Fungus Gnats in a Greenhouse

Fungus gnats can infest soil inside a greenhouse.

Fungus gnats inside a greenhouse damage seedlings, cuttings and young plants.

Elimination of the infestation requires the application of products that specifically target the larvae, which reside in the soil, as well as insecticides that kill adult gnats.

Issues

Adult fungus gnats do not feed, so they do not cause damage to plants inside a greenhouse, according to University of Connecticut Integrated Pest Management. Their larvae live in soil and may feed on the root hairs of plants. This leads to wilting and a failure to thrive.

Control

Fungus gnat larvae may be eliminated with a biological soil drench, such as Bacillus thuringiensis var. israelensis, also called Bt. You might need more than one application to eliminate all generations.

An insecticide that contains permethrin as an active ingredient may be used to kill the adults gnats.

Prevention

You may prevent fungus gnats from infesting your greenhouse by using only sterile soil and inspecting any new plants for gnats before bringing them inside. Avoid overwatering plants: Moist soil attracts fungus gnats. If one plant becomes infested, remove it right away, recommends University of Kentucky Agricultural Communication Services.

References

• University of Connecticut Integrated Pest Management: Fungus Gnats are a Serious Pest
• University of Kentucky Agricultural Communication Services: Greenhouse Insect Management
• University of Minnesota Department of Entymology: Astro