Have you noticed a proliferation of small flies hovering around your houseplants? What a nuisance! Fungus gnats are common pests in the fall and winter. Did you move a potted plant inside for the winter? Most likely that’s how they arrived in your house and now they’ve found plenty of places to reproduce in your houseplants.
The adults are very small flies—only about 1/8 inch and look like mosquitoes. They won’t harm you or your plant. Their sole purpose is to lay eggs. A fungus gnat will only live for about 10 days but in that time, the females will lay about 200 eggs in your plant’s soil. The eggs hatch into larvae that eat fungi, decaying plant matter and roots.
Fungus Gnat Control Strategies
1) Re-pot your plant in sterile soil. The larvae will thrive on soil with organic matter. Check your soil. Does it have a high level of compost or wood chips? Replace it with a sterile potting soil. Always remove dead leaves or flowers that have fallen on the soil surface. Also, because the gnats lay eggs on the top 1 to 2 inches of soil, re-potting the plant will remove larvae and eggs. We recommend Fertilome Ultimate Potting Mix because it’s a sterile, well-drained growing medium that contains the proper mix of nutrients for most plants.
3) Use yellow sticky traps. Lay them on the soil or near the plant to capture the flies. This won’t do anything to stop the eggs from hatching, however.
4) Use insecticidal soap. We recommend the Bonide brand for a great organic option. Its plant-based ingredients kill the adults on contact. So spray the adults when you see them on the leaves or soil surface. You’ll need to use this product regularly.
5) Use a systemic insecticide. A systemic insecticide containing imidacloprid will kill the larvae in the soil. We recommend Bonide Systemic Houseplant Insect Control. Just sprinkle the granular on the soil and mix it into the top 2 inches. This product has no odor and is long-lasting—up to 8 weeks.
6) Be persistent. You may need to use all of the above strategies over a few months to get a profuse infestation under control.