Fungus gnats – the little buggers that find your houseplants. They love to visit seedlings and can even infest your larger houseplants. They fly all over the house and drive gardeners crazy.
In this post I will look at how to get rid of fungus gnats on houseplants and discuss various ways to control them and revel the ones that works best.
Understand the Fungus Gnat
People are always asking me how to control some kind of pest on their plants and my answer usually starts with, “identify the bug and then learn about its life cycle.” The life cycle describes the way the bug lives from egg to full adult. Without this knowledge, it is difficult to figure out the best way to control it.
Listening to other uninformed people on the internet does not work.
Identification of Fungus Gnat
People first notice fungus gnats because they fly around the plants and your head while you are tending the 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 it is also quite small and usually stays underground. It has a black head and a whitish transparent body.
Fungus Gnat Life Cycle
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 on plant roots for about two weeks. They then pupate for 3-4 days before new adults hatch out.
The actual duration of the life cycle depends on temperature. Everything will be 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 you do, they have numerous flies in various stages of its life cycle.
Do Fungus Gnats Harm Plants?
Seems like a simple answer, but its not very clear how much if any damage they cause. The term fungus gnats does describe a number of species that are very difficult to identify without going through a special process. Some species seem to harm plant roots, but others live just on decaying organic matter. It is quite possible that your gnats are doing no 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 fungus gnat flies, they just come back again, because the soil is full of larvae and pupae that have not hatched yet.
On the other hand if you treat the soil to get rid of the larvae, it has no effect on the 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.
Keep Soil Dry
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 soils containing peat moss as it tends to hold water longer than soil. Unfortunately almost all seedling mix uses peat moss. I doubt that coir is any better.
Drying the soil does work.
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.
The DIY suggestions online include coating with Vaseline, honey, glue, and motor oil. I asked if these work in Facebook groups and the answer is no. There is a commercial product called Tangle-trap that might work when it is applied to yellow cards. The commercial products do work to control and monitor populations but they won’t get every last fly.
Yellow sticky cards have no effect on eggs, larvae or pupae, but over a 2-3 week period you will get rid of most of them, as new flies hatch out.
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.
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 also work well in keeping mosquitoes from breeding in rain collection barrels.
This product is considered perfectly safe for humans and pets. It will slowly kill the larvae and therefore future flies. It will not get rid of the flies that have already hatched or pupated.
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 in 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 still they are dead.
Cover Soil with Sand
Cover the surface of the soil with sand. The sand prevents flies from laying eggs in the soil. Some reports say it also prevent 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.
One report suggested that covering with sand does not work, because the flies lay eggs in the drainage holes on pots. That makes sense, so you might want to cover them.
Changing All of the Soil
This makes no 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.
You think you have fungus gnats, but do you? You might have drain flies.
Drain flies are a tiny fly that has a hairy body, and fairly large wings for such a small bug. They look more like a miniature moth than a fly and breed in the gunk found in drains. When they fly around they do look a lot like fungus gnats.
Before trying to solve your problem, determine if you have these guys. Cover drains with sticky tape, sticky side down, and wait a few days to see you catch flies. If you do, you have drain flies.
Fruit flies are also very similar and many people get them confused. Put out a dish of cider vinegar, or red wine, or even a piece of old fruit. If the flies are attracted to any of these, they are fruit flies, not fungus gnats.
- Image of fungus gnat, by gailhampshire: https://commons.wikimedia.org/wiki/File:Fungus_Gnat_._Keroplatidae._-_Flickr_-_gailhampshire_(1).jpg
- Image of drain fly, by Judy Gallagher: https://www.flickr.com/photos/52450054@N04/31373496327