Fungus Gnats on Dieffenbachia: Causes and Cures

The Diagnosis

Black flies emerging from the soil of a Dieffenbachia are almost certainly fungus gnats. Their presence, coupled with yellowing leaves, points to overwatering and root rot.

Why They Appear

Fungus gnat larvae feed on decaying roots and organic matter in soggy soil. Adults are a nuisance but harmless; the larvae signal unhealthy root conditions.

Step-by-Step Eradication

1. Dry the soil: Let the top 1–2 inches dry completely before watering. Empty saucers promptly.
2. Top-dress: Apply a ½-inch layer of coarse sand, diatomaceous earth, or coconut coir to disrupt larval habitat.
3. Potato traps: Place raw potato slices on the soil overnight to attract and remove larvae. Repeat until clear.
4. Biological control: Apply Steinernema feltiae nematodes (Gnat Not, Knock-Out Gnats) as a soil drench for persistent infestations.
5. Repot if severe: In spring, repot in fresh, well-draining mix (add perlite). Remove all loose old soil.

Prevent Recurrence

Use sterile, bark-free potting mix. Water only when the top soil is dry. Ensure good drainage and air circulation. A moisture meter helps avoid guesswork.