Fungus Gnats on Pothos: Dry the Mix, Then Target Larvae

Tiny flies lifting off the soil every time you water a pothos are fungus gnats. They are irritating, but rarely a serious threat to an established plant. University of Maryland Extension calls them annoying but harmless, without significant damage to indoor plants (UMD: Fungus Gnats). The real message is about the pot: the mix is staying wet for too long.

Confirm They Are Fungus Gnats

UC IPM describes adults as tiny dark flies about 1.5–3 mm (1/16–1/8 in) long, with antennae longer than the head (UC IPM: Fungus Gnats). Clemson HGIC notes they run over the soil, fly weakly and are drawn to light (HGIC 2252).

The larvae are the stage that matters. They are whitish, legless and have shiny black heads. Clemson says they grow to about 6 mm (1/4 in) and live in the soil.

Two simple checks:

• Yellow sticky cards set at the pot rim catch adults. UC IPM and UMD both recommend them for monitoring.
• A raw potato chunk pressed into the mix attracts larvae, according to UC IPM. Lift it after a few days and look underneath.

What They Do to a Pothos

Clemson notes the adults do not feed on plants. Larvae feed on decaying organic matter and fungi, and some species feed on roots. Colorado State says the larvae feed on decaying matter and root hairs (CSU: Managing Houseplant Pests).

Gnats are also a sign of overwatering. Virginia Cooperative Extension lists fungus gnats among overwatering symptoms, alongside wilting, yellowing and soft stems (VCE: Properly Watering Container Houseplants). If your pothos is drooping or yellowing in wet mix, check the roots for rot. Our pothos root rot guide covers that check.

What to Do

1. Let the mix dry. UC IPM and Clemson both advise letting the surface of container soil dry between waterings. UMD notes that letting potting soil dry out completely kills larvae. Clemson HGIC’s pothos guide also says to let pothos soil dry between waterings (HGIC 1568).
2. Empty the saucer. Clemson warns against letting water stand in saucers under containers. A pot sitting in water keeps the lower mix wet.
3. Check drainage. UC IPM lists poor drainage as a cause. Make sure the drainage hole is open and the outer cache pot is not holding water.
4. Trap the adults. Sticky cards cut down the flies that lay the next generation. On their own they do not reach the larvae.
5. Treat the larvae with Bti. UC IPM, UMD, Clemson and Colorado State all recommend Bacillus thuringiensis subspecies israelensis watered into the mix. Colorado State calls the Bti drench highly effective for larvae. Follow the product label.
6. Repot if the mix has broken down. Old, soggy, decomposing mix holds water longest. Fresh, airy mix dries faster, and Clemson recommends a well-draining mix for pothos.

Prevent a Return

• Water by the mix, not the calendar. Check moisture below the surface before watering.
• Inspect new plants. UC IPM advises inspecting new plants to avoid bringing gnats home.
• Look for other sources. UMD suggests covering suspected sources with screen or cheesecloth to see where flies come from, and notes gnats can also breed in drains.

For a longer look at stubborn infestations, see our fungus gnat article. For other pests, see the pest treatment overview, or return to the pothos hub.

Related guides

Other pothos problems we cover: mold, overwatering, algae confusion in water propagation, sunburn treatment, aerial roots.

References

• UC IPM: Fungus Gnats
• University of Maryland Extension: Fungus Gnats
• Clemson HGIC 2252: Common Houseplant Insects & Related Pests
• Colorado State University Extension: Managing Houseplant Pests
• Clemson HGIC 1568: How to Grow Pothos Indoors
• Virginia Cooperative Extension: Properly Watering Container Houseplants