How to kill fungus gnat larvae on indoor plants

Written by john haughey
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If your indoor plant is infested with annoying flying pests, you may have a common type of gnat, the fungus gnat. This gnat feeds on fungus which thrives in moist warm conditions. To solve the problem, select a larvicide and insecticide to control the flying insects and remove their food by controlling fungal growth.

Skill level:
Moderately Easy

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Things you need

  • Heavy grade motor oil (SAE 90)
  • Petroleum jelly
  • Bright yellow index card, paper
  • BT larvicide or mosquito dunk
  • Clean sand
  • Pyrethrin or permethrin insecticide
  • Fungicide

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    Identify and Control Gnats

  1. 1

    Trap the pests. Insert yellow cards (yellow attracts the insect) into the soil covered with an adhesive, such as petroleum jelly or heavy grade motor oil (SAE 90), to attract and capture the flying adult insects.

  2. 2

    Identify the pests. The fungus gnat is a small type of fly, approximately 1/8-inch long, with clear wings, long antennae and Y-shaped veins on their wings. They fly in a zigzag pattern and do not stray far from their food source which is the fungus living in the soil.

  3. 3

    Distinguish gnats from other insects. Look for standing water because mosquitoes which may be mistaken for a fungus gnat require water to breed. Look beneath the plant's leaves to find a sticky scale, which can be a sign that the plant is infested with whiteflies, another insect that is commonly mistaken for a gnat.

  4. 4

    Use different pesticides to target different parts of the gnat's life stages. Adults lay eggs that develop into the maggots (larvae). The larvae feed on fungus and moulds. The larvae mature into pupae and then become adult insects. It will take a month or two to control the gnats because the gnat life cycle takes a month.

  5. 5

    Apply a biological larvicide following package instructions. Bacillus thuringiensis (BT) secretes a protein that interferes with larval feeding to kill the larvae. BT will not kill the flying insects that can continue to lay eggs. BT will kill all flies (including whitefly) that have larval stages, as well as mosquitoes.

    Add up to 2 inches of clean sand on top of the potting soil to prevent larval forms in the soil from coming to the surface.

  6. 6

    Apply a general insecticide to kill adult insects such as pyrethrin (derived from chrysanthemums and toxic to bees) or permethrin (toxic to cats, bees, fish and mice). These sprays do not kill the larvae, which continue to feed and become adults. Once the adults lay eggs, the cycle starts over again. Choosing a general pest spray will take several life cycles and is potentially harmful to the ecosystem.

    Identify and Control Fungus

  1. 1

    Look for mushrooms in your plant, a sign of active fungal reproduction. Look for rot in the roots, caused by phytophthora, a type of water mould. Fungus interlaced with the root system requires extreme measures, such as chemical fungicides, repotting the plant using fresh soil and paying close attention to watering frequency and conditions.

    Look for other signs that a plant may be infected with a fungus or a mould, such as sudden death or wilting due to root rot, non-uniform discolouration of plants or leaves, or a fine mould and dew on plant leaves.

  2. 2

    Prevent fungal development by proper culture methods. Avoid overwatering plants. Always provide adequate drainage. Ask your local nearby plant nursery to give advice on the watering and soil needs of your particular plant. Allow air to circulate underneath potted plants.

  3. 3

    Repot a plant infected with fungus. Do not use potting soil that has been kept in a warm, moist environment.

Tips and warnings

  • Fungal or mould infection may spread from plant to plant, isolating a single plant may be necessary to prevent spread to other plants.
  • Pesticides are toxic to cats, harmful to babies and should not be flushed down any drain because they are toxic to aquatic life.
  • Do not store pesticides in unlabeled containers or transfer containers.

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