Check out social media gardening groups and you will find at least one post a day asking about a remedy for fungus gnats. They are perceived to be a huge problem with seedlings and other indoor plants. How do you get rid of them?
There is also a lot of advice online, but much of it is incorrect. Let’s swat some myths about fungus gnats.
Fungus Gnat Life Cycle
One of the biggest mistakes people make with pests and diseases is not taking the time to understand their biology. Understanding a pest usually gives you some good insight into their control which in turn allows you to debunk some common household remedies.
Adults live for about a week and during that time they lay a couple of hundred eggs in moist soil. The eggs hatch in 4-6 days and the larvae feed for two weeks, on mostly dead organic matter but they can also feed on plant roots. They then pupate for 3-4 days before new adults hatch out.
The actual duration of the life cycle depends on temperature. Each stage is slowed down if you are growing seedlings in a cold basement.
Most of the time people don’t notice them right away and by the time they do, they have numerous flies in various stages of its life cycle.
Identification of Fungus Gnats
Gardeners first notice fungus gnats because they fly around your head while you are tending plants. The adult is a small, delicate, mosquito-like fly, about 1/8″ in size, with one pair of clear wings. They are not strong fliers and usually don’t venture too far from the plants.
You might also notice the larvae (or maggot) in the soil, but this is quite small and usually stays underground. It has a black head and a whitish transparent body.
Do Fungus Gnats Harm Plants?
This seems like it should be a simple question, but it’s not very clear how much damage they cause. The term fungus gnats is used for a number of species that are very difficult to identify. Some species seem to harm plant roots, while others live just on decaying organic matter. It is quite possible that your gnats are doing very little harm to the plants.
Killing Fungus Gnats
Now that you understand the life cycle, it becomes clear why people report that some treatments don’t work. If you kill the flies, they just come back again, because the soil is full of larvae and pupae that have not yet hatched.
On the other hand if you treat the soil to get rid of the larvae, it has no effect on the existing flies. You have to wait until they die of old age and in the mean time they are laying more eggs.
You have a couple of options. Kill the flies and larvae at the same time, which probably requires the use of two different techniques, or kill one and wait for the other to die of natural causes. Keep this in mind while we discuss the following potential solutions.
Let Plants Dry Out
Keeping soil dry does two things. It makes it harder for existing larvae to survive and it discourages flies from laying new eggs. It does not get rid of existing flies.
Try to avoid soil containing peat moss as they tends to hold water longer. Unfortunately almost all seedling and potting mixes uses peat moss. I doubt that coir is any better.
Drying the soil does work, and it might even make your plants grow better.
Stop Fungus Gnats With Diatomaceous Earth
Diatomaceous earth (DE) is used to kill insects and it works by absorbing oils and waxes on the outer cuticle. Without this protection the insects dehydrate and die. Does it work for fungus gnats?
A recent study looked at this and concluded diatomaceous earth applied on top of the potting media does not work for controlling fungus gnats! Another study looked at mixing DE into the media and also found it didn’t control fungus gnats.
So why do so many people report DE as working? I think the reason is that DE is only effective when dry and many gardeners know this. So to make it work, they also dry out the soil. The drier soil kills off the larvae and prevents adults from laying eggs. Their positive anecdotal reports may just be a case of misidentified cause and effect.
Peroxide Soil Drench
Some claim peroxide kills the larvae and eggs of fungus gnats, but I could not find a reliable source that confirms this. When peroxide is applied to soil it is quickly deactivated as it reacts with anything organic, including bits of dead plant material, bacteria, fungi and fungus gnat larvae. That sounds promising, but it also reacts with plant roots, harming them.
A low dose will be deactivated quickly with little effect, and a high dose would not be good for plants. I am not convinced this is a good option.
Cover Soil with Sand
Many report that sand on top of the soil prevents flies from laying eggs, thereby solving the problem. Some anecdotal reports say it also prevents flies from hatching out, but I have my doubts about that.
If egg laying is stopped, the gnats should be gone in about 4 weeks.
A study by Purdue University found that a layer of sand did not reduce fungus gnats. This work is reported as part of a summary on fungus gnats and does not seem to have been published.
One report suggested that a covering of sand does not work, because the flies lay eggs in the pot’s drainage holes. That makes sense, so if you try this method cover the holes in the pots.
Numerous anecdotal reports exist, but they generally lack information about the type of sand, or the thickness applied. These reports are almost always accompanied with the suggestion that you should keep plants drier. So was it the dry soil that worked, or the sand?
Smaller grains of sand, and/or a thicker layer may work. The top layer would be relatively dry and contain very little organic matter; not a perfect place to lay eggs. A thicker layer also makes it harder for the small flies to reach the soil. How thick does this need to be? Nobody knows.
Bounce Fabric Dryer Sheets
Bounce® original brand fabric softener dryer sheets were tested to see if they repel fungus gnats and they did work, to some extent, in a special laboratory environment. Fungus gnats were put into the center compartment of a special chamber and allowed to fly towards one of two ends. This resulted in less flies in the end containing the drier sheet.
This is interesting, but not proof that this works in a real world situation, although some nurseries are inserting drier sheets into pots in the hope of controlling this pest.
Change All of the Soil
I see this recommendation a lot but it doesn’t make any sense. If you have flies, they will just lay eggs in the new soil. If you have several plants you would need to change the soil in all pots, and wash all roots thoroughly. Don’t bother.
Coir Keeps Fungus Gnats Away
Some manufacturers of coir claim that it does not allow fungus gnats to grow but a recent study shows this is not true. They grow in both coir and peat moss.
It seems the type of soilless media has little impact on egg laying.
Yellow Sticky Traps for Fungus Gnats
Place yellow sticky traps near plants, and right on the soil surface. Flies are attracted to them, stick, and die. You can buy these or make them yourself, but many home recipes don’t work very well.
These work as a monitoring system to tell you when you have a problem and they will catch some flies, but they are not effective enough to eliminate the problem.
Bacillus thuringiensis Kills Fungus Gnats
Bacillus thuringiensis subsp. israelensis or Bti, is a naturally occurring soil bacteria that is toxic to a variety of insects. When it is eaten by larvae, it kills them. It won’t get rid of existing flies or pupae, but the flies should be gone in about 4 weeks because it controls the larvae.
Bacillus thuringiensis is available in various commercial products. A convenient one that is readily available is a product called Mosquito Dunks. Break it into small pieces, sprinkle a bit on the surface of the soil and water it in. Repeat in 30 days. They can also be used to stop mosquitoes from breeding in rain collection barrels.
Nematodes for Fungus Gnats
Nematodes can also be used, but not everyone reports good results with them. That is due to poor storage of the product, not because they don’t work.
First of all you have to get the right species of nematode, Steinernema feltia.
Secondly, they have to be stored cold and they have a short life span. If you get them shipped, they need to be kept cold, but not frozen, during shipping. Big box stores and Amazon are not a good place to buy live biologicals because they don’t provide the storage needed.
For houseplants and seedlings, you usually need them in winter when nurseries are not open.
You can check to see if your purchased nematodes are alive. Put a small amount of a glass dish, add a couple drop of water at room temperature. Wait a few minutes and use a magnifying glass and a black background to check for activity. They should be swimming. If they are straight and not moving, they are dead.
Controlling Fungus Gnats in the Home
The best option for the home gardener is to control watering. Keep plants drier and you will eliminated most issues with fungus gnats. Be aware that it will take weeks to clear up a problem with any method you use, so don’t give up on it.
A second option is Bti. It is effective if placed in each pot and it’s readily available. It is not a quick fix since it only kills the larvae.
Nematodes do work but are tricky to use and not easily available to home gardeners. The other options listed above either do not work, or there is no scientific evidence to support them.