Pests

Fungus gnats

Also called: sciarid flies, soil gnats

Small dark flies drifting up from wet potting mix; the larvae in the soil are the real story.

What you'd see

Tiny grey-black flies, weak fliers, rising from the soil when you water
Translucent larvae with black heads in the top few centimetres of mix
Seedlings or cuttings wilting despite moist soil (larvae nibbling roots)
Constantly damp soil surface, sometimes with green algae

Where to look

Any pot whose top layer stays wet. Mostly a nuisance to people; a real threat only to seedlings and fresh cuttings.

How it spreads

Adults lay 100–200 eggs in wet mix and live about a week. New bags of compost often carry them.

Is it really this? Look-alikes

Fruit flies — Fruit flies are tan with red eyes and hover around food, not pots.
Shore flies — Stouter, stronger fliers with white spots on the wings; harmless.

What's going on

Fungus gnat larvae eat fungi and decaying organic matter in soggy soil. Wet mix grows the fungus that feeds them; dry mix does not.

Peat-heavy mixes that hold water at the surface are gnat heaven. Chunky, fast-draining mixes are not.

Why?

This is a watering signal dressed as a pest. If gnats are breeding, the top layer of soil is staying wet longer than the plant needs — which also means the roots are getting less air than they want.

Grandmother's remedies

What people have always done — with an honest note on each.

Let it dry
Allow the top 3–5 cm of mix to dry fully between waterings; water from below.
Verdict: The cure. Everything else is support.
Sand or grit mulch
A centimetre of coarse sand or grit on the surface.
Verdict: Helps by keeping the surface dry; adults still find gaps at the pot edge.
Potato slices
Raw potato slices on the soil for a day, then binned with the larvae attached.
Verdict: A charming, honest monitoring trick; removes some larvae, not all.
Vinegar trap
A dish of cider vinegar with a drop of soap.
Verdict: Catches fruit flies well; fungus gnats are largely uninterested.

Modern remedies

What growers and extension services reach for today.

Yellow sticky cards
Placed horizontally at soil level.
Verdict: Excellent for adults and for telling whether numbers are falling.
BTi (Bacillus thuringiensis israelensis)
Mosquito-dunk bacteria dissolved in watering water, used at each watering for a month.
Verdict: Safe, well-evidenced and kills larvae. Combine with drying out.
Beneficial nematodes (Steinernema feltiae)
Watered into the mix.
Verdict: Effective against larvae; needs moist soil to work, so use before the dry-out phase.

Honest verdict

Gnats come from the soil, so leaf sprays do nothing. Drying the mix, sticky cards for adults and BTi for larvae end almost every case in three to four weeks.

Prevention

Water by soil check, not by schedule
Use chunky mixes with bark and perlite
Bottom-water propagation trays
Store opened compost bags sealed and dry

Plants this commonly troubles

Monstera Pothos Philodendron