Fungus Gnats: How to Kill These Annoying Pests

Fungus Gnats are very annoying pests. They are small, black flying insects about 1/8 inch long. They lay their eggs in the moist organic soil of indoor potted plants. Soon after the eggs are laid, the larvae emerge to feed on fungus growing in the damp potting soil.

When in large numbers, the larvae can damage the roots of African violets, carnations, poinsettias, and foliage plants, causing the plants to wilt and drop their leaves.

In a few days, the larvae pupate and become fungus gnats which are extremely annoying pests.

You can see them flying around when you water your indoor plants, or crawling around on the soil surface.

You can also spot them flying around lights or resting on windowpanes.

Use soilless potting mixtures made with coir and perlite. It has great drainage and is not conducive to the pests. When plants sit in waterlogged soil, the roots rot and become food the these soil scavengers.

Treatment for Gnats

1. Water thoroughly and allow the plant to dry out, being careful not to let the plant wilt. The larvae cannot survive in dry soil.

2. Place sticky traps horizontally across the rim of the plant pot or directly on the potting soil so the flies can land on the flat surface. Yellow sticky traps are a nontoxic way to control and monitor the adult and larval populations.

3. Remove dead leaves and apply a layer of diatomaceous earth, which is sold as Safer Brand 5170B Ant and Crawling Insect Killer 4 Pound bag, to the soil. These tiny mineral crystals will cut through the skin of the larvae, so they become dehydrated and die.

4. Use insecticidal soap to kill the adults and other soft-bodied insects. It works only on direct contact with the pests. It is nontoxic to humans and pets.

5 . Apply Bacillus thuringiensis var. israelensis (Bti) as a soil drench. This is a microbial insecticide to specifically kill the larvae. Trade names for Bti-products that allow use on houseplants go under trade names such as “Gnatrol” and “Knock-Out Gnats." Product is available through the internet.

Caution: When treating houseplant pests with insecticides, read the label carefully and use outdoors when possible. Keep all pesticides out of reach of small children and pets.

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