Fungus Gnats: Kill the Larvae, Not the Flies You See

The small dark flies drifting up from a houseplant every time you water it are irritating and essentially harmless. Killing them accomplishes very little.

The larvae are the pest, and they are in the top inch of the compost — up to two hundred eggs per adult, hatching in under a week.

The life cycle is the whole problem

Roughly three to four weeks, and it runs continuously indoors:

1. Eggs — up to 200 per female, laid in damp compost in the top half inch. Hatch in 4 to 6 days.
2. Larvae — tiny, translucent, with a distinct black head. Two weeks feeding on fungi, algae, decaying matter and root hairs. This is the damaging stage.
3. Pupae — 3 to 4 days in the compost.
4. Adults — about a week, flying, mating and laying.

Which means: killing the adults you can see leaves several overlapping generations underground, and the flies are back within days. Any approach that targets only adults will feel like it is failing, because it is.

Why it matters, and when it doesn’t

On seedlings, cuttings and young plants, larvae can be genuinely destructive. Those plants have few roots and no reserves, and larvae grazing the fine root hairs is enough to kill them or stall them permanently.

On an established houseplant, the damage is subtler: unexplained wilting despite damp compost, slow or stalled growth, yellowing lower leaves. The plant is losing root function to something you cannot see.

Heavy infestations can also spread Pythium and other root-rot pathogens between pots.

What to do, in order

1. Let the surface dry. The top inch or two, properly dry, between waterings. This alone removes the habitat. Push a finger in to check rather than judging by the look of it.

2. Water from the bottom. Stand the pot in water for twenty minutes and drain it. The roots drink, the surface stays dry, and the eggs have nowhere to hatch.

3. Add a barrier. Half an inch of horticultural grit, coarse sand or perlite on the surface stops adults reaching the compost to lay. This works well and looks better than the alternatives.

4. Yellow sticky traps. Adults are strongly attracted to yellow. Traps do not solve the problem, but they catch egg-laying females, and — more usefully — they tell you whether the population is going up or down.

5. Biological control, for an established infestation:

6. Repot, if it is severe. Remove as much old compost as you can, check for root rot, and repot into fresh, free-draining mix — see potting soil. Add extra perlite to open it up.

Give it three to four weeks. You are outlasting eggs already in the compost. Numbers should fall noticeably by week two.

Not letting them come back

• Water properly: thoroughly, then not again until the plant needs it.
• Never leave standing water in saucers or cachepots.
• Right-sized pots. An oversized pot holds wet compost the roots cannot use.
• Store compost sealed and dry. Open bags left damp are how infestations arrive.
• Quarantine new plants for two or three weeks. Nursery plants are the other common route in.
• Clear dead leaves off the compost surface — that decaying material is what the larvae eat.

Is it actually a fungus gnat?

• Fungus gnat — dark, slender, long legs, weak erratic flight, comes out of pots.
• Fruit fly — stubby, tan, red eyes, purposeful flight, around fruit, bins and drains.
• Shore fly — darker and stockier than a fungus gnat, short legs, spotted wings, also from damp compost; the same controls work.
• Whitefly — white, on the undersides of leaves, flies up in a cloud when disturbed. A different pest entirely.