Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Constantly moist media is ideal for fungus gnat development. Cut up a potato into half inch strips (like a French fry) and stick 2 inch-long pieces into pots and plugs throughout your range. The top of the strip should be just above the media. Take a look at the strips after 48 hours. The black head capsules of fungus gnat larvae are easy to see against the white potato. If you wait longer than 48 hours to look at the strips, they start to rot and turn brown, making it more difficult to see the larvae. The best way to control fungus gnats is to allow the media to dry out whenever possible. 

If fungus gnats become so abundant that the larvae cause root pruning damage (usually only on cuttings or very small plants) or the adults become a nuisance, apply Azatin XL, Adept, Distance, Marathon or Mesurol as a soil drench. A drench will control larvae for several weeks, preventing new emergence of adults, while a spray only kills adults for a few days and has no effect on the larvae in the soil.