How do I get rid of small black gnats from house plants. I could not figure...

Asked June 18, 2017, 9:17 AM EDT

How do I get rid of small black gnats from house plants. I could not figure where these annoying gnats were coming from and then realized that gnats arise when I stir up the soil of two plants. I place these plants outdoors and thought that letting the soil dry out might get rid of them. Is there anything else I should do now or that I should do if this does not work.

Belmont County Ohio

1 Response

Thanks for your question about the gnats in your plants.

The insect is called a fungus gnat. The fly lays eggs in the soil mix and the maggots feed on decaying material in the soil mix as well as plant roots.

You have begun the first option of control, which is to allow the top 1-2" of soil to dry out before watering again. If this is not sufficient after a couple of weeks, the recommendation is to apply a insecticide for 3 days focused on the adults. This reduces the amount of eggs that can be laid. Chemicals to use contain the following active ingredients: bifenthrin, cyfluthrin, permethrin, and lambdacyhalothrin. These have long enough action to reduce the fly population.

The next control would be directed at the maggots in the soil. You would need to use a soil drench or granular insecticide that gets watered into the soil to kill them. The best insecticide for this contains systemic insecticide imidacloprid. You may also use a natural product, microbial insecticide Bacillus thuringiensis subsp. israelensis (Bti). This may be hard to find on the store shelf as it is not the same as other Bt products, it is a different sub-strain.

To check for maggots in the soil, it is recommended to insert a 1/4 cut potato into the soil. This attracts the insect larva and you can see how large your population is and if you need to do more control.

Here is a fact sheet on Soil Gnats from Colorado State University Extension for your reading pleasure. I tried to condense the material for you.

Thanks for your question. I am sure you will be able to control the gnat population.