Home & Garden Trees & Houseplants

Solutions for Gnats in Potted Plant Soils

    • Fungus gnats are a common problem in potted plants, both indoors and outside. The dark-colored adults are pests that gather around light sources like windows and lamps. They lay their eggs in potted plant soil. The larvae live in the soil, feeding on fungal matter. Gnat larvae will sometimes feed on the roots of a plant, which can cause a plant to fail to thrive. You can control gnats in your potted plant soil using natural, chemical and cultural solutions.

    Natural

    • Use biocontrol options to eliminate gnats in your potted plant soil. Hypoaspis, or the fungus gnat mite, lays eggs in soil infested with fungus gnat larvae. When the eggs hatch, the nymphs that emerge feed on the larvae, eliminating the gnats. Some predatory nematodes target fungus gnats, including Steinernerma carpocapsae and feltiae. These tiny parasitic roundworms invade a gnat's body and release poison that kill it. They will continue to live in plant soil even after the gnats are gone, providing ongoing protection for your plants. Both the mite and the nematodes can be purchased at natural gardening centers. Applying them to plant soil at the beginning of spring and again in winter will best protect your plants.

    Chemical

    • Adult gnats in your potted plant can be killed by spraying them with pyrethrins and pyrethroid insecticides. These insecticides do not linger for very long, and you could have several generations of gnats and larvae living in your plants, which means that, after you kill the adults, more will probably appear. You can use a soil drench on your potted plants to kill the gnat larvae. The biological insecticide Bacillus thuringiensis var. israelensis, or Bti, will kill fungus gnat larvae when used as a soil drench. You can also try using neem oil-based products. You may need to apply the soil drench more than once to eliminate all new generations of gnats in your plants.

    Cultural

    • You can use cultural controls to prevent and eliminate gnats in potted plant soil in your home. Never recycle old potting material. Always use new and sterile potting soil for new plants. Inspect all new plants carefully for gnat infestation before bringing them into your home. Gnat larvae thrive in moist soil, so avoid over watering your potted plants. If gnats do appear, you can kill the larvae by drying out the soil in the plant, which means letting the top few inches of soil become dry and might require stressing the plant as it waits for water. You can try using sticky traps to kill the adult gnats as they fly around your potted plants.

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