil-dwelling stages of fungus gnats, plus one anecdotal way that looks promising.
Mosquito Bits
You may have heard of Bt (Bacillus thuringiensis), a naturally occurring bacteria that is used to kill caterpillars.
Mosquito Bits are granules of Bacillus thuringiensis sp israelensis (Bti), a variation of Bt that kills mosquito larvae, but is harmless to people, pets, and plants. You can mix the bits with water and water your plants as usual, or sprinkle it on top of the soil and water it in. The bits will break down over time.
Don’t let the name Mosquito Bits discourage you, since Bti is extremely effective at killing fungus gnat larvae, not just mosquito larvae.
Bonide Systemic Granules
Before I learned about Mosquito Bits, I used Bonide systemic granules. It worked like a charm, but contains a chemical insecticide, imidacloprid.
Knowing what I know now, I would try Mosquito Bits first. However, since I already have this, I wouldn’t hesitate to use it if I had another gnat outbreak. (I don’t have kids or pets that would get into it, I’m not using it on edible plants, and it breaks down quickly when exposed to water and sunlight.)
Cinnamon (maybe?)
I’ve learned from gardeners in the UK, that Mosquito Bits and Bonide systemic are not readily available everywhere. (I’m as surprised as you are that I have readers in the UK! 🤗)
If that’s the case for you, or you’d just like to try something you have in your kitchen cabinet, try cinnamon. This is one home remedy that shows some real potential for treating fungus gnats.
Cinnamon has known antibacterial and anti-fungal properties. It works as an insect repellent, and in high doses, can prevent insects from laying eggs. It’s speculated that it might also work by killing fungus in the soil, which is the larval food source.
You can either sprinkle powdered cinnamon on top of the soil or dissolve it in water and water with this “cinnamon tea”.
Fungus Gnat Prevention Tips
Once you’ve got your gnat problem under control, you don’t want it to come back. Here are some ways to prevent fungus gnat infestations.
🪴Don’t overwater your plants. The adults lay their eggs on moist soil. Overwatering also increases fungus, the larval food source, in the soil.
🪴Always keep a few sticky traps up as an “early warning detection system” for the flying adults. That way, you’ll know as soon as you have an infestation brewing.
🪴Quarantine any new houseplants. When you bring a new plant home, pop a sticky trap in its soil. Then wait until you’re confident your new plant does not harbor any fungus gnats before introducing it to the rest of your houseplant gang.
🪴Carefully monitor your plants when using new potting soil. Try to buy bags of potting soil that are on the dry side, not sodden with moisture. (I have no doubt that my gnats came from infested potting soil.)
🪴Compost is the perfect food for fungus gnats, so don’t add it to your potting mix.
DIY Home Remedies to Skip
There are tons of DIY remedies that might seem to help… for a while.
Here’s a list of some home remedies that you may be familiar with. I was surprised to see that some of these have actually been studied and proven ineffective.
- Bounce dryer sheets (Ref)
- topping soil with a layer of sand (Ref)
- topping soil with a layer of diatomaceous earth (Ref)
- using coir or peat mass as your growing medium (Ref)
- watering with hydrogen peroxide, soap, or neem oil
Besides being ineffective, here are some other reasons I’m not keen on some of these remedies.
When I tried top dressing with sand, it quickly worked its way into the soil, defeating its purpose.
Diatomaceous earth only has insecticidal properties when wet, and it obviously gets wet every time you water your plants. Also, it’s hazardous to breathe in and can be irritating if it gets on your skin or in your eyes.
Since hydrogen peroxide kills microbes, it’s likely that it will also kill good microbes in your soil. It also kills BT, so don’t use them together!
Nematodes are microscopic worms that eat the larvae. You can buy them and add to your soil. You’ll have to hope they are viable — and they’re pricey. Intentionally adding expensive bugs to my houseplants? Nope. Not for me. 😅
Bottom watering sounds promising, until you find gnats in the damp saucers. I’ve heard anecdotally that they can enter the soil through drainage holes, but I don’t know that for a fact.
Fungus Gnat Problem Solved!
When you’ve beat the gnats, you’ll know it. One day you’ll realize that you haven’t seen any gnats for days, and then weeks, and finally, you can’t remember the last time you saw one.
Since employing this two-pronged approach, I haven’t had a recurrence for well over a year. (Of course, I need to stay vigilant, since any new plant could bring gnats into my home.)
I hope these tips help you get rid of your fungus gnats, if not forever, at least for a long time.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for 15 years.
