Fungus gnats

In short

Fungus gnats are small, dark flies that breed in constantly moist, organic potting mix. The adults are mostly a nuisance, but the larvae live in the top of the mix and can chew roots and the base of soft stems, especially on seedlings, cuttings and young plants.

How it looks

Adults are delicate, dark grey to black flies about 2–3 mm long (1/16–1/8 inch) that look like tiny mosquitoes: long slender legs, long segmented antennae and clear to light grey wings, often with a Y-shaped vein. They run over the surface of the mix and fly weakly around pots. Larvae are legless, whitish to see-through maggots up to about 6 mm (1/4 inch) long with a shiny black head capsule; they live in the top few centimetres of the growing medium and on stems just below the surface. Eggs are tiny, oval and whitish, laid on or in the mix.

Signs

Small flies rising from the pot when it is disturbed or watered, and adults caught on yellow sticky cards. With heavy larval numbers, seedlings and cuttings wilt, stall or collapse because fine roots are eaten and soft stems are tunnelled at or below the mix surface. Larvae and adults can also carry root-rot fungi such as Pythium and Fusarium between pots.

Life cycle

Egg, four larval stages, pupa and adult. At about 22–24 °C (72–75 °F) the whole cycle takes roughly two to three weeks: eggs hatch in about 3 days, larvae feed for about 10 days and pupae produce adults about 4 days later. Adults live about 10 days and a female can lay 100–200 eggs, so generations overlap all year indoors.

What favours it

Growing media that stay wet, especially peat- or bark-rich mixes with plenty of decaying organic matter; overwatering, poor drainage and pots standing in water. Warm indoor and tropical conditions let generations run continuously.

Prevention

• Let the top of the mix dry between waterings and never leave pots standing in water.
• Use a free-draining mix sized to the plant so it does not stay wet for days.
• Use fresh, pasteurised or bagged potting mix and keep it sealed.
• Remove fallen leaves, dead roots and other decaying debris from pot surfaces.
• Check new plants and hold them apart for a few days with a yellow sticky card nearby.

Treatment

Start with the first step and move down only if the problem persists.

1. Dry the mix out as far as the plant tolerates; this is the most important single step.
2. Hang or stake yellow sticky cards to catch adults and track numbers.
3. Push slices or chunks of raw potato into the mix surface for a few days to check for larvae underneath.
4. When repotting, remove as much old, infested mix as possible.
5. Drench with Bacillus thuringiensis subsp. israelensis (Bti), repeating about every five days, or apply Steinernema feltiae nematodes or soil-dwelling predatory mites (Stratiolaelaps/Hypoaspis) to the mix.
6. If still needed, insect growth regulator drenches aimed at the larvae, or short-lived pyrethrins against adults as a temporary measure — follow the product label.

Pros and cons

Look-alikes

Shore flies

Sturdier, housefly-like body with short bristle-like antennae and several pale spots on dark wings; larvae have no black head capsule and feed on algae, not roots.

Fruit flies (vinegar flies)

Tan bodies with bright red eyes; gather around ripe fruit, bins and drains rather than pots.

Affected plants

Any plant in moist organic media; seedlings, cuttings, young plants and plants in propagation are the most vulnerable.

Related

• Shore flies
• Fruit flies
• Root rot
• Damping off
• Overwatering
• Bulb mites

Sources

1. Fungus Gnats — UC IPM Pest Notes
2. Fungus Gnats and Shore Flies (E-111) — Purdue Extension Entomology

Written in our own words from these sources. Check a product label before using any treatment.