How to Control Fungus Gnats | The Three-Prong Approach

Controlling fungus gnats means breaking their life cycle: let the soil dry out, trap adults with yellow sticky traps, and kill larvae in the soil with Bti drenches.

A potted plant’s soil stays damp, and that’s exactly what fungus gnats need to breed. The tiny black flies circling your houseplants are the adults; the real damage happens in the soil, where larvae feed on roots and organic matter. The good news is that you can clear an infestation in two to three weeks with the right sequence — no harsh chemicals required if you start early.

Let the Soil Dry Between Waterings

Fungus gnat eggs and larvae can’t survive in dry soil. Oklahoma State University Extension recommends letting the top inch of potting mix dry out completely before you water again. For moisture-sensitive plants, switch to bottom-watering: set the pot in a tray of water and let the roots drink from below while the soil surface stays dry. This single change cuts the gnat population more than any spray.

Trap Adults Before They Lay More Eggs

Yellow sticky traps are the standard tool. Cornell Cooperative Extension notes that bright yellow traps attract adult gnats and catch them before they can lay eggs in fresh soil. Place one or two traps near the plant leaves, level with the top of the pot. Replace them when they are covered, roughly every week during an active infestation. These traps also work as an early-warning system — if you see gnats on the yellow surface, you know the larvae treatment below is still needed.

The vinegar attracts gnats; the soap breaks the surface tension so they drown. Replace the liquid every few days.

Kill Larvae With Bti Soil Drenches

Bti (Bacillus thuringiensis israelensis) is a naturally occurring bacterium that specifically targets fungus gnat larvae in the soil. You can buy it as granules or liquid — mix it with water according to the package directions and drench the potting mix every five to seven days for three applications. The bacterium kills larvae as they hatch but is harmless to people, pets, and the plant itself. This is the most reliable single chemical-free method. If you want a targeted product list, our fungus gnat larvae control roundup covers the top Bti treatments tested for houseplant use.

When to Use Insecticides

Extension sources reserve chemical insecticides for severe infestations where Bti and traps haven’t kept up. Indoor-label sprays containing pyrethrins or synthetic pyrethroids (like Raid’s houseplant formula or Sevin) can be applied to the soil surface and lower leaves to knock down adults. Michigan State University’s guidance says to use these as soil drenches, not sprayed into the air. Always follow the label’s indoor-use instructions exactly. For most home infestations, the dry-down plus traps plus Bti sequence handles the problem before insecticides are necessary.