Fungus gnats

Fungus gnats are more nuisance than danger: the adults don't bite and live about a week. The larvae live in the top layer of damp soil, feed on organic matter and, in large numbers, on fine roots, which can hurt seedlings and cuttings.

How to tell

• Small, dark, slender flies (2–3 mm) around pots and windows, flying up when you water.
• Weak, zig-zag flight; they're not fruit flies, which are rounder, brown and gather at the fruit bowl.
• Tiny translucent larvae with black heads in the top centimetre of soil.

How to get rid of them

The life cycle takes three to four weeks, so every step needs a month.

1. Dry the soil. Let the top few centimetres dry out between waterings. Larvae die in dry soil. Watering from below, into the saucer, keeps the surface drier.
2. Trap adults with yellow sticky traps pushed into the pots. Fewer adults means fewer eggs.
3. Kill larvae with BTI, a bacterium sold as mosquito bits or dunks: soak it in your watering can and water with it weekly for four weeks. It only affects gnat and mosquito larvae.
4. Optionally, add beneficial nematodes (*Steinernema feltiae*) to the soil, or replace the top few centimetres with fresh soil.

How to prevent them

Don't overwater, use a well-draining mix, keep bags of potting soil sealed (they often arrive in it), and top pots with a thin layer of sand or grit, which the flies can't lay eggs through.