How to Use Diatomaceous Earth for Fungus Gnats and Crawling Insects: Seed Starts & More

Posted by Gary Pilarchik on

by Gary Pilarchik (The Rusted Gardener)

Diatomaceous earth or DE is not a poison. It is basically the sharp fossilized remains of microscopic sea creatures made up of silica. It is not harmful to us as it is a fine powder that feels like chalk. You should take precautions not create a lot of floating dust and breath it in as it can be an irritant in the lungs. However it is perfectly safe to handle. For crawling insects, it can be devastating. The microscopic dust or silica is extremely sharp and it will grind an insect's shell and body down over time.

The key to successful use of DE is dusting the areas the insects most often frequent and keeping it dry. I put this on an ant colony that invade my sunken container garden last summer.  They were gone within 2 days. Fungus gnats and crawling insects can make their way into your seed starts. One strategy to reduce their numbers is to use the DE on the surfaces of your seed starts and on the leaves of the plants.

The whole key is to keep the dust dry. You want the insect to crawl across the dry dust and get the microscopic sharp silica into its joints. The sharp dust will grind in the joints and cause death. The video shows you how I use it on my seed starts.

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  • I have fungus gnats in containers from last summer. Do I need to start over with new soil in all the containers or can I drench with hydrogen peroxide and after allowing them to drain, mix in DE and follow up with neem oil-soap spray treatments weekly?

    Meridith on
  • Thanks so much for this article. I live in the mountains of western NC & grow organically in Earthboxes during summer & winter (in a home built greenhouse). This is the first helpful information I have found for those gnats that crawl in the dirt – especially loving the cabbage family veggies. I will try DE!

    Healing Harper on

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