Fungus Gnats

by Janna
(Edmonton, Alberta, Canada)

Question:

How do I get rid of fungus gnats? They used to make a great fungus gnat powder, but it is no longer available.

I have heard that the flies themselves are harmless, but their larvae in the soil can damage the plant's roots. I have tried insecticidal soap, and have not had any success with it.

Answer:

Janna,

Fungus gnats are small flies that develop in the potting soil of houseplants. The larvae feed on plant roots; however, adult fungus gnats do not bite or feed on the plant.

How to Get Rid of Fungus Gnats

To prevent the fungus gnats from spreading; isolate the affected plant as soon as possible.

Place a small bowl filled with wine or apple cider vinegar near the infected plant. The fungus gnats will be drawn to the fermenting smell, and will eventually drown.

Remove a small amount of topsoil from the plant (approximately ¼ inch), and replace with a thin layer of sand. Fungus gnats are drawn to moist potting medium. They will avoid plants with sand on the soil surface.

If the fungus gnats are still present, repot the plant in fresh, sterile potting medium. Be sure to remove as much of the original soil from around the roots as possible. Remove diseased and/or rotting roots before repotting in the new potting medium.

How to Prevent Fungus Gnats

Inspect plants thoroughly before bringing them home. If necessary, remove any outer wrapping to get a good look at the plant and soil.

Isolate the new plants for a short period of time. Monitor the plants for a couple of weeks. If you don’t notice any problems, move them to their permanent location.

Fungus gnats thrive in warm, moist environments. Allow soil to dry out between watering – ideally, the top 1 – 2 inches.

How to Detect Fungus Gnats

An easy way to check for the presence of larvae is to insert ¼ inch potato wedges or slices into the potting soil. The fungus gnat larvae will start feeding on the potato slices within a couple of days. Be sure to inspect the potato wedges thoroughly.

I hope this helps. Please feel free to return and post a comment, give an update, or ask another question.

Warmest Regards,
Wendy