FUNGUS GNATS

Fungus gnats, flies in the family Sciaridae are generally harmless but can be quite annoying. They are small, dark colored flies that hang out around the surface of the soil and fly around when disturbed. They lay their eggs in the top couple inches of soil and the maggots feed on decaying organic matter. They can also feed on root hairs which can reduce vigor. Once they turn into adults they only live for a couple of days.

Non pesticide options:

  • Impermeable layer – Fungus gnats lay their eggs in the top couple inches of soil. If you exclude access to the soil with an impermeable layer of something like a half-inch of sand they will be unable to lay eggs

  • Bottom watering – If you carefully water from the bottom and avoid wetting the top inch or two of soil fungus gnats will not lay eggs

  • Yellow sticky traps – These will help control some of the adults but mostly act as an early warning system before infestations become noticeable

Insecticides 

  • BTI – BTI is a live culture of Bacillus thuringiensis israelensis, a bacterium that selectively targets the larval stage of certain dipterans like fungus gnats and mosquitos. It will not kill adult fungus gnats but they have very short lifecycles and will die off soon on their own.

    BTI is commonly available for control of mosquitos sold as mosquito bits or mosquito dunks at home depot or lowes. Sold in a 32 oz container of bits for $19

    It is also available online in a more concentrated form specifically for fungus gnats called Gnatrol. You can buy it by the oz, but note that since it is a live bacterium it does have an expiration date (listed on the site): 0.5 oz for $8, 1 oz for $10, 2 oz for $15, or 4 oz for $23

  • Imidacloprid – Imidacloprid is a systemic neonicotinoid that kills everything except for spider mites and hard scale, including soil borne pests like fungus gnats. It is available in granular form that is applied to the soil and provides protection for 90 days. I highly recommend this for all indoor plants. See “Imidacloprid” in the mealybug section for more info