A Soil Drench for Fungus Gnats

Written by faith mcgee
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Fungus gnats infest potting soil and lay eggs. These eggs hatch and eat away at the root system of your plant, according to the University of California. Root damage may be prevented by using a soil drench. A soil drench is an insecticide applied to the root system of the plant.

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Biological Insecticide

Biological insecticides that contain Bacillus thuringiensis var. israelensis or H-14 strain are considered highly effective drenches, according to Colorado State University Extention. Bacillus thuringiensis var. israelensis is a bacterial pathogen that is eaten by the larvae of the fungus gnat, according to the University of Wisconsin. Once the larvae have ingested the bacteria, they get sick and die off. Some gardeners prefer to use biological insecticide, because they do not target beneficial insects in the soil.


Carbamate is an active chemical in insecticidal drenches, according to the University of California. This broad spectrum pesticide can also harm other insects, but it will kill off your fungus gnat larvae. This insecticide drench should be handled with care. There is some evidence that it is harmful to humans when it comes into contact with skin, according to Queensland Government. Wear gloves when using this insecticidal drench and avoid using in a poorly ventilated area.

Neem Products

Neem is an organic pesticide that derives from the neem tree. Neem drenches are biodegradable, safe around humans or pets and can be used to kill off fungus gnat larvae. This organic pesticide is slower to kill off your pests than other chemical pesticides, according to Montana Integrated Pest Management. Gardeners may have to send away for neem drench products, because they are not available in all areas of the country, according to Colorado State University.


Organophosphate drenches containing either acephate or malathion may be used to control your fungus gnat population, according to the University of California. When using organophosphate, take precautionary measures such as using outside, wearing gloves and mask, and drenching the plant without pets or children around. This type of pesticide inhibits the nerve centres of fungus larvae gnats. In addition to being careful with the pesticide, avoid using if you have beneficial insects living in your soil.

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