Fruit flies are tiny, only 1–2 mm / ⅜–⅛ in. They have plump, pale brown or orange bodies, red eyes and a slow, hovering flight. Their maggots are slender, white and about 3–4 mm long, and you’ll find them in soft, fermenting fruit near the surface. Don’t confuse them with fungus gnats, which are black, slender and look like little mosquitoes.

No. Neither the flies nor their maggots harm compost worms. They’re a nuisance, though, especially indoors, and a big swarm tells you there’s more fruit in the bin than the worms can eat.

Most of them come in with the food. Adult flies lay eggs on fruit peels, especially bananas, and the eggs are too small to see. Once they’re in the warm, moist bin they hatch, and within about a week and a half there’s a new generation laying eggs of its own.

1. Freeze fruit scraps overnight before you feed them. This kills the eggs on the fruit and peels. One minute in the microwave also works.
2. Bury the food. Dig a trench through the middle of the bedding, put the scraps in, and cover them with about 5 cm / 2 in of finished worm castings or bedding. The castings act as a natural barrier.
3. Cover the bedding. Lay a sheet of damp newspaper, cardboard or sacking over the whole surface.
4. Feed less fruit at a time. Only add more when the last meal has mostly gone.

Yes. Fruit flies love a bin that is too wet and sour. Lots of fruit makes the bedding acidic, and soggy, airless bedding ferments. Both draw flies in. Keep the bedding as damp as a wrung-out sponge, mix in dry shredded cardboard when it’s soggy, and dust a pinch of agricultural lime or crushed eggshells over the surface to balance the pH. Feed used coffee grounds sparingly, as they add to the acidity. A bin between 15 and 25 °C (59–77 °F) suits the worms and lets them clear the food before the flies can.

1. Take a large, empty plastic soda bottle and cut off the top just where the round shoulder turns into the straight side.
2. With a sharp nail or scissors, punch 5 or 6 small holes of 2–3 mm about 10 cm / 4 in above the bottom.
3. Put a few banana skins into the bottle.
4. Turn the cut-off top upside down and push it into the opening like a funnel. Make sure it fits tightly, and seal it with clear sticky tape.
5. Place the trap a few metres from the worm farm, on something raised.

Then open the worm bin, shoo out as many flies as you can and close it again. The flies follow the smell of the banana into the trap and most can’t find their way out. You should catch most of them within 48 hours. Set the trap again if needed, then feed the dead flies to your worms.

First, take out the worst pocket of fermenting fruit along with its maggots. Bury whatever fruit is left and cover the surface. Stop adding fruit for a week, and set two or three traps. If the bin is wet, mix in dry cardboard. Within a few days the numbers drop sharply.

Fungus gnats are black, slender and look like tiny mosquitoes. They’re attracted to fungi and moisture. Let the surface dry out a little, but remember that the worms still need moist bedding, and cut back on feeding. We’ve never had a big problem with gnats and usually just ignore them.