How get rid of fungus gnats


Getting rid of the gnats

No matter how much I try, I am just not able to get rid of fungus gnats completely.  I have used various methods to control them, but somehow, they just keep reappearing.  Not only are these little pests annoying, but they can also carry diseases like Pythium to seedlings. Pythium can be a real killer, particularly if seedlings share a common source of water. It can knock off a whole batch of seedlings in no time at all

As I have just started a lot of seeds for the 2022 Chili growing season, the time has come to tackle fungus gnats once and for all. I will use my existing methods to limit their numbers, but I also want to find ways to get rid of them forever. To do that, I will be doing a little research

Before doing that, however, the question needs to be asked. Just what are fungus gnats? They are the little black flies you will see hovering around flowerpots in doors, when plants have been overwatered. These little two-to-three-millimetre flies are also known as sciarid flies. They lay their eggs in wet compost or potting soil. When their larvae hatch, they feed on the fungus that develops when soil is too wet. While it is unlikely that the larvae will harm the roots of seedlings, they post the biggest risk when they grow into adult flies.  It is then they are known to carry pathogens like Pythium (which causes damping off) on their feet.  They transfer the disease when they fly from plant to plant

What I have been doing so far

Controlling the numbers

From personal experience, sticky fly paper helps reduce numbers.  The one I use is bright yellow in colour. When I place it near bright light close to my seedlings, the flies get attracted to it and will fly into a thick layer of adhesive on the paper. Once they have done that, they are trapped. It works well, but it doesn’t get rid of the problem, it just reduces the numbers. If the number of flies caught by the paper is any indication of how many of these pests there are around at any one time, then there are definitely plenty.

Another method I have used is to limit the amount of water I give my plants. I also don’t water from the top.  Pouring water on top of potting soil encourages fungus to grow. The larvae of the fungus gnats feed on the fungus. It is how they get their nourishment. By bottom watering and allowing the top of potting to dry out between watering, you can eliminate fungus growing. This deprives the larvae of food, and they begin to die off

This of course is not an exact science. Allowing soil to dry out between watering has to be finely managed. Sometimes you get it wrong, and seedlings will die on you because they have dried too much. It can get tricky, and it is certainly not an ideal way of dealing with things

In spite of the ups and downs, I have had reasonable success using the methods above. I have been able to contain numbers at least.  I wish I could say the same about nematodes. Nematodes are microscopic creatures that act as parasites on other insects. Some swear by them, but I am not a fan. Maybe I didn’t use them properly, but I didn’t find them to work at all. They were also expensive.

What did I learn from my research?

H202 and Bacteria Thuringiensis.

One idea I found appealing was to use 3% hydrogen peroxide mixed in a ratio of one part hydrogen peroxide to four arts waters to kill off the larvae. I have not tried it yet, but apparently if you do this at alternate waterings, it provides an effective way to get rid of the flies. It is certain something I will try in the future.  When I do, it will be the only time I will allow myself to water from the top. If I am going to kill off the larvae, I will want the solution to soak through the seed starting mix from the top right through to the bottom.

I also learned that fungus gnats are suckers for apple cider vinegar. Put a bowl of this vinegar, mixed with sugar, water and detergent, in your greenhouse or grow tent. In time, you will find plenty of fungus gnats swimming in it. In due course, they will be wiped out.  The problem here, though, is the same as the sticky fly paper. You are only killing off the adult gnats. The larvae in the soil will turn into adults, and so the whole cycle will continue. In essence, it is a method of reducing numbers, rather than permanently getting rid of the problem

Something else I discovered made interesting reading.  It concerned a bacteria called Bacteria Thuringiensis. It is a genus that kills many pests. Bacillus Thuringiensis Israelensis is said to be effective in killing off fungus gnats. The bacteria apparently affect the stomach lining of mosquitoes, black flies, and fungus gnats. The larvae of fungus gnats eat the bacteria, and they are killed even before they can turn into adult flies.

These bacteria can be bought online, but are expensive. Something that does a similar job is called Mosquito Dunks (aka mosquito bits). These are bits of mosquito that contain Bacteria Thuringiensis, but just in lower concentrations.  By all accounts, they work well and cost much less. All that is required is to generously sprinkle the mosquito bits on the soil. An infusion should then be made with the dunks, which then be used in the usual way to water plants. Apparently, this permanently removes fungus gnats in two weeks. This is certainly something I am definitely going to try.


I think the first thing I will try will be the Hydrogen Peroxide. It is something I already have on hand. I will be trying it at the 1: 4 ratios, but also at another dosage I have found. The dosage given is to mix 1 part 9% hydrogen peroxide with 6 parts waters.  I will convert this to a comparable ratio for my three percent hydrogen peroxide. Hopefully, I can knock off the fungus gnats once and for all. I will let you know how it goes!

Image credits

Ian Jacobs /CC BY-NC 2.0/ via Flickr

Ian Jacobs/ CC BY-NC 2.0/ via Flickr

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Dealing with fungus gnats

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