Fungus gnats: small flies around wet soil

Start by letting the top of the soil dry out. That one change deals with the conditions they need.

What it looks like

Adults are dark, delicate flies, a bit like tiny mosquitoes, with long antennae. They’re drawn to light, so you may first notice them flying around a window or lamp.

The larvae live in the growing mix. They have a shiny black head and a clear to whitish legless body. In very wet, heavily infested mix, they can leave shiny slime trails on the surface.

In large numbers, larvae can damage roots and stunt growth, mostly in seedlings and young plants (UC IPM).

Why it happens

Larvae feed on fungi, decaying organic matter and root hairs, so damp mix rich in organic material suits them well. Overwatering, poor drainage, leaks and standing water all help.

At 75°F, eggs hatch in about 3 days, larvae pupate after about 10 days, and adults emerge about 4 days after that. That’s about 17 days for a generation, and it goes faster when it’s warmer (UC IPM). Generations overlap, so you won’t clear them with a single treatment.

How to confirm it

1. Hang a yellow sticky trap near the pot to catch adults.
2. Put a chunk of raw potato on the soil, cut side down. Check underneath after a few days for larvae, then replace it (UC IPM).
3. Check the antennae if you can. Shore flies also gather around wet mix, but they’re stouter, with short bristle-like antennae. They’re associated with algae and rotting organic matter.

If your plant is wilting, don’t blame the gnats yet. Larvae can cause it, but unhealthy roots from other causes are more common (UC IPM). Too much or too little water and root rot can look the same. Gnats often appear because the mix is wet, so they may be a sign of the real problem rather than the cause of it.

What to do

1. Let the surface dry between waterings. This is the main fix. Also improve drainage, clean up standing water and fix any leaks.
2. Remove what they’re feeding on, such as fallen leaves, debris and excess organic material on the soil.
3. Use Bti if you need more. Bacillus thuringiensis subspecies israelensis is sold in garden centers and only affects certain fly larvae. It gives temporary control, so expect to reapply.
4. Or try nematodes. Steinernema feltiae works between 60°F and 90°F in moist mix, and it’s applied as a soil drench. Nematodes are perishable and usually bought by mail.
5. Use pasteurized potting mix in your next round.

Insecticide sprays are rarely needed for gnats, and pyrethrins or pyrethroids only give temporary control (UC IPM). Sticky traps catch adults, but they won’t deal with the larvae in the soil.

How much to trust this

Everything here comes from general guidance for houseplants, greenhouses and container plants. We didn’t find a controlled study of fungus gnats on cannabis, or of how much damage they do to it. The advice is low-risk and the biology is general to these flies, but we can’t tell you how serious they are for cannabis specifically.