Fungus gnats on houseplants

Fungus gnats are small flies in the Sciaridae family, attracted to constantly moist potting mix where they lay eggs. The first step in getting rid of fungus gnats is to let the surface of the mix dry.

Their larvae, small white worms with black heads living in the mix, damage the plant by feeding on fine roots and organic matter. An uncontrolled infestation weakens the plant and can cause seedlings and young cuttings to decline.

Signs to check

• Tiny black flies, 2 to 5 mm (about 0.08 to 0.2 in) long, that fly up when the pot is moved.
• Potting mix that stays moist at the surface for several days after watering.
• Translucent white larvae with black heads, 5 to 12 mm (about 0.2 to 0.5 in) long, visible in the top few centimeters (an inch or so) of soil.
• Yellowing lower leaves and general wilting despite regular watering.

What you may notice

• Mold on the soil surface · A white or yellowish, cottony or powdery deposit develops on the potting mix surface.

What to do now

Get rid of fungus gnats

1. Let the potting mix dry. Wait until the top 3 to 5 cm (about 1 to 2 in) are completely dry before watering again. Larvae need moisture to survive, so this simple step disrupts their life cycle.
2. Place yellow sticky traps just above the potting mix. Yellow attracts and catches flying adults, preventing them from laying more eggs.
3. Water the soil with a solution made from 3% hydrogen peroxide (10-volume peroxide). Mix 1 part peroxide with 4 parts water. This solution kills larvae and eggs on contact without harming most plants.
4. Sprinkle a thin layer of diatomaceous earth over dry potting mix. This microscopically abrasive powder damages the cuticle of larvae that touch it, causing them to dehydrate and die.
5. Repeat the hydrogen peroxide drench every 7 to 10 days and replace traps when full. Continue for at least 3 to 4 weeks to eliminate successive generations.

Prevention

• Let the top 3 to 5 cm (about 1 to 2 in) of mix dry between waterings.
• Never leave standing water in saucers.
• Inspect and quarantine new plants for at least 2 weeks.
• Use a 1 to 2 cm (about 0.4 to 0.8 in) layer of sand or gravel to prevent adults from laying eggs in the mix.