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.
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.
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.
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.
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.
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